How to Solve: Reflection of Light

How to Solve: Reflection of Light

(For Students Who Want to Ace Their Exam & Teachers Who Need a Ready-to-Record Script)

Introduction

"If you’ve ever wondered why mirrors flip your image, how periscopes work, or why your exam question asks for the angle of reflection—this is the guide that will get you full marks. Let’s break it down step by step."

What You Need To Know First

Before diving into reflection, make sure you understand: 1. Rays and beams – Light travels in straight lines called rays. A group of rays is a beam. 2. Angles – How to measure angles in degrees (0° to 360°) and identify acute, right, and obtuse angles. 3. Normal line – An imaginary line perpendicular (90°) to a surface at the point where light hits it.

(If any of these are unclear, pause and review them first—reflection builds on these basics!)

Key Vocabulary

Formulas To Know

(Note: No complex formulas here—just the Law of Reflection and mirror properties!)

Step-by-Step Method

How to Solve Any Reflection Problem (4 Steps)

1. Draw the normal line
2. At the point where the incident ray hits the surface, draw a dotted line perpendicular (90°) to the surface.

3. Why? The normal is your reference line for measuring angles.

4. Measure the angle of incidence (i)

5. Use a protractor to measure the angle between the incident ray and the normal.

6. Tip: If the angle is given in the question, write it down (e.g., i = 50°).

7. Apply the Law of Reflection (i = r)

8. The angle of reflection (r) must equal the angle of incidence (i).

9. Example: If i = 35°, then r = 35°.

10. Draw the reflected ray

11. From the point of incidence, draw the reflected ray at angle r from the normal.
12. Check: The reflected ray should be on the opposite side of the normal from the incident ray.

Worked Example (Using the Steps): Question: A light ray strikes a mirror at an angle of 25° to the mirror’s surface. What is the angle of reflection?

1. Draw the normal – Perpendicular to the mirror at the point of incidence.
2. Measure i – The angle between the incident ray and the mirror is 25°, so the angle between the incident ray and the normal is:
   i = 90° – 25° = 65°
3. Apply i = r – r = i = 65°
4. Draw the reflected ray – 65° from the normal, on the opposite side of the incident ray.

Answer: The angle of reflection is 65°.

Worked Examples

Example 1 – Basic (No Tricks)

Question: A ray of light hits a plane mirror at an angle of 40° to the normal. What is the angle of reflection?

Solution: 1. Normal is already drawn (given in the question). 2. Angle of incidence (i) = 40° (given). 3. Law of Reflection: i = r → r = 40°. 4. Reflected ray is 40° from the normal.

Answer: The angle of reflection is 40°.

What we did and why: - We used the Law of Reflection directly because the angle was given relative to the normal. - No calculations needed—just apply i = r.

Example 2 – Medium (Angle Given Relative to Surface)

Question: A light ray strikes a mirror at an angle of 30° to the mirror’s surface. Calculate the angle of reflection.

Solution: 1. Draw the normal – Perpendicular to the mirror. 2. Find i – The angle between the incident ray and the mirror is 30°, so:
i = 90° – 30° = 60° 3. Apply i = r → r = 60°. 4. Draw the reflected ray – 60° from the normal.

Answer: The angle of reflection is 60°.

What we did and why: - The angle was given relative to the mirror, not the normal, so we subtracted from 90° to find i. - Always check if the angle is measured from the surface or the normal!

Example 3 – Exam Style (Disguised Question)

Question: A student stands 2 m in front of a plane mirror. She moves 0.5 m closer to the mirror. What is the new distance between the student and her image?

Solution: 1. Understand the setup – Plane mirror image distance = object distance. 2. Initial distance – Student is 2 m from mirror → image is 2 m behind the mirror.
Total distance = 2 m (student to mirror) + 2 m (mirror to image) = 4 m. 3. After moving – New distance to mirror = 2 m – 0.5 m = 1.5 m.
Image distance = 1.5 m behind mirror. 4. New total distance = 1.5 m (student to mirror) + 1.5 m (mirror to image) = 3 m.

Answer: The new distance is 3 m.

What we did and why: - The question tested image properties in a real-world scenario. - We used the rule: Image distance = Object distance for plane mirrors. - The "trick" was recognising that the total distance includes both the object and image distances.

Common Mistakes

Exam Traps

1-Minute Recap

(Spoken naturally, addressing the student directly)

"Okay, let’s lock this in for your exam. Here’s the one-minute recap for reflection of light:

1. Law of Reflection is simple: Angle in (i) = Angle out (r). Memorise this.
2. Always draw the normal—it’s your 90° reference line. No normal? No marks.
3. If the angle is given to the mirror (not the normal), subtract from 90° to find i.
4. Plane mirrors produce virtual images—same size, same distance behind the mirror, but left and right are swapped.
5. Watch for exam traps: Angles to the surface, missing normals, or questions about image properties.

That’s it. Draw the normal, measure i, set r = i, and you’ve got full marks. Now go practice—you’ve got this!