General Science Review: Physics

Thermodynamics
Thermodynamics is a branch of physics that studies the conversion of energy into work and heat. It is especially concerned with variables such as temperature, volume, and pressure.  Thermodynamic equilibrium refers to objects that have the same temperature because heat is transferred between them to reach equilibrium.  Thermodynamics takes places within three different types of systems; open, isolated, and closed systems.  Open systems are capable of interacting with a surrounding environment and can exchange heat, work (energy), and matter outside their system boundaries. A closed system can exchange heat and work, but not matter. An isolated system cannot exchange heat, work, or matter with its surroundings. Its total energy and mass stay the same.  In physics, surrounding environment refers to everything outside a thermodynamic system (system). The terms 'surroundings' and 'environment' are also used. The term 'boundary' refers to the division between the system and its surroundings.

The laws of thermodynamics are generalized principles dealing with energy and heat.

The zeroth law of thermodynamics states that two objects in thermodynamic equilibrium with a third object are also in equilibrium with each other. Being in thermodynamic equilibrium basically means that different objects are at the same temperature.

The first law deals with conservation of energy. It states that neither mass nor energy can be destroyed; only converted from one form to another.

The second law states that the entropy (the amount of energy in a system that is no longer available for work or the amount of disorder in a system) of an isolated system can only increase. The second law also states that heat is not transferred from a lower-temperature system to a higher-temperature one unless additional work is done.

The third law of thermodynamics states that as temperature approaches absolute zero, entropy approaches a constant minimum. It also states that a system cannot be cooled to absolute zero.

Thermal contact refers to energy transferred to a body by a means other than work. A system in thermal contact with another can exchange energy with it through the process of heat transfer. Thermal contact does not necessarily involve direct physical contact.  Heat is energy that can be transferred from one body or system to another without work being done.

Everything tends to become less organized and less useful over time (entropy).

In all energy transfers, therefore, the overall result is that the heat is spread out so that objects are in thermodynamic equilibrium and the heat can no longer be transferred without additional work.

The laws of thermodynamics state that energy can be exchanged between physical systems as heat or work, and that systems are affected by their surroundings. It can be said that the total amount of energy in the universe is constant. The first law is mainly concerned with the conservation of energy and related concepts, which include the statement that energy can only be transferred or converted, not created or destroyed. The formula used to represent the first law is ∆U = Q – W, where ∆U is the change in total internal energy of a system, Q is the heat added to the system, and W is the work done by the system. Energy can be transferred by conduction, convection, radiation, mass transfer, and other processes such as collisions in chemical and nuclear reactions. As transfers occur, the matter involved becomes less ordered and less useful. This tendency towards disorder is also referred to as entropy.

The second law of thermodynamics explains how energy can be used. In particular, it states that heat will not transfer spontaneously from a cold object to a hot object. Another way to say this is that heat transfers occur from higher temperatures to lower temperatures. Also covered under this law is the concept that systems not under the influence of external forces tend to become more disordered over time. This type of disorder can be expressed in terms of entropy. Another principle covered under this law is that it is impossible to make a heat engine that can extract heat and convert it all to useful work. A thermal bottleneck occurs in machines that convert energy to heat and then use it to do work. These types of machines are less efficient than ones that are solely mechanical.

Conduction is a form of heat transfer that occurs at the molecular level. It is the result of molecular agitation that occurs within an object, body, or material while the material stays motionless. An example of this is when a frying pan is placed on a hot burner. At first, the handle is not hot. As the pan becomes hotter due to conduction, the handle eventually gets hot too. In this example, energy is being transferred down the handle toward the colder end because the higher speed particles collide with and transfer energy to the slower ones. When this happens, the original material becomes cooler and the second material becomes hotter until equilibrium is reached. Thermal conduction can also occur between two substances such as a cup of hot coffee and the colder surface it is placed on. Heat is transferred, but matter is not.

Convection refers to heat transfer that occurs through the movement or circulation of fluids (liquids or gases). Some of the fluid becomes or is hotter than the surrounding fluid, and is less dense. Heat is transferred away from the source of the heat to a cooler, denser area. Examples of convection are boiling water and the movement of warm and cold air currents in the atmosphere and the ocean. Forced convection occurs in convection ovens, where a fan helps circulate hot air.


