Lecture 3 – Chapter 1
We are
here
•Some history of technology (Lec.1)
•How vision works (Lec.2)
•What is light, what is a wave (Lec. 2)
•Wavelength and Frequency: c = f λ
Scientific notation and metric units
Electromagnetic spectrum
Transmission and opacity
•Speed of light
•Amplitude
•Direction: Rays and wavefronts
•Polarization
•Phase difference
We only did the first 19 of
•Resonance
these 28 viewgraphs.
•Sources of light
1
Review
How vision works:
• Self luminous object: light goes to eye
• Diffuse scattering at a surface
glossy and matte surfaces
• Specular reflection at a surface
• Volume scattering (fog)
• Transparent
Demos: laser pointer, water spray, white paper, blank transparency, mirror
2
Review
Light is an electromagnetic wave
• Frequency, f, measured in Hz (cycles/s)
• Wavelength, λ (meters)
• Scientific notation is needed because for
light f is big and λ is small
3
Some radio wave frequencies
•
•
•
•
•
•
Submarine communication, 76 Hz
AM radio, 520 – 1610 kHz
FM radio, 87.5 – 108 MHz
GPS, 1575 MHz or 1.575 GHz
Wireless internet, 2.4 GHz
Security scanner: 1000 GHz = 1 THz
The frequency of light is very high, ~1015 Hz,
and is off this scale.
4
Wavelength λ of wave
λ
λ
Could be measured in m, cm, mm, km, etc.
5
Demo: copper tube bent as wave
Wavelengths of x-rays, light, infrared, etc.
Demo: prism
Low frequency
Long wavelength
High frequency
Short wavelength
nanometers
6
Visible spectrum: 400-700 nm
7
How big is 500 nm?
• 500 nm = 0.5 μm
• Human hair is 17 – 180 μm in diameter.
A 50 μm hair is 100 wavelengths.
• There are 2000 wavelengths of green light
per millimeter.
• A “light” microscope cannot see things
smaller than about 0.5 μm.
Demo: divide 1 mm by 500 nm
8
Transmission of glass
UV is
blocked.
No
sunburn
indoors.
Some IR is
blocked.
Opacity is the opposite of transmission.
0% transmitting is 100% opaque.
9
Opacity of atmosphere
Radio
communication
X ray telescope above
atmosphere
IR telescope above
the atmosphere
10
For THz radiation, skin is opaque and clothing is transmitting.
THz scan:
11
Lecture 3 – Chapter 1
•Some history of technology (Lec.1)
•How vision works (Lec.2)
•What is light (Lec. 2)
•Wavelength and Frequency: c = f λ
We are
here
Scientific notation and metric units
Electromagnetic spectrum
Transmission and opacity
•Speed of light
•Amplitude
•Direction: Rays and wavefronts
•Polarization
•Phase difference
•Resonance
•Sources of light
12
Properties of light
1. Light travels in vacuum.
Sound travels in air (no sound in vacuum).
2. Light moves with a particular speed in vacuum,
but moves less rapidly in other materials (water, glass).
3. Light carries energy. (Sunlight warms, generates electricity.)
4. Light travels in vacuum in straight lines (rays).
Rays can be bent by materials.
5. Light has wavefronts (crests), just like water waves.
If one wave is a little ahead of another, the distance one
crest is in front of the other is called the phase difference.
6. Light has amplitude (intensity).
7. Light has a polarization direction (or is unpolarized).
13
Speed of light
• Measurements based on the relation
distance = velocity x time
• velocity = distance / time or v = d/t
Early observation: lightning precedes thunder.
So light is faster than sound.
(Sound goes 1000 ft / s, or 1 mi. in 5 s)
14
Early measurements of c
• 1600s: Galileo, lanterns on hilltops
(“null” result)
• 1800s: Roemer, light crosses earth’s
orbit in 16 mins
• 1800s: Fizeau, rotating gear
• 1900s: Michelson, rotating mirror
c = 300,000 km/s = 3 x 108 m/s
7 times around the earth in 1 s.
15
Roemer’s 1676 measurement of c
Earth
Sun
Earth 6 mos. later
4 astronomical units distance
6 astronomical units distance
16 light minutes
Roemer realized light from Jupiter would travel 4 a.u. distance when
earth is closest to Jupiter and 6 a.u. distance when earth is furthest
from Jupiter. By timing eclipses of Jupiter’s moons, it was possible
to time the delay caused by the extra distance travelled by light.
The extra travel time is about 16 minutes.
16
Fizeau’s 1849 measurement of c
The gear has to turn at the right speed for the light to pass
between the teeth of the gear both going and returning.
17
Michelson’s 1879 measurement of c
18
The frequency-wavelength formula
c = f λ = frequency x wavelength
f=c/λ
λ=c/f
Using the formula:
FM radio station: f = 100 MHz = 108 Hz
then the wavelength is
λ = [3 x 108 m/s] / [108 /s] = 3 m
19
Demo: frequency for 1 mm wavelength
Amplitude A of a water wave
A = amplitude
= half height
2A
Twice the
amplitude as
the wave
above
Could be measured in m, cm, mm, km, etc.
20
Lecture 3 – Chapter 1
•Some history of technology
•How vision works
•What is light
•Wavelength and Frequency: c = f λ
Scientific notation and metric units
Electromagnetic spectrum
Transmission and opacity
We are
here
•Speed of light
•Amplitude
•Direction: Rays and wavefronts
•Polarization
•Phase difference
•Resonance
•Sources of light
21
Wavefronts
Wave crests
or
wavefronts
Wave trough
22
Rays are perpendicular to fronts
Wave crests
or
wavefronts
Rays
http://en.wikipedia.o
rg/wiki/File:Two_so
urces_interference.
gif
http://en.wikipedia.org/wiki/Wavefronts
Right click
23
Polarization
Vertical polarization has
electric field “up and down”
Horizontal polarization has
electric field “left and right”
24
Demo sunglasses, grill
Polarizing filter
Ordinary light has both polarizations.
Polarizing filters pass only one polarization.
We return to this in Chapter 13.
25
Review
• c = 300,000 km/s =3 x 108 m/s
• c = fλ, units are Hz for f and meters for λ
• wavelength of light spans 400 – 700 nm
(blue to red)
• Atmosphere transmits light and radio,
is opaque to UV, x-rays, some IR
• Amplitude of a water wave is the half-height
• Wavelength is distance from crest to crest
26
Intensity of sunlight
We will do this better, later.
Units are watts/square meter or W/m2.
Watts is a unit of power.
Power is energy per unit time.
Sunlight delivers about 1000 Watts/m2.
27
Sunlight and the earth
• Sunlight is constant at 1000 W/m2
(UV and x-ray output varies with the 11year sunspot cycle)
• Earth’s temperature is a balance between
sunlight in and infrared radiation out
• Carbon dioxide blocks infrared going out
and has a warming effect.
28