Question

State the laws of reflection. Describe an experimental method to verify the laws of reflection of light.

Accepted Answer

The returning of light in the same medium after striking a surface, is called reflection of light. The reflection of light from a plane surface or from a spherical surface takes place according to two laws called the laws of reflection of light. The two laws of reflection of light are as follows:

1) The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

2) The angle of incidence $(i)$ is always equal to the angle of reflection $(r)$ , then

$∠ i = ∠ r$

The laws of reflection can be verified by the following simple experiment.

1) Take a drawing board and a white sheet of paper. Fix the white sheet of paper on the drawing board. Draw a straight line $M M_{1}$ on the paper as shown in the following figure

2) On the line $M M_{1}$ , take a point $O$ nearly at its middle and draw a line $O A$ such that $∠ M O A$ is less than $90 °$ . Draw a normal $O N$ on $M M_{1}$ , at the point $O$

3) Now take a small plane mirror and place its long edge along the line $M M_{1}$ . Use plasticine to fix the mirror on the paper so that it stands upright

4) Now fix two pins $P$ and $Q$ vertically on the line $O A$ as shown in the above figure

5) Then set eye on the other side of the normal $O N$ , along the direction as shown in the diagram. The images $P'$ and $Q'$ of the pins $P$ and $Q$ in the mirror can be observed.

6) Set eye in line with the two images $P'$ and $Q'$ and fix two more pins $R$ and $S$ on the paper in front of the mirror such that both of them are in line with $P'$ and $Q'$ .

7) Now remove the mirror. Draw small circles around the pins $P, Q, R$ and $S$ to mark their positions as shown in the following figure.

8) Draw a line $O B$ joining the points $R$ and $S$ . $A O$ is the incident ray, $O B$ is the reflected ray and $O N$ is the normal at the point of incidence $O$ .

9) Now using a protractor, measure the angles $i = ∠ A O N$ and $r = ∠ B O N$ . We can find that the two angles are equal.

This verifies that the angle of incidence $i$ is equal to the angle of reflection $r$ . Since the incident ray, normal and the reflected ray all lie in the same plane of the paper, this verifies the second law of reflection.

If a ray of light incident normally or perpendicularly on a mirror, it is reflected back along the same path. This is because the angle of incidence as well as the angle of reflection is zero.

State the laws of reflection. Describe an experimental method to verify the laws of reflection of light.

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