The law of reflection is one of the simplest ideas in GCSE Physics: the angle of incidence equals the angle of reflection. So why do so many students get reflection questions wrong? It’s all about the normal.
What is the normal?
The normal is an imaginary line drawn at 90° to the surface, at the point where the ray hits. You draw it as a dashed line, and every angle is measured from it, not from the mirror.
The trap
A ray hits a mirror at 20° to the mirror’s surface. What is the angle of reflection?
Lots of students write 20°. But the angle of incidence is measured from the normal, so it’s 90° − 20° = 70°. The angle of reflection is 70°.
Reflect, absorb or transmit?
When any wave meets a boundary it can be reflected, absorbed or transmitted, and often all three happen at once. What happens depends on the wavelength and the materials involved.
Specular vs diffuse
A smooth mirror reflects all the light in one direction (specular reflection), so you see an image. Rough paper scatters it in all directions (diffuse reflection), so you don’t. Every ray still obeys the law of reflection; the normals just point different ways.
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