Surface roughness controls reflected-ray directions
Definition
Specular reflection
Reflection from a smooth surface in which parallel incident rays remain parallel after reflection.
Definition
Diffuse reflection
Reflection from a rough surface in which parallel incident rays are reflected in many directions.
- A smooth surface has normals pointing in almost the same direction at neighbouring points.
- Parallel rays therefore strike at the same angle of incidence and leave at the same angle of reflection.
- The reflected rays remain ordered, so a clear image can form in a mirror, still water or polished metal.
- Each ray obeys i=ri=ri=r.
- A rough surface has microscopic slopes, so the normal points in a different direction at each point.
- Each ray still obeys i=ri=ri=r relative to its own local normal, but the reflected rays travel in different directions.
- The scattered rays do not preserve the ordered pattern needed for a clear image.
- Paper, unpolished wood and a painted wall usually produce diffuse reflection.
Diffuse reflection makes objects visible from many positions
- A smooth surface directs most reflected light into a narrow range of directions, so glare or a bright reflected image may be seen from only certain positions.
- A rough surface sends reflected light towards observers in many positions, which is why writing on paper can be read from different angles.
- Diffuse reflection does not mean that light is absorbed; it means that the reflected light leaves in many directions.
Example
Comparing two surfaces
- Parallel rays striking a plane mirror meet nearly parallel local normals.
- The rays reflect in parallel directions, so the mirror produces specular reflection and can form an image.
- The same rays striking matt paper meet differently oriented local normals.
- The rays obey i=ri=ri=r at each point but spread out, so the paper produces diffuse reflection and no clear image.
Common Mistake
- Do not say that diffuse reflection breaks the law of reflection; each ray still obeys i=ri=ri=r at its point of incidence.
- Do not describe a visibly rough object only by its large-scale shape; the relevant roughness is on the scale of the light-reflecting surface.
- Do not confuse diffuse reflection with absorption.
Exam technique
Explaining the difference
- Compare the surfaces first: smooth for specular reflection and rough for diffuse reflection.
- Then compare the rays: parallel rays remain parallel after specular reflection but leave in many directions after diffuse reflection.
- State that the law of reflection applies to every individual ray in both cases.
Self review
- Define specular reflection.
- Define diffuse reflection.
- Explain why a rough surface sends reflected rays in different directions.
- State whether the law of reflection applies during diffuse reflection.
- Explain why matt paper can be viewed from many positions.
