Today’s lesson
• Refraction of light• Refraction of light by a
prism is called dispersion • Please remember to
remain in you seats unless given permission
• Mirror Island Homework due in
Refraction
Glass block
Beam of lightLight going from air to glass bends towards the normal
Light going from glass to air bends away from the normal
Refraction
Glass block
Ray of light
A light ray entering along the normal continues in a straight line.
Do Now Can you
carefully add this diagram & notes
to your book?
Today’s lesson
• Refraction of light• Refraction of light by a
prism is called dispersion • Please remember to
remain in you seats unless given permission
• Mirror Island Homework due in today
Light waves
Light slows down as it goes from air to glass/water
its speed changes from 3 x 108 m/s to
2 x 108 m/s
or from 300 000 km/s to 200 000 km/s
Refraction – a few notes
The wavelength changes, the speed changes, but the frequency stays the same.
when a waves direction is changed it is deviated (deviation)
Ripple Tank Simulation
Refraction experiment
Typical results air to glass:angle of
incidence / °angle of
refraction / °deviation / °
0 0 0
15 10 5
30 19 11
45 28 17
60 35 25
75 40 35
No deviation occurs when the angle of incidence is zero.
Increasing the angle of incidence increases the deviation.
Refraction experiment Air to Perspex
Typical results air to Perspex :angle of
incidence / °angle of
refraction / °deviation / °
0
15
30
45
60
75
prism
Refraction by a prism – copy pleaseA prism splits the colours of white light
into the spectrum.
This is called dispersion.
Violet light deviates the most it has the shortest wavelength,
red the deviates the least it has the longest wavelength.
spectrum
whitelight
Choose appropriate words to fill in the gaps below:
Refraction occurs when a wave changes ______ as it crosses the boundary between two regions. The _________ of the wave also usually changes.
Light rays deviate ________ the normal when they pass from less dense air to more dense _________. The greater the angle of incidence the greater is the _________.
Different ______ of light deviate by different amounts. Violet deviates the _____. A prism can be used to split the colours of white light into a spectrum. This is called _________.
most speeddirectiondeviation coloursperspextowards
WORD SELECTION:
dispersion
most
speed
direction
deviation
colours
perspex
towards
dispersion
Refraction
When a wave changes speed (normally when entering another medium) it may refract (change direction)
Ripple Tank Simulation
Choose appropriate words to fill in the gaps below:
Refraction occurs when a wave changes ______ as it crosses the boundary between two regions. The _________ of the wave also usually changes.
Light rays deviate ________ the normal when they pass from less dense air to more dense _________. The greater the angle of incidence the greater is the _________.
Different ______ of light deviate by different amounts. Violet deviates the _____. A prism can be used to split the colours of white light into a spectrum. This is called _________.
most speeddirectiondeviation coloursperspextowards
WORD SELECTION:
dispersion
most
speed
direction
deviation
colours
perspex
towards
dispersion
Do Now and finish reflection and refraction sheet
Solar bottle
27 of 45 © Boardworks Ltd 2009
Refractive index is calculated by comparing speed of light in a vacuum to that in a given medium:
The speed of light in a vacuum is 300,000,000 m/s, and the speed of light in water is 225,000,000 m/s. What is the refractive index of water?
refractive index = 300,000,000 225,000,000
refractive index = speed of light in vacuum speed of light in medium
Refractive index is a measure of how much a substance slows down light. The higher its value, the more a medium slows light. The more the light is slowed, the more it bends towards the normal.
Refractive index
= 1.33
28 of 45 © Boardworks Ltd 2009
Use the information in the diagram to find the refractive index of glass.
refractive index (n) = sin i sin r
refractive index = sin 45° sin 28°
The refractive index can also be calculated using Snell’s Law, which uses the angle of incidence (i) and angle ofrefraction (r) to establish how much a medium slows light.
Snell’s Law
refractive index = 1.5
normal
29 of 45 © Boardworks Ltd 2009
Using Snell’s Law
If a ray of light enters water at an angle of 15° and has an angle of refraction of 11.2°, find its refractive index.
refractive index = sin 15 sin 11.2
Use Snell’s Law to answer the following:
refractive index = 1.33
31 of 45 © Boardworks Ltd 2009
Using Snell’s Law
Perspex has a refractive index of 1.5. If a ray of light passing into a perspex block has an angle of refraction of 24°, find the angle of incidence.
sin i = sin r × refractive index
Use Snell’s Law to answer the following:
sin i = (sin 24°) × 1.5 = 0.61
i = sin-1 0.61
i = 37.6°