Purpose of this Minilab

1. Purpose of this Minilab • Experiment with and learn about - Light intensity - Polarization - Diffraction - Interference

2. 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!!!

3. Activity 1: Light Intensity Light (and other electromagnetic radiation) carries energy.

4. Activity 1: Light Intensity Example: The Sun The 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?

5. Activity 1: Light Intensity Earth with satellite (they both are about the same distance from the sun). • All 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 (4pR2). R Note: Due to reflection at the earth’s atmosphere only 250W/m2 arrive at the earth’s surface.

6. Activity 1: Light Intensity Photometer: Compares the light intensities entering the two side windows. Side windows Look through the eyepiece in the center: Unequal color in the two half circles indicates different light intensities.

7. Activity 1: Light Intensity Equal colors in the two half circles indicates equal light intensities.

8. Activity 1: Light Intensity 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).

9. Activity 1: Light Intensity 1.1 Measure how the light intensity changes as the light source is placed further and further away. Optics mounts (empty lens holders) filters Point source (hole) on this side Photometer Pasco light source Flash Light Optics Bench Leave some room (maybe 10cm) between filter and flash light

10. Optics Bench Activity 1: Light Intensity r Procedure for Activity 1.1: • Rotate filter to 100% I0 setting ( = no filtering). • Move point source such that photometer shows even color. • Record distance r. • Rotate filter to 75% I0 setting. • Move point source such that photometer shows even color. • Record distance r. • Etc.. I I I (arb. units) r r2 1 0.75 0.50 0.25 … … … … … … … … r2 r

11. Activity 1: Light Intensity Question 1: What is the relationship between intensity and distance from a point source? Hint: Think about the example we gave with the sun.

12. Activity 1: Light Intensity 1.2 Measure how much light intensity is transmitted by a polarizer. Add a polarizer. (Don’t change distance of flash light to photometer) Filters Point source (hole) on this side Photometer Pasco light source Optics Bench r • Insert a polarizer between photometer and flash light (but do not change the distance • between photometer and flash light). • Select 100% filter. • Move the Pasco light source until photometer shows equal intensity. • Record distance r. • Use I versus r (or I versus r2) table to determine what I is with polarizer inserted..

13. Activity 2: Polarization Light has wave characteristics. y Electric field vectors a short time later x Direction of propagation z Electric field vectors

14. y y y y x x x x E E E E y x 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”): even later even later a little later Etc…. goes up and down t = 0 This light is called “linearly polarized” (in the y-direction). Let’s symbolize it as:

15. y y y x x x Activity 2: Polarization Unpolarized light (a superposition of many “light waves” that are polarized in a random direction). Linearly polarized in the x-direction Linearly polarized in the y-direction

16. y y y x x x Activity 2: Polarization A polarizer (often that is a thin sheet of material) only passes light that is polarized in a certain direction: Indicates polarizer orientation. Polarizer Light before passing through the polarizer. Light after passing through the polarizer (no change).

17. y y y x x x Activity 2: Polarization Polarizer Light before passing through the polarizer. All the light is blocked by the polarizer.

18. y y y x x x Activity 2: Polarization Polarizer • Only the component of • that is aligned with the • polarizer passes. • Reduced intensity • Changed direction of polarization. Light before passing through the polarizer.

19. y y y y y x x x x x Activity 2: Polarization Ecos (Q) E = + (vector addition) E cos(Q) E sin (Q)

20. y y y x x x Activity 2: Polarization …and this is why it’s called a “polarizer” Polarizer After the light passes through the polarizer: Light is polarized. Unpolarized light before passing through the polarizer.

21. Activity 2: Polarization 2.1 Rotate polarizers with respect to each other and observe the intensity of the light after passing through both polarizers. Polarizers eye Optics Bench Answer Questions 4 and 5.

22. Activity 2: Polarization 2.2 Measure intensity I versus Q, where Q is the relative angle between the two polarizer orientations. Polarizers Point source (hole) on this side Photometer Pasco light source Filters Optics Bench Here’s an idea on how to do 2.2 (feel free to improvise otherwise): • Insert two polarizers between photometer and flash light. • Align the two polarizer orientations so they are the same • Put the filter on the side facing the Pasco light source and select the 100% filter. • Move the Pasco light source until photometer shows equal intensity.

23. Activity 2: Polarization 2.2 Measure intensity I versus Q, where is the relative angle between the two polarizer orientations. Polarizer 1 Polarizer 2 Point source (hole) on this side Photometer Pasco light source Filters Optics Bench • Select the 75% filter. • Slowly rotate polarizer 2 while observing the photometer. • Find and record all orientations Q of polarizer 2 for which you see equal intensity. • Repeat steps 4-6 for the 50% and 25% filters. • Create a table with two columns: Qand intensity. • Create a graph of intensity versus Q. • Try other plots (e.g. intensity versus cos(Q) or versus cos2(Q)…etc.) to try to find the • relationship between angle and intensity.

24. Activity 3: Diffraction and Interference Shining coherent light (e.g., laser) through a small slit (or multiple slits) causes interference (a fancy word for “wave addition”) of the “light waves”. Wave fronts of light Dark screen • The wave going through this slit has to travel just a bit • further 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. Double slit

25. Activity 3: Diffraction and Interference Dark Bright • The waves going through both slits travel the same distance • to 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.

26. Activity 3: Diffraction and Interference Dark • The light exits the slits in all directions • simultaneously. • A pattern of bright and dark spots appears. (called “Interference pattern”). Bright Dark Bright Dark Bright Dark

27. Activity 3: Diffraction and Interference The pattern of interference depends on the slit sizes, slit number, and slit separation, etc.. Single slit Double slit Multiple slits a (slit width) a d: separation between slits • 3.1 Look at interference patterns of: • Single slits (use different slit widths) (Q7). • Double slits (use different slit separations) (Q8). • Multiple slits (keep a and d constant and vary number of slits) (Q9).

28. Activity 3: Diffraction and Interference Laser Disk with different slit patterns (rotate to select). Screen Laser light Optics Bench Laser power supply

29. Activity 3: Diffraction and Interference Hint for Question 10: For a double slit there is not only interference between the two slits but also within each slit (each slit has its own single slit interference).  Think of superposition of two effects.

30. Activity 3: Diffraction and Interference 3.2 Determine the wavelength of the laser light. Bright 2nd order maximum (m=2) Dark Bright 1st order maximum (m=1) y (for m=1) Dark 0th order maximum (m=0) Bright D Dark 1st order maximum (m=1) Bright Dark 2nd order maximum (m=2) Bright multi slit

31. Activity 3: Diffraction and Interference Bright Dark Bright 1st order maximum (m=1) Dark Hint: It is more accurate to measure 2y and then divide by 2. 2 y (for m=1) Bright Dark 1st order maximum (m=1) Bright Dark Bright

32. Activity 3: Diffraction and Interference 3.3 Determine the distance d between the “grooves” of a CD d

33. Activity 3: Diffraction and Interference Method: Reflection on grooves produces also interference pattern. Dark Bright CD (with grooves) reflected light Dark Bright screen Dark Laser Bright Dark screen behind laser D

34. Activity 3: Diffraction and Interference Optics mount Screen CD attached Laser light Laser Optics Bench  10 cm  10 cm

35. Activity 3: Diffraction and Interference Again: Use to determine d. Question 15: 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?

36. Using the Desk Lamp Lamp Plug (black) must be plugged into dimmer plug. Dimmer plug (white) must be plugged into power outlet. Dimmer On/Off switch of lamp