Radiation is heat transfer that occurs through the emission of electromagnetic waves, which carry energy away from the emitting object. 

All objects with temperatures above absolute zero radiate heat.

Temperature is a measurement of an object's stored heat energy. More specifically, temperature is the average kinetic energy of an object's particles. When the temperature of an object increases and its atoms move faster, kinetic energy also increases. Temperature is not energy since it changes and is not conserved. Thermometers are used to measure temperature.

There are three main scales for measuring temperature.  Celsius uses the base reference points of water freezing at 0 degrees and boiling at 100 degrees.   Fahrenheit uses the base reference points of water freezing at 32 degrees and boiling at 212 degrees. 

Celsius and Fahrenheit are both relative temperature scales since they use water as their reference point.  The Kelvin temperature scale is an absolute temperature scale.  Its zero mark corresponds to absolute zero.  Water's freezing and boiling points are 273.15 Kelvin and 373.15 Kelvin, respectively. 

Where Celsius and Fahrenheit are measured is degrees, Kelvin does not use degree terminology.

- Converting Celsius to Fahrenheit:



· Converting Fahrenheit to Celsius:



- Converting Celsius to Kelvin:



- Converting Kelvin to Celsius:



Heat capacity, also known as thermal mass, refers to the amount of heat energy required to raise the temperature of an object, and is measured in Joules per Kelvin or Joules per degree Celsius. The equation for relating heat energy to heat capacity is

, where Q is the heat energy transferred, C is the heat capacity of the body, and ∆T is the change in the object's temperature.  Specific heat capacity, also known as specific heat, is the heat capacity per unit mass. Each element and compound has its own specific heat. For example, it takes different amounts of heat energy to raise the temperature of the same amounts of magnesium and lead by one degree. The equation for relating heat energy to specific heat capacity is

, where m represents the mass of the object, and c represents its specific heat capacity.

Heat Capacity
Some discussions of energy consider only two types of energy: kinetic energy (the energy of motion) and potential energy (which depends on relative position or orientation). There are, however, other types of energy. Electromagnetic waves, for example, are a type of energy contained by a field. Another type of potential energy is electrical energy, which is the energy it takes to pull apart positive and negative electrical charges.

Chemical energy refers to the manner in which atoms form into molecules, and this energy can be released or absorbed when molecules regroup. Solar energy comes in the form of visible light and non-visible light, such as infrared and ultraviolet rays. Sound energy refers to the energy in sound waves.

Energy is constantly changing forms and being transferred back and forth.  An example of a heat to mechanical energy transformation is a steam engine, such as the type used on a steam locomotive. A heat source such as coal is used to boil water. The steam produced turns a shaft, which eventually turns the wheels.  A pendulum swinging is an example of both a kinetic to potential and a potential to kinetic energy transformation. When a pendulum is moved from its center point (the point at which it is closest to the ground) to the highest point before it returns, it is an example of a kinetic to potential transformation. When it swings from its highest point toward the center, it is considered a potential to kinetic transformation. The sum of the potential and kinetic energy is known as the total mechanical energy.  Stretching a rubber band gives it potential energy. That potential energy becomes kinetic energy when the rubber band is released.

Motion and Force
Mechanics is the study of matter and motion, and the topics related to matter and motion, such as force, energy, and work.  Discussions of mechanics will often include the concepts of vectors and scalars. Vectors are quantities with both magnitude and direction, while scalars have only magnitude. Scalar quantities include length, area, volume, mass, density, energy, work, and power.  Vector quantities include displacement, direction, velocity, acceleration, momentum, and force.

Motion is a change in the location of an object and is the result of an unbalanced net force acting on the object.  Understanding motion requires an understanding of three basic quantities: displacement, velocity, and acceleration.

Displacement
When something moves from one place to another, it has undergone displacement.  Displacement along a straight line is a very simple example of a vector quantity.  If an object travels from position x = -5 cm to x = 5 cm, it has undergone a displacement of 10 cm.  If it traverses the same path in the opposite direction, its displacement is -10 cm.  A vector that spans the object's displacement in the direction of travel is known as a displacement vector.

Velocity
There are two types of velocity to consider: average velocity and instantaneous velocity.  Unless an object has a constant velocity or we are explicitly given an equation for the velocity, finding the instantaneous velocity of an object requires the use of calculus. 

