← Ray Model of Light

Lesson 6 of 8 · Textbook 5.3

Smooth and rough surfaces

You can see yourself in a mirror but not in a sheet of paper. Both reflect light — the difference is entirely about which way the surface is facing.

The two cases

When parallel rays of light fall on a smooth surface, each ray is reflected in the same direction. When they fall on a rough surface, each ray is reflected in a different direction.

Parallel rays meeting a surface

The dashed lines are the normals. On the rough surface they point in different directions — and that, not any change in the law, is what scatters the light.
The point the exam actually tests For both smooth and rough surfaces, the angle of reflection of each light ray is equal to the angle of incidence. The law of reflection is never broken.

What changes on a rough surface is the direction of the normal at each point. The surface tilts differently from place to place, so each ray meets its own normal at its own angle and leaves in its own direction. Saying “on a rough surface i is not equal to r” is wrong, and it is the answer examiners are watching for.

Where each one happens

Reflection on a smooth surface Reflection on a rough surface
Each ray reflected in the same direction Each ray reflected in a different direction
Clear images are formed Light is scattered; no image forms
A plane mirror · a highly polished metallic surface · the calm surface of a water body A sheet of paper · a wall · most everyday objects

Although a piece of white paper looks smooth, its surface is uneven and rough when viewed under a microscope. Since the rough surface of the paper scatters light, we are not able to see our reflection in it.

Why a reservoir works like a mirror On a non-windy day a still body of water forms a smooth, flat surface — much like a plane mirror — so you see the trees and the sky reflected in it. Add wind, and the surface becomes uneven: the normals scatter, and the reflection breaks up into shimmer.

Same water, same law, different roughness. That is the whole of this lesson in one example.

Check yourself