Purpose of this Minilab

Optics 3: Physical Optics

Purpose of this Minilab

• Experiment with and learn about

- Light intensity- Polarization- Diffraction- Interference


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!!!


Light (and other electromagnetic radiation) carries energy.

TimeEnergyPower

TimeAreaEnergy

AreaPowerIntensity

Activity 1: Light Intensity


Example: The SunThe sun radiates 4x1026 Joules of energy every second.

The sun is 1.5x1011m (93.2 million miles) away from the earth.


What is the intensity of solar radiation on the solar panel of a satellite?


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.



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.


Side windows



Equal colors in the two half circles indicates equal light intensities.


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).



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




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



Problem 2: What is the relationship between intensity and distance from a point source?

Hint: Think about the example we gave with the sun.


Measure how much light intensity is transmitted by a polarizer.

Optics Bench

Pasco light source

PhotometerFiltersPoint source (hole)

on this side


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


Activity 2: Polarization

Light has wave characteristics.

Electric fieldvectors

Direction of propagation

Electric field vectorsa short time later

z

x

y


x

y


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


x

y


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).



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).



x

y


Polarizer

x

y

x

y

All the light is blocked by the polarizer.



x

y


Polarizer

x

y

x

y

Only the component of that is aligned with thepolarizer passes.Reduced intensityChanged direction of polarization.

E



x

y

=

x

y

x

y

x

y

x

y

+(vector addition)

E

E cos (Q)

Q

E sin (Q)

E cos (Q)



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.


Rotate polarizers with respect to each other and observe the intensity of the light after passing through both polarizers.

Optics Bench


Answer Questions 4 and 5.

Polarizers

eye


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


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):


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


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


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



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



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”).



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 Bench

Laser Disk with different slit patterns(rotate to select).

Laser light

Screen

Laser power supply



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)


Bright

Bright

2nd order maximum (m=2)

2nd order maximum (m=2)

D

y (for m=1)



Dark

Dark

Dark

Bright

Bright

Dark

Bright

yDmd



Bright

Bright

2 y (for m=1)Hint:It is more accurate tomeasure 2y and then divide by 2.



Determine the distance d between the “grooves” of a CD (Problem 14 -17

d



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 Bench

Laser

Optics mount

CD attached

Laser light

10 cm 10 cm

Screen



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?


Using the Desk Lamp

Dimmer

Lamp Plug (black) must be pluggedinto dimmer plug.Dimmer plug (white) must be pluggedinto power outlet.

On/Offswitchof lamp

Purpose of this Minilab

Documents

Transcript of Purpose of this Minilab