If we want to calculate the average velocity of an object, we need to know two things: the displacement, or the distance it has covered, and the time it took to cover this distance.  The formula for average velocity is simply the distance traveled divided by the time required.  In other words, the average velocity is equal to the change in position divided by the change in time. 

Average velocity is a vector and will always point in the same direction as the displacement vector (since time is a scalar and always positive).

Acceleration
cceleration is the change in the velocity of an object.  On most test questions, the acceleration will be a constant value. 

Like position and velocity, acceleration is a vector quantity and will therefore have both magnitude and direction.

Most motion can be explained by Newton's three laws of motion:

Newton's first law
An object at rest or in motion will remain at rest or in motion unless acted upon by an external force.  This phenomenon is commonly referred to as inertia, the tendency of a body to remain in its present state of motion.  In order for the body's state of motion to change, it must be acted on by an unbalanced force.

Newton's second law
An object's acceleration is directly proportional to the net force acting on the object, and inversely proportional to the object's mass.  This law is generally written in equation form F=ma, where F is the net force acting on a body, m is the mass of the body, and a is its acceleration.  Note that since the mass is always a positive quantity, the acceleration is always in the same direction as the force.

Newton's third law
For every force, there is an equal and opposite force.  When a hammer strikes a nail, the nail hits the hammer just as hard.  If we consider two objects, A and B, then we may express any contact between these two bodies with the equation FAB= -FBA, where the order of the subscripts denotes which body is exerting the force.  At first glance, this law might seem to forbid any movement at all since every force is being countered with an equal opposite force, but these equal opposite forces are acting on different bodies with different masses, so they will not cancel each other out.

Energy

The two types of energy most important in mechanics are potential and kinetic energy.  Potential energy is the amount of energy an object has stored within itself because of its position or orientation.  There are many types of potential energy, but the most common is gravitational potential energy.  It is the energy that an object has because of its height (h) above the ground.  It can be calculated as PE = mgh, where m is the object's mass and g is the acceleration of gravity.  Kinetic energy is the energy of an object in motion, and is calculated as KE = mv2/2, where v is the magnitude of its velocity.  When an object is dropped, its potential energy is converted into kinetic energy as it falls.  These two equations can be used to calculate the velocity of an object at any point in its fall.

Work

Work can be thought of as the amount of energy expended in accomplishing some goal.  The simplest equation for mechanical work (W) is W = Fd, where F is the force exerted and d is the displacement of the object on which the force is exerted.  This equation requires that the force be applied in the same direction as the displacement. 

If this is not the case, then the work may be calculated as W = Fd cos(θ), where θ is the angle between the force and displacement vectors.  If force and displacement have the same direction, then work is positive; if they are in opposite directions, then work is negative; and if they are perpendicular, the work done by the force is zero. A. an example, if a man pushes a block horizontally across a surface with a constant force of 10 N for a distance of 20 m, the work done by the man is 200 N-m or 200 J.  If instead the block is sliding and the man tries to slow its progress by pushing against it, his work done is -200 J, since he is pushing in the direction opposite the motion.  If the man pushes vertically downward on the block while it slides, his work done is zero, since his force vector is perpendicular to the displacement vector of the block.

Friction
Friction is a force that arises as a resistance to motion where two surfaces are in contact. The maximum magnitude of the frictional force (f)can be calculated as f = Fcµ, where Fc is the contact force between the two objects and µ is a coefficient of friction based on the surfaces' material composition.  Two types of friction are static and kinetic.  To illustrate these concepts, imagine a book resting on a table.  The force of its weight (W) is equal and opposite to the force of the table on the book, or the normal force (N).  If we exert a small force (F) on the book, attempting to push it to one side, a frictional force (f) would arise, equal and opposite to our force.  At this point, it is a static frictional force because the book is not moving.  If we increase our force on the book, we will eventually cause it to move.  At this point, the frictional force opposing us will be a kinetic frictional force. 

Generally, the kinetic frictional force is lower than static frictional force (because the frictional coefficient for static friction is larger), which means that the amount of force needed to maintain the movement of the book will be less than what was needed to start it moving.



