P5g - Refraction of wavesRefraction is…

Refraction

Refraction

• When waves slow down, the wavelength gets shorter• When waves speed up, the wavelength gets longer• This causes the wave to change direction unless it is traveling along the normal.

Refraction investigation

Intro

You are going to calculate the refractive indices and critical angles for glass and perspex blocks

Refraction investigation

Method

1. Watch the brief intro

2. Draw up 2 results tables, one for glass and one for perspex

3. Collect your apparatus

4. Repeat each experiment at least 5 times using the same angle i.

Refractive index (n)

n = speed of light in a vacuum

speed of light in the medium

n = 300 000 000 m/s

speed of light in the medium

Refractive index (n)Medium Speed of

light (m/s)

Vacuum 300 000 000

Air 299 900 000

Water 225 000 000

n = speed of light in a vacuum speed of light in the medium

Q ~ What is n for a) air b) water

Snell’s law

i

r

n = sin i sin r

medium i

medium r

“Angle of refraction depends on the refractive index”

Snell’s law n = sin i sin r

Q ~ If the medium is water (n = 1.33) and angle i is 30, what is angle r ?

n = sin i sin r

1.33 = sin 30 sin r

sin r = sin 30 1.33

sin r = 0.5 = 0.38 1.33

r = 22

Snell’s law n = sin i sin r

Q ~ In another medium, if i is 30 and r is 12, what is n?

Q ~ If the medium is water (n = 1.33) and angle i is 40, what is angle r ?

Refraction investigation

Analysis:Calculate n for air/glass and air/perspex

interfaces using your experimental results

1. Pick one result from each material and use angles i and r

2. Use Snell’s law (n = sin i / sin r) to work out your values for n.

Refractive index between 2 media

i

r

medium i

medium r

n = nr

ni

Refractive index between 2 media

i

r

water (n = 1.33)

air (n = 1.0) n = nair = 1.0 nwater 1.33

= 0.75

n = nr

ni

Refractive index between 2 media

i

r

water (n = 1.33)

glass (n = 1.5)

n = nglass = 1.5 nwater 1.33

= 1.12

n = nr

ni

Refraction investigation

Analysis

You can now calculate the refractive indices (n) for glass and perspex

1. Use your results nair/glass and nair/perspex

2. Assume that nair is 1.0

Refraction investigation

i

r

air (n = 1.0)

glass (n = ?)

n = nglass = nglass

nair 1.0

n = nr

ni

nglass = ?

Refraction investigation

i

r

air (n = 1.0)

perspex (n = ?)

n = nperspex = nperpex

nair 1.0

n = nr

ni

nperspex = ?

Refractive index and critical angle

“Total internal reflection”

Refractive index and critical angle

n = nr

ni

= sin i sin r

= sin c sin 90

nr

ni

= sin c

nr

ni

sin 90 = 1

Refractive index and critical angle

= sin c

nr

ni

Refraction investigationNow, using your results you can calculate:

1) The critical angle for glass

2) The critical angle for perspex

We will check your answers with a practical next lesson