Refraction and refractive index
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- The bent straw
- What refraction is
- Which way does it bend?
- What stays the same
Read the transcript of the teaching part
Last time
Before we start: last time, in Reflection, refraction and total internal reflection, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
The bent straw
Put a straw in a glass of water, and it looks broken at the surface. The straw is fine. Light changes speed when it goes from water into air, and that bends its path. The bending follows a rule you can calculate.
What refraction is
Refraction is the change in direction of a wave when it passes from one medium into another. It happens because the speed of the wave changes. Light is slower in glass and water than in air. If the ray goes in along the normal, the direction does not change, but the speed still does.
Which way does it bend?
Going from air into glass, the light slows down, and bends towards the normal. Going from glass into air, the light speeds up, and bends away from the normal. The angles are always measured from the normal.
What stays the same
When light enters glass, its speed decreases, and its wavelength decreases. The frequency stays the same, because it is set by the source. That follows from v equals f lambda: with f fixed, v and lambda change together.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Refraction and refractive index · Refraction and refractive index.
Chapter 3.2: Light
- Reflection by a plane mirror 7 min
- Refraction and refractive index 5 min
- Critical angle and total internal reflection 5 min
- Thin lenses, ray diagrams and magnification 6 min
- Uses of lenses and correcting sight 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.