Gravitational force
Gravitational force is a universal force that causes every object to exert a force on every other object.  The gravitational force between two objects can be described by the formula, F = Gm1m2/r2, where m1 and m2 are the masses of two objects, r is the distance between them, and G is the gravitational constant, G = 6.672 x 10-11N-m2/kg2.  In order for this force to have a noticeable effect, one or both of the objects must be extremely large, so the equation is generally only used in problems involving planetary bodies.  For problems involving objects on the earth being affected by earth's gravitational pull, the force of gravity is simply calculated as F = mg, where g is 9.81 m/s2 toward the ground.

Electrical force
Electrical force is a universal force that exists between any two electrically charged objects. Opposite charges attract one another and like charges repel one another. The magnitude of the force is directly proportional to the magnitude of the charges (q)and inversely proportional to the square of the distance (r) between the two objects: F = kq1q2/r2, where k = 9 x 109 N-m2/C2.  Magnetic forces operate on a similar principle.

Buoyancy
Archimedes's principle states that a buoyant (upward) force on a submerged object is equal to the weight of the liquid displaced by the object.  Water has a density of one gram per cubic centimeter.  Anything that floats in water has a lower density, and anything that sinks has a higher density.  This principle of buoyancy can also be used to calculate the volume of an irregularly shaped object.  The mass of the object (m) minus its apparent mass in the water (ma) divided by the density of water (ρw), gives the object's volume: V = (m-ma)/ρw.

Machines

Simple machines include the inclined plane, lever, wheel and axle, and pulley. These simple machines have no internal source of energy. More complex or compound machines can be formed from them. Simple machines provide a force known as a mechanical advantage and make it easier to accomplish a task.

The inclined plane enables a force less than the object's weight to be used to push an object to a greater height. A lever enables a multiplication of force.

The wheel and axle allow for movement with less resistance. Single or double pulleys allow for easier direction of force. The wedge and screw are forms of the inclined plane. A wedge turns a smaller force working over a greater distance into a larger force. The screw is similar to an incline that is wrapped around a shaft.

A certain amount of work is required to move an object. The amount cannot be reduced, but by changing the way the work is performed a mechanical advantage can be gained. A certain amount of work is required to raise an object to a given vertical height. By getting to a given height at an angle, the effort required is reduced, but the distance that must be traveled to reach a given height is increased. An example of this is walking up a hill.

One may take a direct, shorter, but steeper route, or one may take a more meandering, longer route that requires less effort. Examples of wedges include doorstops, axes, plows, zippers, and can openers.

A lever consists of a bar or plank and a pivot point or fulcrum. Work is performed by the bar, which swings at the pivot point to redirect the force. There are three types of levers: first, second, and third class. Examples of a first-class lever include balances, see-saws, nail extractors, and scissors (which also use wedges). In a second-class lever the fulcrum is placed at one end of the bar and the work is performed at the other end. The weight or load to be moved is in between. The closer to the fulcrum the weight is, the easier it is to move. Force is increased, but the distance it is moved is decreased. Examples include pry bars, bottle openers, nutcrackers, and wheelbarrows. In a third-class lever the fulcrum is at one end and the positions of the weight and the location where the work is performed are reversed. Examples include fishing rods, hammers, and tweezers.

The center of a wheel and axle can be likened to a fulcrum on a rotating lever. As it turns, the wheel moves a greater distance than the axle, but with less force. Obvious examples of the wheel and axle are the wheels of a car, but this type of simple machine can also be used to exert a greater force. For instance, a person can turn the handles of a winch to exert a greater force at the turning axle to move an object. Other examples include steering wheels, wrenches, faucets, waterwheels, windmills, gears, and belts. Gears work together to change a force. The four basic types of gears are spur, rack and pinion, bevel, and worm gears. The larger gear turns slower than the smaller, but exerts a greater force. Gears at angles can be used to change the direction of forces.





A single pulley consists of a rope or line that is run around a wheel. This allows force to be directed in a downward motion to lift an object. This does not decrease the force required, just changes its direction. The load is moved the same distance as the rope pulling it. When a combination pulley is used, such as a double pulley, the weight is moved half the distance of the rope pulling it. In this way, the work effort is doubled. Pulleys are never 100% efficient because of friction.

Examples of pulleys include cranes, chain hoists, block and tackles, and elevators.

