Physics 1230: Light and Color

Transcription

1 Physics 1230: Light and Color Chuck Rogers, Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 EXAM 1 is Thursday in-class 3:30PM. You can bring an 8.5x11 cheat sheet, calculator, pencil. See the website for details. Covers HWKs 1-5 and Lectures 1-9 (through next week, Tuesday). It is a two-stage exam, we will form group for you. Look for seat assignments on D2L Thursday AM. 1

2 Physics 1230: Light and Color Chuck Rogers, Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Lecture 9: Electromagnetic waves! How we talk about them, making them, how they travel, how they are detected. Polarization! 2

3 Last Time: E and B and forces Electric fields Magnetic fields Light is an electromagnetic wave E B 3

4 Last Time: Electric Fields Charges feel a force: ELECTRIC FIELD due to the rest of the universe. Fq r qe The charge value. Properties of the rest of the universe. 4

5 Last Time: Maxwell (1861) found that changing electric fields create magnetic fields. AND Changing magnetic fields create electric fields (Faraday). Together, E and B can make a traveling wave! E AND move Charges. B 5

6 Polarization demonstration 6

7 Properties of light Interaction with matter Electric fields exert forces on charges (e s and p s in atom) X + F=qE X _ F=qE E E Force = charge x electric field F= qe Light is an oscillating E-field (and B-field). It interacts with matter by exerting forces on the charges the electrons and protons in atoms.

8 How do you generate light (electromagnetic radiation)? a. Stationary charges b. Charges moving at a constant velocity c. Wiggling charges d. b and c e. a, b, and c Hint 1: Stationary charges constant E-field, no magnetic (B)-field E + Hint 2: Charges moving at a constant velocity Constant current through wire creates a B-field But B-field is constant and no E-field. I B Accelerating charges changing E-field and changing B-field (EM radiation both E and B are oscillating)

9 Go to radiowave sim How do you generate light (electromagnetic radiation)? a. Stationary charges b. Charges moving at a constant velocity c. Wiggling charges d. b and c e. a, b, and c Ans. is c. Accelerating charges create EM radiation. The Sun Surface of sun- very hot! Whole bunch of free electrons whizzing around like crazy. Equal number of protons, but heavier so moving slower, less EM waves generated.

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12 Remember, as earlier: Some things are self-luminous, like the SUN. They burn energy and release light. Some things are NOT self-luminous. Their charges are forced to wiggle by light from other things. The MOON 12

13 Group Activity: Making sense of the Sine Wave What does the curve tell you? -For Water Waves? -For Sound Wave? -For E/M Waves?

14 A sound wave is represented below. You can listen to the wave with your ear at positions 1, 2, or 3, which are all at the same X-axis position. The wave goes by at the speed of sound. The loudest tone is at: A) 1 B) 2 C) 3 D) 1 and 2, but not 3 E) None of the above. 1, 2, and 3 are equally loud! 14

15 Continue with Part 3: EM waves 15

16 EM radiation often represented by a sinusoidal curve. OR What is the wave showing? What does the curve tell you? a.the spatial extent of the E-field. At the peaks and troughs the E-field is covering a larger extent in space b.the E-field s direction and strength along the center line of the curve c. The actual path of the light travels d.more than one of these e.none of these.

17 EM radiation often represented by a sinusoidal curve. OR What is the wave showing? What does the curve tell you? a.the spatial extent of the E-field. At the peaks and troughs the E-field is covering a larger extent in space b.the E-field s direction and strength along the center line of the curve c. The actual path of the light travels d.more than one of these e.none of these.

18 Making sense of the sine wave Here is an electric wave. You should know: E X What is varying over the length of the wave? (What aspect of the electromagnetic field does the amplitude of the wave represent?) What would happen to a negative electric charge that is hit by this wave?

19 Continue with the last page! Detecting EM Waves.

20 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

21 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 B. Yes, to the right C. No This is why radio antenna are oriented up and down

22 Suppose you move the antenna to Here? 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

23 PhET Radio Waves

24 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

25 Wavefront Electromagnetic radiation

26 Good time for Questions!

27 We ve covered a lot, here are the points for what we learned today Define the two types of charge and whether they feel attractive or repulsive forces Recognize that a charged object feels a force in an electric field Recognize that electromagnetic waves are oscillating electric and magnetic fields Predict what happens when an electromagnetic wave hits a charged object, and how the frequency and amplitude of the wave affects this outcome.

28 Electromagnetic waves can jiggle in different directions = Polarization

29 Polarization Vertical polarization has electric field up and down Horizontal polarization has electric field left and right 29 29

30 Most light is made of several of these polarizations at once: called un-polarized

31 Can also have other states such as circularly polarized Movie at: tion_(waves)

32 Polarizing filters select out one direction of polarization (other is blocked) Ordinary light has both polarizations. Polarizing filters pass only one polarization. Another indication that light is a wave! Demos Slits Overhead filter 32 32

33 Polarizers are used in modern 3D glasses Old style 3D: red and blue filters. Different image to different eyes = stereoscopic vision, illusion of depth.

34 Turn your Real 3D glasses, you ll see the effects of polarized filters New style 3D uses polarizers (usually circularly), so can see color images. But 3D movies are about 35% dimmer than standard films.

35 Polarizing filter Ordinary light has polarization in all directions (unpolarized). Polarizing filters pass only one polarization

36 How does a polarizing filter work? Polarizers are made of long-chain molecules, stretched in one direction. The waves that hit the polarizer, oriented parallel to those molecules, vibrate those molecules and are ABSORBED. So a vertical polarizer has the longchain molecules aligned horizontally.

37 Light reflected off surfaces can become mostly horizontally polarized

38 Let s get out our polarizer squares! Light on the table is it producing polarized light? A) YES B) NO Light on the table is its reflection polarized? A) YES B) NO How about my laptop is its screen producing polarized light? A) YES B) NO

39 Light reflected off surfaces can become mostly horizontally polarized

40 Clicker Question Clicker question Which are the filters on polarizing sunglasses?

41 Clicker Question Clicker question Which are the filters on polarizing sunglasses? C - Vertical filters block horizontally polarized light

42 Polarized sunglasses reduce horizontally polarized light by using vertical polarizing filter 42

43 Clicker Question Clicker question What do you see if you tilt your head sideways while wearing polarized sunglasses? A. Looks the same B. Gets brighter C. Gets dimmer D. Can t see anything

44 Clicker Question Clicker question What do you see if you tilt your head sideways while wearing polarized sunglasses? A. Looks the same B. Gets brighter C. Gets dimmer D. Can t see anything

45 Polarizing filters on cameras use the same principle No filter Polarizing filter

46 Two Polarizing Filters When the pickets of both fences are aligned in the vertical direction, a vertical vibration can make it through both fences When the picket of the second fence is horizontal, vertical vibrations are blocked

47 Clicker Question Clicker question What happens if a third polarizer is added at 45 degrees between two perpendicular (crossed) polarizers? A. Gets dimmer B. Looks the same C. Gets lighter Demo other items between crossed polarizers

48 Liquid Crystal Liquid-crystal Displays display use Polarizers applications