Download Physics 1230: Light and Color

Physics 1230: Light and Color
Chuck Rogers, [email protected]
Matt Heinemann, [email protected]
www.colorado.edu/physics/phys1230
Matt apologizes, but he will miss office hours today
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Individual Exam Average: 88 +/- 8 of 100 points
Group Exam Average: 97+/-3 of 100 points. Great!1
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Physics 1230: Light and Color
Chuck Rogers, [email protected]
Matt Heinemann, [email protected]
www.colorado.edu/physics/phys1230
Lecture 6
How do we produce light waves?
3
Last Time: Electromagnetic wave
•
•
•
•
Charge a piece of tape.
Now, wiggle your tape up and down
You’ve made an electromagnetic wave!
Why can’t you see it?
Wiggle
Last Time: Electromagnetic wave
•
•
•
•
Charge a piece of tape.
Now, wiggle your tape up and down
You’ve made an electromagnetic wave!
Why can’t you see it?
X
Wiggle
Last Time: Electromagnetic wave
• Now, wiggle your tape up and down
• You’ve made an electromagnetic wave!
• Why can’t you see it? TOO SLOW FOR LIGHT
Wiggle
PhET Radio Waves
https://phet.colorado.edu/en/simulation/legacy/radio-waves
Clicker question
Would the electric field in this electromagnetic
wave cause electrons in the wire to move?
A. Yes, along the length of the wire
B. Yes, to the right
C. No
Clicker question
Wave moves up and down, disturbance to the right.
Would the electric field in this electromagnetic
wave cause electrons in the wire to move?
A. Yes, along the length of the wire This is why radio
B. Yes, to the right
antenna are
oriented up and
C. No
down
Electromagnetic radiation
Wave fills all space
Wave can travel outward
in different directions
Wavefront
Clicker question
We just saw an example of how to make radio
waves
To make visible light I should wiggle electrons:
A) slower
B) faster
C) same speed
D) visible light needs to be made differently
Goals for today
• Identify the general mechanism through which an
incandescent bulb creates light
• Learn how to draw a spectral distribution curve
• Be able to use the relationship between temperature and
peak wavelength of emission in incandescent light
emission to predict how the peak wavelength will change
given a change in temperature
• Understand the concept of color temperature in everyday
lighting
How is light (electromagnetic waves)
created?
There are two main ways to make light
1. Incandescence: Heat
an object up, and it
glows. (“Blackbody
radiation”)
2. Luminescence: For
example gas discharge
lamp
First: Incandescence
How does a lightbulb glow?
Hot things glow = incandescence:
• Current runs through the filament
• This makes the filament hot
• Electrons jiggle in hot things  emit light
PhET radio
waves
Heating objects
causes their
electrons to jiggle
and they emit
light.
Jiggle electrons at different frequencies,
we get different kinds of electromagnetic
radiation (light).
Which part of this glowing
piece of metal in a
blacksmith shop is hotter?
(Use your common sense
for this one)
A) redder part
B) yellower part
C) the whole thing must be
the same temperature
Color temperature
The hotter the temperature of a thing, the faster
the charges in it wiggle.
So, the hotter the object, the:
A. Shorter the wavelength of light it emits
B. Longer the wavelengh of light it emits
Temperature affects color emitted
High temperature  Short wavelength
Hotter
Cooler
UV
Infrared
(IR camera)
White hot glows over visible range, eye mixes to make white
Temperature affects color emitted
Wien’s Law:
Brightest color wavelength is just
proportional to the absolute temperature.
T brightest  3 10
3
meters.Kelvin
Tell me the brightest color of a hot object
and I can tell you how hot it is!
Temperature affects color emitted
Example:
What is the temperature of the Sun?
T
3 103 m*K
brightest
brightest  500nm
3
3 10 m*K
3 103 m*K
T

9
7
500 10 m
5 10 m
4
 0.6 10 K
 6000K
0 Celsius is the same as 273 Kelvin.
3 103 7

5
Color temperature
I have an incandescent light bulb at a
temperature of 3000 K. What will be the
peak wavelength of emission?
A) 1000 nm
B) 100 nm
C) 3000 nm
D) 300 nm
E) None of these
T brightest  3 10
3
m*K
Review clicker question
The star Betelgeuse is reddish when you look at it in the night sky
Is Betelgeuse hotter or colder
than our yellow star (the sun):
A) hotter
B) colder
C) same temperature
Color temperature
This is why incandescent lightbulbs are
so inefficient
There are two main ways to make light
1. Incandescence: Heat
an object up, and it
glows. (“Blackbody
radiation”)
2. Luminescence: For
example discharge
lamp
More light sources
To understand fluorescent light bulbs and light
emitting diodes (LEDs) we need to understand
mechanisms other than blackbody radiation
Look at light tower with diffraction gratings
Key points about atoms
• Electrons are “bound” to
atoms
• There are only certain
amounts of energy the
electrons can have (energy
levels)
• The more energy the
electron has, the further
away it is from nucleus
(higher energy level)
These are both simplified models of atom
30
Atoms can absorb energy (e.g., light)
• Then, the electron moves up an energy level
+
31
This is (basic idea of) how LEDs, lasers,
and fluorescent lights work
One color (f) only
Spacing between energy levels leads to the color emitted: E = hf
Which type of light has the highest
energy per photon?
A.
B.
C.
D.
Red light
Blue light
Infrared
Radio
E = hf
High frequency light has higher energy
E = hf
Low E
High E
Ionizing vs. non-ionizing radiation
This is why we limit our exposure to uv and X-rays
Fluorescence
Absorbs in one wavelength and emits in another: Must
absorb shorter wavelength (high energy) and emit
higher wavelength (low energy)
High E
(e.g, UV)
Low E
(e.g., visible)
Fluorescence
Shine UV light on these; they emit light of another color
Phosphorescence is just delayed fluorescence; it continues to glow.
The lighting in this room
• Tube is filled with argon and
mercury; spark in gas creates UV –
but we can’t see UV
• The UV light is absorbed by the
coating and emits visible wavelength
of a lower energy/frequency
This process is called:
A) Lasing
B) Fluorescence
C) Discharge
D) Blackbody radiation
The latest in lighting: LED
Another example of luminescence:
Lasers:
Light Amplification by
Stimulated Emission
You don’t have to know how they work.
But very important part of modern
technology.
In the category of luminescence.