Electrical Charges

A glass rod and a plastic rod can illustrate the concept of static electricity due to friction. Both start with no charge. A glass rod rubbed with silk produces a positive charge, while a plastic rod rubbed with fur produces a negative charge. The electron affinity of a material is a property that helps determine how easily it can be charged by friction.

Materials can be sorted by their affinity for electrons into a triboelectric series. Materials with greater affinities include celluloid, sulfur, and rubber. Materials with lower affinities include glass, rabbit fur, and asbestos. In the example of a glass rod and a plastic one, the glass rod rubbed with silk acquires a positive charge because glass has a lower affinity for electrons than silk. The electrons flow to the silk, leaving the rod with fewer electrons and a positive charge. When a plastic rod is rubbed with fur, electrons flow to the rod and result in a negative charge.

The attractive force between the electrons and the nucleus is called the electric force. A positive (+) charge or a negative (-) charge creates a field of sorts in the empty space around it, which is known as an electric field. The direction of a positive charge is away from it and the direction of a negative charge is towards it. An electron within the force of the field is pulled towards a positive charge because an electron has a negative charge. A particle with a positive charge is pushed away, or repelled, by another positive charge. Like charges repel each other and opposite charges attract. Lines of force show the paths of charges. The electric force between two objects is directly proportional to the product of the charge magnitudes and inversely proportional to the square of the distance between the two objects. Electric charge is measured with the unit Coulomb (C). It is the amount of charge moved in one second by a steady current of one ampere (1C = 1A × 1s).

Insulators are materials that prevent the movement of electrical charges, while conductors are materials that allow the movement of electrical charges. This is because conductive materials have free electrons that can move through the entire volume of the conductor. This allows an external charge to change the charge distribution in the material. In induction, a neutral conductive material, such as a sphere, can become charged by a positively or negatively charged object, such as a rod. The charged object is placed close to the material without touching it. This produces a force on the free electrons, which will either be attracted to or repelled by the rod, polarizing (or separating) the charge. The sphere's electrons will flow into or out of it when touched by a ground. The sphere is now charged. The charge will be opposite that of the charging rod.

Charging by conduction is similar to charging by induction, except that the material transferring the charge actually touches the material receiving the charge. A negatively or positively charged object is touched to an object with a neutral charge. Electrons will either flow into or out of the neutral object and it will become charged. Insulators cannot be used to conduct charges. Charging by conduction can also be called charging by contact. The law of conservation of charge states that the total number of units before and after a charging process remains the same. No electrons have been created. They have just been moved around. The removal of a charge on an object by conduction is called grounding.

Circuits

Electric potential, or electrostatic potential or voltage, is an expression of potential energy per unit of charge. It is measured in volts (V) as a scalar quantity. The formula used is V = E/Q, where V is voltage, E is electrical potential energy, and Q is the charge. Voltage is typically discussed in the context of electric potential difference between two points in a circuit.  Voltage can also be thought of as a measure of the rate at which energy is drawn from a source in order to produce a flow of electric charge.

Electric current is the sustained flow of electrons that are part of an electric charge moving along a path in a circuit. This differs from a static electric charge, which is a constant non-moving charge rather than a continuous flow. The rate of flow of electric charge is expressed using the ampere (amp or A) and can be measured using an ammeter. A current of 1 ampere means that 1 coulomb of charge passes through a given area every second.

Electric charges typically only move from areas of high electric potential to areas of low electric potential.  To get charges to flow into a high potential area, you must connect it to an area of higher potential, by introducing a battery or other voltage source.

Electric currents experience resistance as they travel through a circuit.  Different objects have different levels of resistance.  The ohm (Ω) is the measurement unit of electric resistance. The symbol is the Greek letter omega. Ohm's Law, which is expressed as I = V/R, states that current flow (I, measured in amps) through an object is equal to the potential difference from one side to the other (V, measured in volts) divided by resistance (R, measured in ohms). An object with a higher resistance will have a lower current flow through it given the same potential difference.

Movement of electric charge along a path between areas of high electric potential and low electric potential, with a resistor or load device between them, is the definition of a simple circuit. It is a closed conducting path between the high and low potential points, such as the positive and negative terminals on a battery. One example of a circuit is the flow from one terminal of a car battery to the other. The electrolyte solution of water and sulfuric acid provides work in chemical form to start the flow. A frequently used classroom example of circuits involves using a D cell (1.5 V) battery, a small light bulb, and a piece of copper wire to create a circuit to light the bulb.

