Physics 1230: Light and Color
Chuck Rogers, [email protected]
Matt Heinemann, [email protected]
www.colorado.edu/physics/phys1230
HWK 3 is due TODAY at 5PM on D2L.
EXAM 1 is Wednesday in-class 11AM. See the
website for details. Covers HWKs 1, 2, 3, and
Lectures 1-5 (through tomorrow).
1
Physics 1230: Light and Color
Chuck Rogers, [email protected]
Matt Heinemann, [email protected]
www.colorado.edu/physics/phys1230
Lecture 4:
The wave picture of light: Traveling waves,
their wavelength, period, frequency, speed.
Waves can diffract (bend around things).
Waves can add and/or subtract. We call this
fact ‘superposition’; it causes interference
2
Last Time: How to think about light?
Ray model
Light travels as
rays in all
directions
Wave model
Light travels as
a wave.
Rays from point source
Waves from point source 3
Last Time: Wave basic properties
Wave repeats its shape each time you
move left or right by a special distance.
X
l
l is called the ‘wavelength’.
Last Time: Wave basic properties
Wave travels at some speed.
X
l
l is called the ‘wavelength’.
c is the speed.
Last Time: Wave basic properties
After traveling some time, T, the period,
the wave looks the same as initially.
X
l
l is called the ‘wavelength’.
c is the speed.
T is the period.
Why do we think about light as a wave??
The light ray model is GREAT! Why do we
want another model??
A) We don’t want another model
B) A ray is a ray, they predict everything
C) Wait… I can think of something rays
don’t PREDICT.
D) I’m confused.
E) Blue
A good model lets you
PREDICT behavior.
Demo: Laser and a reaaally tiny pinhole
What do you PREDICT you will see on
the screen when we turn on the laser?
A) A bright dot
B) A dot, but not bright (a small pinhole
only lets through a tiny bit of laser light)
C) A smear of light
D) A bulls-eye pattern
E) Something else
This is what it looks like
(And the hole is 0.5 mm)
The ray model
predictions are
not working!
THAT is a good
reason for a
better model!
Note, we wouldn’t see this if it weren’t a single color of light, all in phase (coherent)
Another observation:
A prism spreads
out white light
into the colors
of the rainbow
Question: Why are the colors always in this order? In what
sense is yellow “between” red and blue?
Diffraction of water and sound
We need to build on our model:
Actually, light is a wave!
• Waves bend around a tiny
hole and build up in patterns
(diffraction)
• Colors have different
wavelengths, and those
wavelengths are affected
differently by going through a
material, and spread out
(dispersion)
Again: Wave basic properties
Height of the wave is the Amplitude
l is called the wavelength.
c is the speed. T is the period.
l
X
We see 1 ‘cycle’
each T seconds.
Frequency is the number
of cycles each second.
Frequency and Period
• Frequency is the number of
full cycles per second
• Period is the time to swing
forth and back (one cycle)
• If f = 2 Hz,
then 2 swings per second,
and period = 0.5 s.
OR: f = 1/T
14
Again: Wave basic properties
Height of the wave is the Amplitude
l is called the wavelength.
c is the speed. T is the period.
l
X
c
l
T
OR
clf
Clicker questions
A
Question 1:
Which has the highest
amplitude?
Question 2: Which has the
smallest frequency?
B
b)
c)
C
So this is how yellow is “between” red and blue
Yellow has a larger wavelength than blue, smaller than red.
Wavelength or frequency tells you the color.
Red = 700 nanometers (nm)
Yellow = 600 nm
c=fλ
Violet = 400 nm
A nanometer is a billionth of a meter, or 1 x 10-9 meters
What is color?
Color is our brain’s interpretation of light of different
wavelengths/frequencies entering our eyes
Light with wavelength
of 650
nm
Which
of these
has
appears red when
it enters
a viewers eye
the
largest
wavelength?
A. Red
Light with wavelength
of 520 nm
B. Green
appears green when it enters a viewers eye
C. Blue
Light with wavelength of 470 nm
appears blue when it enters a viewers eye
Clicker Question
c=lf
An FM radio station transmits at a frequency of:
f = 100 MHz = 108 Hz
then the wavelength is :
A) 1 m B) 0.3 m C) 3 m D) 100 m
E) None of these.
19
Clicker Question
An FM radio station transmits at a frequency of:
f = 100 MHz = 108 Hz
then the wavelength is :
A) 1 m B) 0.3 m C) 3 m D) 100 m
E) None of these.
c=lf
f=c/l
l=c/f
l = [3 x 108 m/s] / [108 /s] = 3 m
20
Clicker Question
c=lf
An FM radio station transmits at a frequency of:
f = 100 MHz = 108 Hz. We already found that
the wavelength is 3m.
