IGCSE Physics - 3.2.1 Reflection of Light

This section covers the reflection of light by plane mirrors, including using constructions, measurements, and calculations.

1. Law of Reflection

The law of reflection states that the angle of incidence is equal to the angle of reflection. This means the angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal.

Let's denote:

• θ_i = Angle of incidence (angle between the incident ray and the normal)
• θ_r = Angle of reflection (angle between the reflected ray and the normal)

According to the law of reflection: θ_i = θ_r

2. Using a Mirror and a Straightedge to Find the Point of Reflection

We can use a simple construction to find the point where a ray of light reflects from a plane mirror. This involves using a straightedge to draw a line that passes through the point of incidence and the reflected ray.

1. Draw a straight line on a piece of paper. This represents the mirror.
2. Mark a point on the line to represent the point where the light ray strikes the mirror (the point of incidence).
3. From the point of incidence, draw a line in the same direction as the incident ray.
4. Now, draw a line that passes through the point of incidence and intersects the line representing the mirror. The point where these two lines intersect is the point of reflection.

3. Angle of Incidence and Angle of Reflection

The angle of incidence (θ_i) is the angle between the incident ray and the normal to the mirror. The normal is a line perpendicular to the mirror at the point of incidence.

The angle of reflection (θ_r) is the angle between the reflected ray and the normal to the mirror.

As stated by the law of reflection, θ_i = θ_r.

4. Measuring Angles of Incidence and Reflection

To measure the angle of incidence and reflection, you can use a protractor. Place the protractor along the incident ray and the normal, and read the angle. Do the same for the reflected ray and the normal.

5. Calculating Angles of Incidence and Reflection

In many situations, we can calculate the angles of incidence and reflection using trigonometry. Consider a ray of light making an angle θ_i with the normal. The angle of reflection will also be θ_i.

Consider a scenario where the incident ray makes an angle θ with the mirror. The angle of incidence is θ. The angle of reflection is also θ.

We can relate the angle of incidence to the angle with the mirror using the following relationship:

$$ \theta = \text{Angle of incidence} = \text{Angle with the mirror} $$

6. Example Calculation

A ray of light strikes a plane mirror at an angle of 30° with the normal. What is the angle of incidence and the angle of reflection?

Angle of incidence = 30°

Angle of reflection = 30°

7. Summary

Understanding the law of reflection and how to construct a mirror to find the point of reflection are fundamental to understanding the reflection of light. Using a protractor to measure angles and trigonometric relationships allows us to calculate these angles in various scenarios.