How do I control light
(color, phase, brightness)?
Phys 1020, Day 14:
Questions?
Sunlight 14.1
Making light, discharge 14.2
LASERS: 14.3 (next week)
Next Week:
Finish (start?) Lasers
Move to Optics
1
Light
• Where does light come from
• Why do colors appear as they do: scattering
– Why is the sky blue
– And the sunset red
• Light interacting with matter
– Reflection
– Refraction
• Putting it together: What about reflection and
light bending?
– Snow / ice
– Stars twinkling
2
The Sun
Power = 4*1024 W
73% Hydrogen
25 % Helium
Spins once per 24 days (equator), > 29 at poles.
3
• Where does energy come from?
nuclear fusion, like in hydrogen bomb.
p + n
pn +
energy
pn +pn
hydrogen nuclei
pn +
np
helium nucleus
energy
hydrogen nuclei fusion “burning” into helium,
helium nuclei fusion to form other elements more slowly up to
iron. Energy goes into thermal energy- means atoms and electrons
VERY HOT--moving and shaking a lot!
4
Sun is VERY HOT (5500 C) —atoms and electrons moving ‘n shaking alot!
Surface of sun- whole bunch of free electrons whizzing around like crazy
Get electrons hot (5500 C)
- shake around with a particular range of frequencies
- give off EM radiation at those frequencies.
“Black body” or “thermal” radiation.
++
+
+
What if sun were hotter?
Sunlight would look:
a. same as now,
b. redder
c. bluer
c. bluer.
Hotter  more rapid shaking
 more light (photons) at all
wavelengths but even more
high energy photons
5
Note two different ways to make
light
• Atomic energy level shift (absorption /
emission)
• Oscillations (of mobile electrons)
++
+
+
6
E/M wave sim…
7
Atom discharge lamps-- neon signs.
Energy levels
metal, bulb filament,
or not stuck in atom
(like sun). If hot,
jump between all
diff. levels. Wiggle
around, all colors.
Energy levels in
isolated atom.
kick up,
only certain
wavelengths when
come down.
In discharge lamps, lots of electrons given bunch of energy
(voltage). Bash into atoms. (“discharge tube”)
120 V or more with long tube
8
Discharge Lamps
9
Electromagnetic waves. Oscillating electric force field.
All go at speed of light c.
Energy = color is determined by wavelength.
wavelength related to frequency of oscillation by
c = wavelength x freq.
Green

 (or f)
Blue
Red
+
+
=
Many photons at same time,
larger amplitude wave (brighter).
Same wavelength.
10
Energy in light
Light comes in little chunks of energy, called photons
Each wavelength has own size chunk,
Energy of one photon = h x speed of light
h = plank’s constant = 6.6 10-34 J s

c = speed of light =3.0 108 m/s
Energy per photon = h c / h f
Total energy = number of photons x energy per photon
= N h c / 
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Power, Energy, Intensity
• What’s the difference between 1 Joule of energy
(# of blue photons?) in 1 second and 1 year?
• Energy / Time = Power
• Watts = measure of power Energy / second
• What about how much area that Power goes into?
– What’s the difference between being close and far to a
100W light bulb?
• Intensity = Power / Area = Energy / (time x area)
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Three ways light can interact
with matter
• Absorption:
• Reflection / Scattering
• Transmission:
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Color of objects:
it’s all about what the electrons in its atoms are doing.
What colors they absorb? What colors do they reflect?
1. Leaves look green to us. What colors do they absorb? What
colors do they reflect?
a.
b.
c.
d.
Absorbs green. Reflects reds, yellows, blues
Absorbs blue. Reflects reds, yellows, greens.
Absorbs reds, yellows, blues. Reflects greens.
Absorbs reds, yellows. Reflects greens and blues.
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Color of objects
1. Absorbing a particular colors light out of white sunlight, leaving
other colors to reflect off. We see what is reflected.
c. Reflects green …
that’s what we see!
absorbs other colors
(reds, yellows, blues),
One type of leaf
molecule (absorbs blue)
Energy
Types (colors) of photons absorbed depends
on atomic energy levels in material. Here
differences in energy levels of molecules in
leaf match energy of reds, yellows, and blues.
15
Look at paper in bright sunlight, it looks white.
Means that
a. paper is absorbing all colors of light from the sun equally
b. scatters only green and yellow
c. scatters all colors from sun equally
d. absorbs all but red
e. scatters white photons.
Graded group question.
c. scatters all colors from sun equallywhite- lots of light coming off it and same
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spectrum as sun.
Q: Why is the sky blue?
– Light scatters off atoms in the atmosphere.
– Blue light scatters more than red,
• Scattering process depends upon wavelength (blue better)
• closer to matching energy levels of nitrogen and oxygen.
(Not quite same as book, but right.)
SUN
Red goes through
Blue scatters
excited level of
nitrogen (O close)
Earth surface
Energy of light E = hc/ ,
Red bigger wavelength, smaller energy photons.
Blue closer to match, more absorption and spitting out
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= scattering.
If the atmosphere looks blue, why does
the sunset look red?
a. Atmosphere is better at
scattering red at dusk
b. Blue is scattered away, red is
leftover
c. At low angles, the scattering
isn’t as good, so more red
left to get to eye
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sunset light, gone through so much air all
the blue is scattered out. Only red left.
Why is sky brighter red if smoky?
If smoke light get
to eye, only red left
b. Blue is scattered
away, red is leftover
19
Simulated sun
creamer
water
Predict what will happen to the light passing through the tank?
What will happen to light coming out side of tank?
question to ponder: when leaves dry up and turn from green to
yellow and red, what does it mean about changes to in leaves
in terms of what energy levels must be to absorb different
colors of light?
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Absorption, Scattering, Reflection
• Sky blue and sunset red
because of how light scatters off atoms and dust in the air.
• Dyes different colors
because of which colors absorbed, the rest scatter off.
Other ways light can interact with matter:
reflection and refraction
why snow white, water and ice shiny, stars twinkle, glass lenses bend
light.
21
Speed of light through space = c = 3 x 108 m/s
Light travels slower in all other materials (air, glass, diamonds, water)
Speed of light in a material = c/(index of refraction) = c/n
Speed of light in space > speed in air > speed in water > speed in glass
What happens when light suddenly slows down when hits glass?
Air, velocity = .9999c
(n = 1.0001)
Some
reflected
Glass
n glass ~ 1.5
Slower velocity
Velocity of light =
wavelength x frequency
(frequency fixed …
wavelength changes,
smaller in glass)
Some
Refracted
(heading in different
Direction).
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