Light Waves
3 marks · NormalThe diagram shows a light ray travelling from air into glass at an angle of incidence of 0°.
Explain, using physics ideas, why the ray does not change direction as it enters the glass.
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(a) When a light ray strikes the boundary at 0° (along the normal), it does not bend because there is no component of its speed that is across the boundary. It still slows down as it enters the glass, but it continues in the same direction. Refraction only causes a change in direction when the light ray is at an angle to the normal.
Common mistakes: Saying the light slows down but not addressing direction
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Marking points
M1 (a) · 1 mark · Explains that when the light ray enters along the normal, no change in direction occurs.
M2 (a) · 1 mark · Mentions that the ray slows down as it enters glass.
B1 (a) · 1 mark · States that a change in direction only occurs for rays not on the normal / angle not 0°.
Worked solution
When the light ray enters the glass at 0° to the normal (straight on), the part of the wave front in both media changes speed together and the ray does not bend. The speed decreases, but the direction is unchanged. Refraction only changes the ray's direction when it enters at an angle to the normal.