Optics 3: Physical Optics
Purpose of this Minilab
• Experiment with and learn about
- Light intensity- Polarization- Diffraction- Interference
Optics 3: Physical Optics
WARNING – Lasers Used in this Lab
Lasers can cause permanent damage to the eye.
Do not look directly into the laser beam!!!
Do not aim the laser towards others!!!
Optics 3: Physical Optics
Light (and other electromagnetic radiation) carries energy.
TimeEnergyPower
TimeAreaEnergy
AreaPowerIntensity
Activity 1: Light Intensity
Optics 3: Physical Optics
Example: The SunThe sun radiates 4x1026 Joules of energy every second.
The sun is 1.5x1011m (93.2 million miles) away from the earth.
Activity 1: Light Intensity
What is the intensity of solar radiation on the solar panel of a satellite?
Optics 3: Physical Optics
WxsJx
TimeEnergysunthebyemittedPower 26
26
104104
2211
26
2
26 1600105.14
1044104satelliteatintensityRadiation
mW
mxWx
RWx
AreaPower
Earth with satellite (they both are about the same distance from the sun).
RAll the solar power must pass through a virtual sphere (with the earth at the surface of that sphere).The power from the sun is spread out over the surface area of that sphere (4R2).
Note: Due to reflection at the earth’s atmosphere only 250W/m2 arrive at the earth’s surface.
Activity 1: Light Intensity
Optics 3: Physical Optics
Photometer: Compares the light intensities entering the two side windows.
Look through the eyepiece in the center:Unequal color in the two half circles indicates different light intensities.
Activity 1: Light Intensity
Side windows
Optics 3: Physical Optics
Activity 1: Light Intensity
Equal colors in the two half circles indicates equal light intensities.
Optics 3: Physical Optics
You can attach a variable filter disk to one side to vary the intensity on that side.4 different filters are available: 100%, 75%, 50%, and 25% (% indicates the amount of light transmitted by the filter).
Activity 1: Light Intensity
Optics 3: Physical Optics
Measure how the light intensity changes as the light source is placed further and further away (Problem 1)
Optics Bench
Pasco light source
Photometer
Optics mounts(empty lens holders)
Flash Light
filtersPoint source (hole)on this side
Leave some room (maybe 10cm)between filter and flash light
Activity 1: Light Intensity
Optics 3: Physical Optics
Activity 1: Light Intensity
Procedure for Problem 1:
Optics Bench
1. Rotate filter to 100% I0 setting ( = no filtering).2. Move point source such that photometer shows even color.3. Record distance r.4. Rotate filter to 75% I0 setting.5. Move point source such that photometer shows even color.6. Record distance r.7. Etc..
r
I (arb. units) r
10.750.500.25
………… r
Ir2
………… r2
I
Optics 3: Physical Optics
Activity 1: Light Intensity
Problem 2: What is the relationship between intensity and distance from a point source?
Hint: Think about the example we gave with the sun.
Optics 3: Physical Optics
Measure how much light intensity is transmitted by a polarizer.
Optics Bench
Pasco light source
PhotometerFiltersPoint source (hole)
on this side
Activity 1: Light Intensity
Add a polarizer.(Don’t change distance offlash light to photometer)
1. Insert a polarizer between photometer and flash light (but do not change the distance between photometer and flash light).2. Select 100% filter.3. Move the Pasco light source until photometer shows equal intensity.4. Record distance r.5. Use I versus r (or I versus r2) table to determine what I is with polarizer inserted..
r
Optics 3: Physical Optics
Activity 2: Polarization
Light has wave characteristics.
Electric fieldvectors
Direction of propagation
Electric field vectorsa short time later
z
x
y
Optics 3: Physical Optics
x
y
Activity 2: Polarization
Now looking at the electric field vector at one particular point in space in the direction of propagation (light travels “into the screen”):
t = 0
x
y
x
y
x
ya littlelater
evenlater
evenlater Etc….
goes up and down
This light is called “linearly polarized” (in the y-direction).
EE
EE
E
Let’s symbolize it as: x
y
Optics 3: Physical Optics
x
y
Activity 2: Polarization
Linearly polarized in the y-direction
x
y
x
y
Linearly polarized in the x-direction
Unpolarized light (a superpositionof many “light waves” that are polarized in arandom direction).
Optics 3: Physical Optics
Activity 2: Polarization
x
y
A polarizer (often that is a thin sheet of material) only passes light that ispolarized in a certain direction:
Light before passingthrough the polarizer.
Polarizer
x
y
Indicates polarizerorientation.
x
y
Light after passingthrough the polarizer(no change).
Optics 3: Physical Optics
Activity 2: Polarization
x
y
Light before passingthrough the polarizer.
Polarizer
x
y
x
y
All the light is blocked by the polarizer.
Optics 3: Physical Optics
Activity 2: Polarization
x
y
Light before passingthrough the polarizer.
