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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
