Reflection and total internal reflection: the law of reflection, specular versus diffuse reflection, the critical angle, and the conditions for total internal reflection.

What this dot point is asking

Edexcel statements 5.1 and 5.2 (separate physics) want you to explain reflection with the aid of ray diagrams, including the law of reflection and the critical angle, the difference between specular and diffuse reflection, and the conditions for total internal reflection.

The law of reflection

Both angles are always measured from the normal, not from the surface, which is a frequent source of lost marks. On a ray diagram you draw the normal first, then the incident and reflected rays making equal angles with it. This law holds at every point on any surface, smooth or rough.

Specular and diffuse reflection

The law of reflection is obeyed in both cases; the difference is the surface. At a rough surface the normal points in slightly different directions at each tiny bump, so the reflected rays head off at many angles and the image is scattered. This is why a mirror gives a sharp reflection but a sheet of paper, lit by the same light, does not.

Total internal reflection

As the angle of incidence increases towards the critical angle, the refracted ray bends further from the normal until, at the critical angle, it runs along the boundary. Beyond that angle there is no refracted ray and the light is entirely reflected. This is used in optical fibres (light bounces along the fibre by repeated TIR, carrying data or images in endoscopes) and in reflecting prisms in binoculars and periscopes.

How Edexcel examines this

These statements are separate-physics only and are examined on both tiers within that route. Reflection appears as a definition question (the equal-angles law) and a ray-diagram task where you must mark the normal and equal angles; the mark scheme penalises angles measured from the surface. The specular-versus-diffuse contrast is a reliable two or three mark question rewarding the smooth-versus-rough surface distinction and the effect on the image. Total internal reflection is a higher-value question (three to four marks) asking for both conditions, light going from a denser to a less dense medium and an angle of incidence greater than the critical angle, plus a use such as optical fibres or reflecting prisms. Examiners reward stating both conditions explicitly; giving only "the angle is big enough" without the denser-to-less-dense condition loses a mark. You may also be asked to define the critical angle as the angle giving an angle of refraction of $90^{\circ}$, so learn that precise wording.

Try this

Q1. State the law of reflection. [1 mark]

• Cue. The angle of incidence equals the angle of reflection (measured from the normal).

Q2. State the two conditions needed for total internal reflection. [2 marks]

• Cue. Light going from a denser to a less dense medium, and an angle of incidence greater than the critical angle.