Refraction through a glass block

Waves

GCSE Physics (8463)

ORIGINAL

Describe the ray diagram step by step — normals, angles from the normal, the bend in and out — then, for Higher tier, explain it with wave speed.

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Question

A ray of light in air strikes one face of a rectangular glass block at an angle of incidence of

40^\circ

. It passes through the block and leaves through the opposite, parallel face. Describe how to draw the ray diagram and the path of the ray. Higher tier: explain why the ray changes direction at the first face.

Every step worked, with the reasoning.

1
Draw the normal: a dashed line at $90^\circ$ to the surface of the block, at the point where the ray enters.
Every angle in a ray diagram is measured from the normal, not from the surface.
2
Measure $40^\circ$ from the normal and draw the incident ray, with an arrow towards the block.
Centre the protractor on the point where the ray meets the surface.
3
Inside the glass the ray bends towards the normal: the angle of refraction is less than $40^\circ$ .
Going from air into glass, a ray bends towards the normal (a ray that arrives along the normal goes straight on).
4
At the second face draw another normal. The ray bends away from the normal as it leaves, and emerges parallel to the incident ray but shifted sideways.
Going from glass back into air, the ray bends away from the normal — the reverse of the first face; with parallel faces the two changes of direction cancel.
5
Higher tier: one side of each wavefront enters the glass first and slows down first, while the rest is still moving faster in air, so the wavefront changes direction. The frequency stays the same, so the wavelength in the glass is shorter.
Refraction is caused by the change in the wave's speed between the two media — not by a change in frequency.

Answer: The ray bends towards the normal as it enters the glass (angle of refraction less than $40^\circ$ ), bends away from the normal as it leaves, and emerges parallel to the incident ray. Higher tier: it changes direction because light slows down when it enters glass.

Question

Stuck on refraction?