PhysicsChapter 5: Optics
Refraction and Snell's Law
Refraction is the change of speed (and usually direction) when light enters a new medium.
Learning objectives
- Define refraction and snell's law in clear scientific words
- Explain REFRACTIVE INDEX n and SNELL'S LAW
- Use the key relation or reasoning in a short exam problem
Statement
Refraction is the change of speed (and usually direction) when light enters a new medium. In glass or water, light is slower than in air. Towards the normal if it slows; away from the normal if it speeds up.
Mathematical form
i = 30 degrees
n = 1.5
r = sin 301.5
Where / symbols
- Refraction And Snell'S Law
- Refraction is the change of speed (and usually direction) when light enters a new medium. In glass or water, light is slower than in air. Towards the normal if it slows;…
- Refractive Index N
- n = speed in vacuum (or air) / speed in the medium. Air is about 1, water about 1.33, glass about 1.5. Larger n means slower light in that medium.
- Snell'S Law
- n1 sin i = n2 sin r, with i and r from the normal. From air into glass: sin i / sin r = n_glass. Frequency stays the same; wavelength and speed drop in the slower medium.
Overview
Index and Snell's law:
REFRACTIVE INDEX n
n = speed in vacuum (or air) / speed in the medium. Air is about 1, water about 1.33, glass about 1.5. Larger n means slower light in that medium.
Examples Air n ~ 1 · Water n ~ 4/3 · Glass n ~ 3/2
Snell's Law
n1 sin i = n2 sin r, with i and r from the normal. From air into glass: sin i / sin r = n_glass. Frequency stays the same; wavelength and speed drop in the slower medium.
Examples Straw looking bent in a glass · Pool looking shallower · Light into a prism
REFRACTIVE INDEX n
n = speed in vacuum (or air) / speed in the medium. Air is about 1, water about 1.33, glass about 1.5. Larger n means slower light in that medium.
SNELL'S LAW
n1 sin i = n2 sin r, with i and r from the normal. From air into glass: sin i / sin r = n_glass. Frequency stays the same; wavelength and speed drop in the slower medium.
Worked idea and exam use
Worked example: i = 30 degrees, n = 1.5, sin r = sin 301.5 = 0.333, r ~ 19 degrees. Apparent depth of a pool is less than real depth because rays bend at the surface.
Worked example
- Worked example: i = 30 degrees, n = 1.5, sin r = sin 301.5 = 0.333, r ~ 19 degrees. Apparent depth of a pool is less than real depth because rays bend at the surface.
In simple words
Draw the normal first. Students lose marks by measuring from the surface. A ray along the normal does not bend (i = 0).
Key points
- change of speed
- towards the normal if slower
- c / v
- larger n -> slower
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Refraction and Snell's Law
Sticky Notes · Quick Revision
Core Idea
- Refraction is the change of speed (and usually direction) when light enters a new medium. In glass or water, light is slower than in air. Towards the normal if it slows; away from the normal if it speeds up.
Explain It in Steps
- Name the idea: REFRACTION AND SNELL'S LAW.
- State the key point: change of speed.
- Worked example: i = 30 degrees, n = 1.5, sin r = sin 301.5 = 0.333, r ~ 19 degrees. Apparent depth of a pool is less than real depth because rays bend at the surface.
Compare the Cases
- REFRACTIVE INDEX n: n = speed in vacuum (or air) / speed in the medium. Air is about 1, water about 1.33, glass about 1.5. Larger n means slower light in that medium.
- SNELL'S LAW: n1 sin i = n2 sin r, with i and r from the normal. From air into glass: sin i / sin r = n_glass. Frequency stays the same;…
Examples and Uses
- Air n ~ 1
- Water n ~ 43
- Glass n ~ 32
- Straw looking bent in a glass
- Pool looking shallower
Exam Answer Pattern
- Start with the definition of refraction and snell's law.
- Write the key relation or condition: change of speed · towards the normal if slower.
- Show units, direction, or a labelled diagram whenever the question needs them.
Avoid This Mistake
- Draw the normal first. Students lose marks by measuring from the surface. A ray along the normal does not bend (i = 0).
60-second revision
- Refraction is the change of speed (and usually direction) when light enters a new medium. In glass or water, light is slower than in air. Towards the normal if it slows; away from the normal if it speeds up.
- n = speed in vacuum (or air) / speed in the medium. Air is about 1, water about 1.33, glass about 1.5. Larger n means slower light in that medium.
- n1 sin i = n2 sin r, with i and r from the normal. From air into glass: sin i / sin r = n_glass. Frequency stays the same; wavelength and speed drop in the slower medium.
- Worked example: i = 30 degrees, n = 1.5, sin r = sin 301.5 = 0.333, r ~ 19 degrees. Apparent depth of a pool is less than real depth because rays bend at the surface.
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