Polarizer
x
y
x
y
Only the component of that is aligned with thepolarizer passes.Reduced intensityChanged direction of polarization.
E
Optics 3: Physical Optics
Activity 2: Polarization
x
y
=
x
y
x
y
x
y
x
y
+(vector addition)
E
E cos (Q)
Q
E sin (Q)
E cos (Q)
Optics 3: Physical Optics
Activity 2: Polarization
Unpolarized light before passingthrough the polarizer.
Polarizer
x
y
x
y
…and this is why it’s called a “polarizer”
x
y
After the light passesthrough the polarizer:Light is polarized.
Optics 3: Physical Optics
Rotate polarizers with respect to each other and observe the intensity of the light after passing through both polarizers.
Optics Bench
Activity 2: Polarization
Answer Questions 4 and 5.
Polarizers
eye
Optics 3: Physical Optics
Measure intensity I versus Q, where Q is the relative angle between the two polarizer orientations (for Problem 7)
Optics Bench
Pasco light source
Photometer
Filters
Point source (hole)on this side
Activity 2: Polarization
1. Insert two polarizers between photometer and flash light.2. Align the two polarizer orientations so they are the same3. Put the filter on the side facing the Pasco light source and select the 100% filter.4. Move the Pasco light source until photometer shows equal intensity.
Polarizers
Here’s an idea on how to do this (feel free to improvise otherwise):
Optics 3: Physical Optics
Measure intensity I versus Q, where is the relative angle between the two polarizer orientations (for Problem 7)
Optics Bench
Pasco light source
Photometer
Filters
Point source (hole)on this side
Activity 2: Polarization
4. Select the 75% filter.5. Slowly rotate polarizer 2 while observing the photometer.6. Find and record all orientations Q of polarizer 2 for which you see equal intensity.7. Repeat steps 4-6 for the 50% and 25% filters.8. Create a table with two columns: Q and intensity.9. Create a graph of intensity versus Q.10. Try other plots (e.g. intensity versus cos(Q) or versus cos2(Q)…etc.) to try to find the relationship between angle and intensity.
Polarizer 1Polarizer 2
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Shining coherent light (e.g., laser) through a small slit (or multiple slits) causesinterference (a fancy word for “wave addition”) of the “light waves”.
Wave fronts of light
Double slit
screenThe wave going through this slit has to travel just a bitfurther to get to this particular place on the screen. The waves from the two slits are out of phase by half a wavelength.The two waves annihilate each other. (“destructive interference”). There will be darkness on that place on the screen.
Dark
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
The waves going through both slits travel the same distanceto the screen. The waves from the two slits are in phase.The two waves add together and have twice the amplitude (“constructive interference”). There will be a bright spot on that place on the screen.
Dark
Bright
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Dark
Dark
Dark
Bright
Bright
Dark
Bright
The light exits the slits in all directionssimultaneously.
A pattern of bright and dark spots appears. (called “Interference pattern”).
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
The pattern of interference depends on the slit sizes, slit number, and slit separation, etc..
Single slit
a (slit width)
a
Double slit
d: separation between slits
Multiple slits
Look at interference patterns of:1) Single slits (use different slit widths) (Problem 8).2) Double slits (use different slit separations) (Problems 9).3) Multiple slits (keep a and d constant and vary number of slits)
(Problems 10, 11, 12).
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Optics Bench
Laser Disk with different slit patterns(rotate to select).
Laser light
Screen
Laser power supply
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Dark
Dark
Dark
Bright
Bright
Dark
Bright
Determine the wavelength of the laser light (Problem 13).
yDmd
0th order maximum (m=0)
multi slit
1st order maximum (m=1)
1st order maximum (m=1)
Bright
Bright
2nd order maximum (m=2)
2nd order maximum (m=2)
D
y (for m=1)
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Dark
Dark
Dark
Bright
Bright
Dark
Bright
yDmd
1st order maximum (m=1)
1st order maximum (m=1)
Bright
Bright
2 y (for m=1)Hint:It is more accurate tomeasure 2y and then divide by 2.
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Determine the distance d between the “grooves” of a CD (Problem 14 -17
d
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Method: Reflection on grooves produces also interference pattern.
Laser
CD (with grooves) screen
D
Dark
Dark
Dark
Bright
Bright
Dark
Bright
screen behind laser
reflected light
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Optics Bench
Laser
Optics mount
CD attached
Laser light
10 cm 10 cm
Screen
Optics 3: Physical Optics
Activity 3: Diffraction and Interference
Again: Use to determine d.yDmd
Problem 17:How many grooves are on the CD?Yes, technically there is only 1 groove on the CD that snakes its way from the outside to the center. The proper question you should answer is:How many times does this groove go around the CD?
Optics 3: Physical Optics
Using the Desk Lamp
Dimmer
Lamp Plug (black) must be pluggedinto dimmer plug.Dimmer plug (white) must be pluggedinto power outlet.
On/Offswitchof lamp
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