Snell's Law

Snell's Law

What is it?

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.

In simple words

• 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.

Important terms

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.

What you should learn

• 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

Examples

• Straw looking bent in a glass
• Pool looking shallower
• Light into a prism

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.

Full school-level explanation

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.

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.

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.

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.

Remember

• n1 sin i = n2 sin r
• frequency unchanged

Examples

• Straw looking bent in a glass
• Pool looking shallower
• Light into a prism

Remember

• n1 sin i = n2 sin r
• frequency unchanged
• change of speed
• towards the normal if slower
• n1 sin i = n2 sin r
• frequency unchanged
• c / v
• larger n -> slower

How the ideas connect

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.

Deeper understanding

Go beyond memorising Snell's Law. Connect the definition to reasons, relationships, exceptions, and exam wording.

How the ideas connect

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.

Common mistakes

Exam focus

Core Idea

Explain It in Steps

Compare the Cases

Examples and Uses

Exam focus

Quick revision

• n1 sin i = n2 sin r
• frequency unchanged
• change of speed
• towards the normal if slower
• Refraction And Snell'S Law: Refraction is the change of speed (and usually direction) when light enters a new medium. In glass o
• Refractive Index N: n = speed in vacuum (or air) / speed in the medium. Air is about 1, water about 1.33, glass about 1.

Draw the normal first. Students lose marks by measuring from the surface. A ray along the normal does not bend (i = 0).

For long answers on Snell's Law: define clearly → explain the main parts → give one example → state one exam caution.