Seven original Cambridge-style questions. They target the explanation marks students lose: the bending rule, optical versus mass density, and the one property that does not change when light refracts.
Define refraction, and state what causes it.
If you only mention "bending" without the change in speed, you usually miss the second mark.
A ray of light passes from air into a glass block, and later leaves the glass block back into air.
(a) State which way the ray bends as it enters the glass, and explain why. [2] (b) State which way it bends as it leaves the glass, and explain why. [2](a) Toward the normal. Glass is optically denser than air, so the light slows down, which bends it toward the normal. ✓✓
(b) Away from the normal. Going back into air the light speeds up, which bends it away from the normal. ✓✓
Always tie the bending direction to the change in speed. Slower → toward. Faster → away.
A student writes: "Light slows down in glass because glass is heavier than air." Explain why this answer would not earn the explanation mark, and give a correct version.
Optical density, not mass density. This single word is the difference between a mark and a cross.
When light passes from air into glass, state what happens to each of the following: its speed, its wavelength, and its frequency.
Frequency is fixed by the source. The classic error is to say frequency changes. It never does at a boundary.
A ray of light travels along the normal as it meets the flat surface of a glass block (angle of incidence = 0°).
(a) Does the ray change direction? Explain. [2] (b) Does the ray change speed? [1](a) No. At 0° incidence the whole wavefront reaches the boundary at the same instant, so there is nothing to pivot the ray. It passes straight through. ✓✓
(b) Yes. It still slows down in the glass, even though its direction is unchanged. ✓
A favourite trap: students assume "refraction" must mean bending. Along the normal, the speed still changes but the direction does not.
The diagram shows a ray of light in air striking the flat surface of a glass block. The normal is drawn at the point where the ray meets the surface.
(a) Refracted ray drawn bending toward the normal, steeper than the incident ray. ✓✓
(b) The angle of refraction is smaller than the angle of incidence. ✓
Use a ruler. The refracted ray is a single straight line with an arrow, bending toward the normal.
A straight straw standing in a glass of water appears to be bent at the point where it enters the water. Explain this observation in terms of refraction.
The straw is not actually bent. It is the refraction of light leaving the water that shifts the apparent position.
Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.