Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Laws of Reflection of Light

Introduction

The Laws of reflection are (1) The incident ray, reflected ray and normal remains in a plane (2) The angle of incidence and angle of reflection are equal. These laws are introduced at middle school level. Students find it difficult to comprehend these laws because they involve three dimensional geometry.

Procedure

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Place the plane mirror vertically on the table. Place a laser torch at some height \(h_1\) in front of the mirror so that light falls on the mirror traversing a path in horizontal plane. Locate the spot made by the reflected beam and measure its height \(h_2\).

Check that \(h_1=h_2\). Since the source is at height \(h_1\) and incident ray is in horizontal plane, the point of reflection should be at that same height. As the mirror is vertical, the normal lies in the horizontal plane at height \(h_1\). This shows that incident ray, reflected ray and normal lies in same plane (horizontal plane in this case).

Variant: Take a transparent plastic box of approximate size approx 12 in by 18 in by 6 in. Paste a plane mirror on one of the vertical face (from inside the box) with the help of adhesive (quickfix etc). Make a small hole to push a incense stick (dhoop, agarbatti). A big protector placed on the top can be used to measure angle. You can mark horizontal lines on the mirror so that height of the spot where incident ray strikes is known. This setup can be used to show both laws of reflection.