5.2P-5.3P - Surfaces, filters and colour

Specular and diffuse reflection

A polished mirror gives a clear image, but a sheet of paper can be seen from many directions without acting like a mirror. The difference is the orientation of the surface on a small scale.

• In specular reflection, a smooth surface has nearly parallel local normals. Parallel incident rays are reflected in one orderly direction, so an image can remain clear.
• In diffuse reflection, a rough surface has differently oriented local normals. Parallel incident rays leave in many directions, so no clear image is formed.

Every ray in diffuse reflection still obeys the law of reflection at its own point of incidence. The rays do not break the law; their local normals point in different directions.

Surfaces, filters and colour

Colour adds a different question: which colours of the incident light are absorbed, reflected or transmitted? White light contains a range of visible colours.

At an opaque surface, differential absorption means that different colours are absorbed by different amounts. A red surface in white light absorbs most other colours and reflects red light towards an observer. The colour seen therefore depends on both the surface and the colours present in the incident light. Under blue-only light, that same red surface has no red light available to reflect and appears very dark.

A filter acts on transmitted light. An ideal red filter transmits red and absorbs other colours. It does not add red to the beam or turn every colour into red. Blue-only light incident on an ideal red filter is absorbed, so almost no light emerges.