If the frequency doubles, what is the new
wavelength? (No calculators!)
A) 6 m B) 0.3 m C) 1.5 m D) 100 m
E) It does not change
21
Clicker Question
An FM radio station transmits at a frequency of:
f = 100 MHz = 108 Hz. We already found that
the wavelength is 3m.
If the frequency doubles, what is the new
wavelength? (No calculators!)
A) 6 m B) 0.3 m C) 1.5 m D) 100 m
E) It does not change
c=lf
f=c/l
l=c/f
c=lf
c = (½ l)(2f)
22
Clicker Question
c=lf
Your microwave oven operates at a frequency
of: f = 3 GHz = 3x109 Hz
then the wavelength is :
A) 1 cm B) 0.3 cm C) 3 cm D) 30 cm
E) None of these.
100 cm = 1 m
23
Clicker Question
Your microwave oven operates at a frequency
of: f = 3 GHz = 3x109 Hz
then the wavelength is :
A) 1 cm B) 0.3 cm C) 3 cm D) 30 cm
E) None of these.
c=lf
f=c/l
l=c/f
l = [3 x 108 m/s] / [3x109 /s]
l = 0.1 m
= 0.1 m x 100 cm/m
= 10cm
24
Visible spectrum is just part of the
electromagnetic spectrum
Short wavelength
High frequency
Long wavelength
Low frequency
Different colors are just different wavelengths of
electromagnetic radiation!
“Electromagnetic radiation” = energy created by oscillating
electric and magnetic fields: We’ll learn what that means
Short wavelength
High frequency
Long wavelength
Low frequency
c=fλ
A nanometer is a billionth of a meter, or 1 x 10-9 meters
Model of Light as a wave…
• Wave travels at 3x108 m/sec, called c
• Waves have amplitude (bigger is brighter).
• Waves have wavelength, l, period, T, and/or
frequency f = 1/T , AND
c
l
T
lf
Breathe in… Breathe out…
QUESTIONS!!
Good time for a 5 minute break!
Please click in when you’re back…
Next goals for waves: Explain this!
• Explain what is diffraction
and what is interference?
• What causes the bright
and dark rings?
Waves bend around a boundary
(diffraction) – let’s see how this gives rise
to dark/light bands (due to interference)
PhET “wave interference” sim:
Diffraction
At a particular instant of time, wave 1 and wave 2 are as shown.
What does the sum of the two waves look like? Add them up.
Wave 1
Wave 2
A)
B)
C)
D)
At a particular instant of
time, 2 sinusoidal waves,
labeled 1 and 2 are
shown. The two waves
are exactly in phase and
have the same amplitude
and wavelength. What
does the sum of the two
waves look like?
So waves add and subtract depending
on whether they’re in phase or out of
phase
In phase. Waves add = constructive interference
Light
Out of phase (offset). Waves subtract = destructive interference
Dark
Interference makes the
bright and dark areas!
PhET “wave interference” sim:
Diffraction
Two slit illustration
x
In phase = Waves move together.
Waves add = constructive interference
(Young’s double slit experiment, do not need
to know how to do trigonometry of this)
Why do we see the light/dark bands in
diffraction patterns?
When waves travel the same distance, or path length difference is an
integer number of wavelengths, end up in phase at the screen
Light
When waves travel different distance,or path length difference is halfinteger number of wavelengths, waves end up out of phase at screen
Dark
Clicker question
What type of waves are we most likely to see
diffraction from in everyday life?
A. Radio waves
B. Visible light waves
C. Gamma waves
Do we see diffraction every day?
• Usually see diffraction when wavelength of
light is comparable to size of the object or slit.
• Wavelengths of visible light are tiny, so we
usually don’t see evidence of diffraction with
light
• Larger waves make diffraction: Ocean waves
around a jetty, sound waves around objects
(loud/soft spots)
Often shows up in multi-colored objects!
Diffraction separates out colors
• Colors have different wavelengths
• White light is made of all colors
• Different wavelengths bend at
different angles
CD’s have lots of
closely spaced pits,
act as a diffraction
grating
Peacocks features have
lots of ridges
100x
Ridges and pits give interference
Diffraction Gratings make diffraction
more noticeable
• We often use diffraction gratings – lots and
lots of little slits – results in larger separations
so we can more clearly see the pattern
• Why do we see colors?
Examples:
Colors on a CD
Holograms on credit card
A good place to stop today…
HWK 3 is due today at 5PM
in the D2L dropbox!
Exam 1 coming on Wednesday in-class.
Enjoy the rest of your day.
See you tomorrow.