Push a straw into a glass of water and it looks broken at the surface. The light from the underwater part bends as it leaves the water, and your eye is fooled. That bending, caused by a change of speed, is refraction.
Light changes speed when it crosses between materials of different optical density, and that change of speed bends the ray. Entering a denser material it slows and bends towards the normal; leaving it speeds up and bends away. A ray along the normal does not bend.
Definition · learn the exact words
Refraction is the change in direction of light as it changes speed on crossing a boundary between two materials.
into denser medium: bends towards the normal
A ray travelling along the normal changes speed but does not change direction.
Section 01
Send a ray into the glass.
Change the angle of the incoming ray and watch how it bends as it enters and leaves the block.
Stage 1 · Learn
Check refraction
Four quick checks. Each correct answer earns XP and lights this skill on your star map.
Quick check+10 XP
Refraction is the change in direction of light when it...
Quick check+10 XP
When light passes from air into glass it...
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When light passes from glass into air it...
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A ray travelling along the normal as it enters glass will...
Section 02
Which way it bends.
The direction of bending depends on whether the light is speeding up or slowing down.
Into a denser medium: light slows down and bends towards the normal (for example air into glass).
Into a less dense medium: light speeds up and bends away from the normal (for example glass into air).
Along the normal: at 0 degrees incidence the light changes speed but carries straight on, and the frequency never changes.
Examiner trap
A ray travelling along the normal does not bend, even though it does change speed. And refraction is caused by a change of speed: the frequency of the light stays the same throughout.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Light enters a glass block from air. Inside the glass, compared with the air, the light travels...
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During refraction, which quantity stays the same?
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A ray strikes a glass surface exactly along the normal. It...
Skill unlocked
Refraction, mastered.
Refraction mastered. Total internal reflection is next.