Edexcel IGCSE Physics · Spec 3.15-3.17
Reflection & Refraction of Light
How light reflects and refracts, shown with ray diagrams.
Physics revision video
Reflection & Refraction of Light
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Explained
Reflection and refraction, and the line everything is measured from
Every angle in this topic is measured from the normal, which is an imaginary line drawn at ninety degrees to the surface at the point where the ray meets it. Not from the surface. Getting that wrong turns a 30 degree angle into a 60 degree one, and it is the most common way to lose marks here.
Reflection
The law of reflection is that the angle of incidence equals the angle of reflection, both measured from the normal. The incident ray, the reflected ray and the normal all lie in the same plane.
When you draw it, mark the normal as a dashed line, label both angles, and use a ruler. Examiners accept a small tolerance on the angle but not a freehand sketch.
Refraction, and why it happens
Refraction is a change in direction caused by a change in speed at a boundary. Light travels more slowly in glass than in air, and it is that change in speed which bends the ray.
Entering glass from air, light slows down and bends towards the normal. Leaving glass into air, it speeds up and bends away from the normal. Learn it as a pair, because a glass block question requires both.
A ray that meets the boundary along the normal, at ninety degrees to the surface, does not change direction at all. It still changes speed, but there is nothing to bend, and questions use this case to check that you know speed and direction are separate things.
The glass block investigation
Place a rectangular glass block on paper and draw round it. Shine a ray at the surface and mark the incoming ray with two points, then mark where it leaves the block. Remove the block, join the marks, and draw the normal where the ray entered.
Measure the angle of incidence and the angle of refraction from the normal, and repeat for several different angles. A ray leaving a rectangular block emerges parallel to the ray that entered it, but displaced sideways, which is a useful check that the drawing is right.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Physics name two errors directly. In one ray diagram question, a common error was drawing refraction instead of reflection, or drawing an incorrect normal.
The reports also note that candidates could usually state a condition for total internal reflection, but that their attempts to describe total internal reflection itself were often too imprecise to be credited. If you are asked what it is, say that all of the light is reflected back into the denser material and none is refracted out.
Read the command word before drawing anything. Reflect and refract look similar on a diagram and describe opposite things.
Spec 3.15-3.17
What you need to know
- State the law of reflection
- Explain why light refracts at a boundary
- Describe investigating rays with a glass block
Active recall
Quick check
Answer each question before opening the answer.
State the law of reflection.
The angle of incidence equals the angle of reflection.
What is refraction and why does it happen?
The change in direction of light as it passes between media, caused by a change in its speed.
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