IGCSE Physics (0625)

Refraction and refractive index

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

The refractive index compares the speed of light in a vacuum (or air) to its speed in the material, not the other way around.
Common mistake: Students often mistakenly invert this ratio.
The refractive index is defined as the ratio of the sine of the angle of incidence (in the first medium) to the sine of the angle of refraction (in the second medium), both measured from the normal.
When light enters a medium with higher optical density (where it travels slower), it bends towards the normal.
Common mistake: Students often reverse this, incorrectly saying it bends away from the normal when entering a denser medium.
Applying Snell's law requires using the correct form (sin i / sin r = n) and recognizing that when light enters a denser medium, the angle of refraction is smaller than the angle of incidence.
Common mistake: Students often mistakenly make it larger.
When light enters a denser medium, the refracted ray bends towards the normal (not away), and the angle of refraction is always measured from the normal inside the new medium.
When light passes from a denser to a less dense medium at an angle less than the critical angle, it refracts away from the normal (angle increases).
Common mistake: A ray along the normal does not change direction—students often forget the normal ray does not bend.

Define the refractive index of a transparent material in terms of the speeds of light.

Try it first, then show the working

State what kind of quantity refractive index is: The refractive index n is a ratio comparing two speeds of light.
Identify the numerator and denominator of the ratio: n = speed of light in a vacuum (or air) ÷ speed of light in the material.
Write the definition as one statement: The refractive index is the ratio of the speed of light in a vacuum (or air) to the speed of light in the material.

Answer: The refractive index is the ratio of the speed of light in a vacuum (or air) to the speed of light in the material.

13 exam-style questions on Refraction and refractive index, each marked to the scheme.