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# Light intro: Waves

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by

## Emma van Leeuwen

on 8 June 2015

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#### Transcript of Light intro: Waves

Waves
http://www.acs.psu.edu/drussell/Demos/waves/Lwave-v8.gif
longitudinal
transverse
http://www.acs.psu.edu/drussell/Demos/waves/Twave.gif
standing (safari) http://www.walter-fendt.de/ph14e/stwaverefl.htm
harmonics http://www.walter-fendt.de/ph14e/stlwaves.htm
Reflection (echo)
http://www.acoustics.salford.ac.uk/schools/teacher/lesson3/flash/whiteboardcomplete.swf
Waves can be:
Surfing standing waves:
Hydraulic jump
Waves transmit energy, not matter
2 types of waves:
Longitudinal
Transverse
Freeze time and draw wave as pressure variation
Properties of waves
Light?
When light with a frequency of 5.6E14 Hz travels through a vacuum, what is its:
(a) period
(b) wavelength (in nanometres)?
(c) the colour?
Examples
The Sun is 150x10 m from the Earth. How long does sun light take to get here?
1. Find the frequency of the following rays of light.
a) 500 nm
b) 920 m
c) 2.95 m
2. Find the wavelength of the following waves in air.
a) 100.3 MHz
b) 774 kHz
c) 665 THz
3. Provide a definition for:
a) Wavelength b) Period c) Frequency d) Amplitude
12. 600 THz, 326.1 GHz, 101.69 MHz
13. 2.99 m, 387.6 m, 451 nm
14. Length of one complete wave, time taken for one wave to pass, number of waves passing a point each second, height of the disturbance
Questions
Electromagnetic wave spectrum
Electromagnetic model of light
Electromagnetic wave model of light
Polarisation of light waves
Electromagnetic wave spectrum
Dutch flag under white light
Dutch flag under pure orange light
Example 1)
Most surfaces aren't 'pure' colours
Filters
Appears yellow
Appears green
Practice Qs:
Light
Paint
Red cones + Green cones = Yellow
Brain:
-> Yellow light OR red light + green light
Yellow
Red
Green
This weird thing the brain does.
Colour subtraction
mixing paints
3 types of cones sensitive to different ranges of wavelengths
Surfaces can be:
opaque
translucent
transparent
Make transverse and longitudinal waves
Measure/estimate:
period
amplitude
(transverse only)
wavelength
Calculate:
frequency
speed of wave
Extension:
send a wave from either end towards each other and observe what happens
Try and create a series of 'standing waves'
Direction of wave
motion
Wave properties
Same for longitudinal
Amplitude
: maximum displacement from equilibrium
Wavelength
: length of one complete wave

Frequency
: number of complete waves passing a point per second
Period
: time for one complete wave to pass a point
Velocity
: speed of wave
Calculate/measure (not on graph):
Glare: Light reflected off surfaces is horizontally polarised
Light is a transverse wave
http://www.walter-fendt.de/ph14e/emwave.htm
Electric and magnetic fields oscillating perpendicular to each other
Same formulas but v=c
1.9x10 s
-15
540 nm
yellow/green
t = x/v = x/c = 500 s = 8 min
gif: http://www.astronomynotes.com/light/freqwavl.gif
= c (3x10 m/s) for light in a vacuum
8
9
Compression
Rarefaction
Peak
Trough
Add paint -> less light reflected
(blue light)
(green light)
(red light)
Where is magenta???!!!!
Yellow light is an
actual
wavelength of light.
Why do we see red and green light as yellow too?
Magenta is a colour our brains made up!!
pic: http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Spectra/travelwave.gif
Wave properties
V in water = 1500 m/s
speed of sound only depends on the medium it is traveling in
t = 0.5 s
= c (3x10 m/s) for light in a vacuum
8
Amplitude
: maximum distance from rest position
Wavelength ( )
: length of one complete wave

Can measure from photo of wave:
Frequency (f)
: number of complete waves passing a point per second
Period (T)
: time for one complete wave to pass a point

Can measure from moving wave:
Velocity
: speed of wave
speed =
Can calculate:
wavelength
period
_________
v =
T
__
....................................................................................................
amplitude
T =
1
f
_
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