Question

State the laws of reflection and describe an experiment to verify them.

Answer 1

Laws of reflection:

1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane.

Experiment to verify the laws of reflection:

Fix a white sheet of paper on a drawing board and draw a line MM₁ as shown in figure. On this line, take a point O nearly at the middle of it and draw a line OA such that ∠MOA is less than 90°. Then draw a normal ON on line MM₁ at the point O, and place a small plane mirror vertical by means of a stand with its silvered surface along MM₁.

Next fix two pins P and Q at some distance (≈5 cm) apart vertically on line OA, on the board. Keeping eye on the other side of normal (but on the same side of mirror), see clearly images P’ and Q’ of the pins P and Q. Next fix a pin R such that it is in line with the images of pins P and Q as observed in the mirror. Next, fix one more pin S such that the pin S is in line with the pin R as well as images P’ and Q’ of pins P and Q.

Draw small circles on paper around the positions of pins as shown in figure. Remove the pins and draw a line OB joining the pin points S and R, which meets the surface of mirror at O. The angles AON and BON are measured and recorded.

The experiment is then repeated for the angle of incidence ∠AON equal to 40°, 50°, 60°.

From results, it is observed that angle of incidence is equal to the angle of reflection. This verifies the first law of reflection.

The experiment has been performed on a flat drawing board, with mirror normal to the plane of board on which white sheet of paper is being fixed. Since the lower tips of all the pins also lie on the same plane (i.e., the plane of paper), it proves the second law of reflection.