Magnets
A magnet is a piece of metal, such as iron, steel, or magnetite (lodestone) that can affect another substance within its field of force that has like characteristics. Magnets can either attract or repel other substances. Magnets have two poles: north and south. Like poles repel and opposite poles (pairs of north and south) attract. The magnetic field is a set of invisible lines representing the paths of attraction and repulsion.

Magnetism can occur naturally, or ferromagnetic materials can be magnetized.

Certain matter that is magnetized can retain its magnetic properties indefinitely and become a permanent magnet. Other matter can lose its magnetic properties. For example, an iron nail can be temporarily magnetized by stroking it repeatedly in the same direction using one pole of another magnet. Once magnetized, it can attract or repel other magnetically inclined materials, such as paper clips. Dropping the nail repeatedly will cause it to lose its charge.

The motions of subatomic structures (nuclei and electrons) produce a magnetic field. It is the direction of the spin and orbit that indicates the direction of the field. The strength of a magnetic field is known as the magnetic moment. As electrons spin and orbit a nucleus, they produce a magnetic field. Pairs of electrons that spin and orbit in opposite directions cancel each other out, creating a net magnetic field of zero. Materials that have an unpaired electron are magnetic. Those with a weak attractive force are referred to as paramagnetic materials, while ferromagnetic materials have a strong attractive force. A diamagnetic material has electrons that are paired, and therefore does not typically have a magnetic moment. There are, however, some diamagnetic materials that have a weak magnetic field.



A magnetic field can be formed not only by a magnetic material, but also by electric current flowing through a wire. When a coiled wire is attached to the two ends of a battery, for example, an electromagnet can be formed by inserting a ferromagnetic material such as an iron bar within the coil. When electric current flows through the wire, the bar becomes a magnet. If there is no current, the magnetism is lost. A magnetic domain occurs when the magnetic fields of atoms are grouped and aligned. These groups form what can be thought of as miniature magnets within a material. This is what happens when an object like an iron nail is temporarily magnetized. Prior to magnetization, the organization of atoms and their various polarities are somewhat random with respect to where the north and south poles are pointing. After magnetization, a significant percentage of the poles are lined up in one direction, which is what causes the magnetic force exerted by the material.


Waves
Waves have energy and can transfer energy when they interact with matter. Although waves transfer energy, they do not transport matter. They are a disturbance of matter that transfers energy from one particle to an adjacent particle. There are many types of waves, including sound, seismic, water, light, micro, and radio waves.

The two basic categories of waves are mechanical and electromagnetic. Mechanical waves are those that transmit energy through matter. Electromagnetic waves can transmit energy through a vacuum. A transverse wave provides a good illustration of the features of a wave, which include crests, troughs, amplitude, and wavelength.

There are a number of important attributes of waves. 

Frequency is a measure of how often particles in a medium vibrate when a wave passes through the medium with respect to a certain point or node. Usually measured in Hertz (Hz), frequency might refer to cycles per second, vibrations per second, or waves per second. One Hz is equal to one cycle per second.

Period is a measure of how long it takes to complete a cycle. It is the inverse of frequency; where frequency is measure in cycles per second, period can be thought of as seconds per cycle, though it is measured in units of time only.

Speed refers to how fast or slow a wave travels. It is measured in terms of distance divided by time. While frequency is measured in terms of cycles per second, speed might be measured in terms of meters per second.

Amplitude is the maximum amount of displacement of a particle in a medium from its rest position, and corresponds to the amount of energy carried by the wave. High energy waves have greater amplitudes; low energy waves have lesser amplitudes. Amplitude is a measure of a wave's strength.

Rest position, also called equilibrium, is the point at which there is neither positive nor negative displacement.

Crest, also called the peak, is the point at which a wave's positive or upward displacement from the rest position is at its maximum. 

Trough, also called a valley, is the point at which a wave's negative or downward displacement from the rest position is at its maximum. A wavelength is one complete wave cycle. It could be measured from crest to crest, trough to trough, rest position to rest position, or any point of a wave to the corresponding point on the next wave.

