5.2P Explain the difference between specular and diffuse reflection

The idea

Specular and diffuse reflection differ in the smoothness of the reflecting surface and therefore in how parallel incident rays leave it. At a smooth surface such as a plane mirror, parallel rays strike at the same angle and reflect as a parallel beam in one direction; this is specular reflection and produces a clear image. At a rough surface such as paper or a wall, the surface normal points in many directions, so parallel rays strike at different angles and reflect in many directions; this is diffuse reflection and no clear image forms. In both cases the law of reflection still holds at each point: the angle of incidence equals the angle of reflection measured from the local normal. The difference is the orientation of normals across the surface, not a breakdown of the law.

What earns the marks

• Identifies specular reflection as occurring at a smooth surface where parallel rays reflect as a parallel beam in one direction.
• Identifies diffuse reflection as occurring at a rough surface where parallel rays reflect in many directions.
• Explains that the local normal points in different directions on a rough surface, so rays strike at different angles.
• States that the law of reflection still applies at each point in both specular and diffuse reflection.
• Links specular reflection to the formation of a clear image and diffuse reflection to no clear image.

Common mistakes and how to put them right

• Claims the law of reflection does not apply to rough surfaces.

  Put it right: State that it applies at each point but the normals differ.

• Says diffuse reflection produces a clear image.

  Put it right: State that scattered rays give no clear image.

• Describes specular reflection as light reflecting in all directions.

  Put it right: State that parallel rays stay parallel in one direction.

• Confuses the surface type with the law.

  Put it right: Identify smoothness of the surface as the cause of the difference.

Exam tips

• Read the options carefully and eliminate any that say the law of reflection fails on rough surfaces.
• Look for the key contrast: parallel beam from a smooth surface versus scattered rays from a rough surface.
• Check whether the option correctly links a clear image to specular reflection only.
• Confirm the option mentions the direction of the normal on the surface, since that explains the difference.