Sound is a pressure disturbance that moves through a medium in the form of mechanical waves, which transfer energy from one particle to the next. Sound requires a medium to travel through, such as air, water, or other matter since it is the vibrations that transfer energy to adjacent particles, not the actual movement of particles over a great distance. Sound is transferred through the movement of atomic particles, which can be atoms or molecules. Waves of sound energy move outward in all directions from the source. Sound waves consist of compressions (particles are forced together) and rarefactions (particles move farther apart and their density decreases). A wavelength consists of one compression and one rarefaction. Different sounds have different wavelengths. Sound is a form of kinetic energy.

The electromagnetic spectrum is defined by frequency (f) and wavelength (λ). Frequency is typically measured in hertz and wavelength is usually measured in meters. Because light travels at a fairly constant speed, frequency is inversely proportional to wavelength, a relationship expressed by the formula f = c/λ, where c is the speed of light (about 300 million meters per second). Frequency multiplied by wavelength equals the speed of the wave; for electromagnetic waves, this is the speed of light, with some variance for the medium in which it is traveling. 

Electromagnetic waves include (from largest to smallest wavelength) radio waves, microwaves, infrared radiation (radiant heat), visible light, ultraviolet radiation, x-rays, and gamma rays. The energy of electromagnetic waves is carried in packets that have a magnitude inversely proportional to the wavelength. Radio waves have a range of wavelengths, from about 10-3 to 105 meters, while their frequencies range from 103 to about 1011 Hz.

Spectrum
Atoms and molecules can gain or lose energy only in particular, discrete amounts. Therefore, they can absorb and emit light only at wavelengths that correspond to these amounts. Using a process known as spectroscopy, these characteristic wavelengths can be used to identify substances.

Light is the portion of the electromagnetic spectrum that is visible because of its ability to stimulate the retina. It is absorbed and emitted by electrons, atoms, and molecules that move from one energy level to another. Visible light interacts with matter through molecular electron excitation (which occurs in the human retina) and through plasma oscillations (which occur in metals). Visible light is between ultraviolet and infrared light on the spectrum. The wavelengths of visible light cover a range from 380 nm (violet) to 760 nm (red). Different wavelengths correspond to different colors. The human brain interprets or perceives visible light, which is emitted from the sun and other stars, as color. For example, when the entire wavelength reaches the retina, the brain perceives the color white. When no part of the wavelength reaches the retina, the brain perceives the color black.

When light waves encounter an object, they are either reflected, transmitted, or absorbed.  If the light is reflected from the surface of the object, the angle at which it contacts the surface will be the same as the angle at which it leaves, on the other side of the perpendicular. 

If the ray of light is perpendicular to the surface, it will be reflected back in the direction from which it came.  When light is transmitted through the object, its direction may be altered upon entering the object.  This is known as refraction.  The degree to which the light is refracted depends on the speed at which light travels in the object.  Light that is neither reflected nor transmitted will be absorbed by the surface and stored as heat energy.  Nearly all instances of light hitting an object will involve a combination of two or even all three of these.

When light waves are refracted, or bent, an image can appear distorted. Sound waves and water waves can also be refracted.  Diffraction refers to the bending of waves around small objects and the spreading out of waves past small openings. The narrower the opening, the greater the level of diffraction will be. Larger wavelengths also increase diffraction. A diffraction grating can be created by placing a number of slits close together, and is used more frequently than a prism to separate light. Different wavelengths are diffracted at different angles.  The particular color of an object depends upon what is absorbed and what is transmitted or reflected. For example, a leaf consists of chlorophyll molecules, the atoms of which absorb all wavelengths of the visible light spectrum except for green, which is why a leaf appears green.  Certain wavelengths of visible light can be absorbed when they interact with matter.

Wavelengths that are not absorbed can be transmitted by transparent materials or reflected by opaque materials.


The various properties of light have numerous real-life applications.  For example, polarized sunglasses have lenses that help reduce glare, while non-polarized sunglasses reduce the total amount of light that reaches the eyes. Polarized lenses consist of a chemical film of molecules aligned in parallel. This allows the lenses to block wavelengths of light that are intense, horizontal, and reflected from smooth, flat surfaces.  The 'fiber' in fiber optics refers to a tube or pipe that channels light. Because of the composition of the fiber, light can be transmitted greater distances before losing the signal. The fiber consists of a core, cladding, and a coating.

Fibers are bundled, allowing for the transmission of large amounts of data.