Physics 9 Wednesday, February 1, 2012

Transcription

1 Physics 9 Wednesday, February 1, 2012 learningcatalytics.com class session ID: Today: repeat soap bubble; measure λ for laser Today: telescope, human eye Friday: first of 3 days on fluids (liquids, gases) Thursday/Bill study session: 7pm to 9:30pm Sunday/Zoey: 2pm to 5pm (just this week superbowl)

2 Question: For the soap film below, the rainbow strips of varying thickness are visible because...

3 (Reflected wave: opposite sign for n 2 > n 1, same sign for n 3 < n 2.)

4 Giancoli s optical instruments chapter It was interesting to see how various optical instruments work, such as telescopes and goggles, and the lens of the eye. I liked the stuff about eyes I wish it had gone on to discuss color vision. I thought it was interesting the way the eye works as a lens with a specific index of refraction. I find the human eye fascinating, so I enjoyed reading a bit about how it works. The discussion of human vision was interesting, but the material in this chapter was generally rather confusing. Learning about telescopes doesn t seem so useful to me. I found this reading to be extraordinarily technical and I m not sure I will retain much of the specifics.

5 I ve always wondered how cameras work, so it was fascinating to see a more in depth look on how all of the different components come together to form the image. Also, I wear contacts and have never thought about how they work to fix my vision, but that was really interesting to understand. I still have a really hard time understand how Xrays work. I enjoyed the explanations of how x-rays and CAT scans work. It s interesting to learn about the physics behind practical everyday items. The section on X-ray diffraction was interesting, but also complex and difficult to understand. I m confused by a few of the equations and what they mean. I liked the section about telescopes because my dad just bought a refractor and I was talking about the optical differences between that and his reflector with him recently.

6 Why do you need swim goggles to see underwater? Our eyes work as a lens that is adapted from view through air. Water has about the same index as the cornea (about 1.33), which essentially eliminating the cornea s focusing properties. Goggles have a flat surface that separates yours eyes from water with a layer of air. When light moves from one medium to another of different n, the light bends. Water has similar n to the human cornea, eliminating the cornea s ability to focus. Goggles placing a layer of air between the cornea and the water, allowing the light to focus. It is difficult for the human eye to see under water because the index of refraction of the cornea is so close to the index of refraction of water that light does not bend properly. Swim goggles create an air filled space in front of your eyes to bring light into the eye properly.

7 Why do you need swim goggles to see underwater? Instead of the image being focused on the retina, as it is in air, the image is formed somewhere behind, which makes everything blurry. This is because of the refractive index of water is almost the same as the cornea, so it doesn t bend very much when it enters the eye. We can t see under water since our eyes can t focus under water. When light travels from one medium to another like air to water light is refracted. Light is bent because light travels at different speeds in different media. The refractive indicies of water and the cornea are so similar that light is hardly bent at all when it enters the eye; it is bent only by the lens, so that the image is not focused on the retina, but somewhere behind the retina, causing objects to appear blurry. Swim goggles work by providing a pocket of air in front of you that enables light rays to bend properly as they enter the cornea.

8 Smallest details that visible light can resolve Because of diffraction, it is not possible to discern details smaller than the wavelength of the radiation being used. This limits the useful magnification of a light microscope to about 1000x. The smallest object that can be seen is the wavelength of the radiation being used. A few hundred nanometers. 200nm since the Rayleigh criterion says that anything past a certain magnification will only result in you seeing the diffraction pattern of light instead of details of the object. ( It seems interesting to compare X-rays with ultrasound. ) ultrasound: f 5 MHz, c 1500 m/s, λ 0.3 mm. visible light: f Hz, c = m/s, λ 500 nm. X-rays: f Hz, c = m/s, λ 0.03 nm.

9 Let s measure the wavelength of red light, by running the laser beam through two narrow slits! But first, a question: Is this pattern the result of a horizontal slit or a vertical slit?

10 Q: What do we expect to see on the screen for two slits? Two spots? One spot? Many spots, wildly varying? Many evenly-distributed spots?

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12 Distance y between two adjacent spots on screen corresponds to l = λ, i.e. where the two path lengths differ by one wavelength λ = y δ L where δ is slit spacing and L is distance to screen

13 Distance to screen: Slit spacing: L = 612 cm δ = mm

14 14.4 cm Red: Distance between minima: y = 9 = 1.60 cm. (On Wednesday, I counted incorrectly: I got 18 cm should have got 11 = 1.64 cm.) So we get λ = y δ L = 18 cm 10 = 1.8 cm, but I 1.60 cm 0.25 mm 612 cm = 650 nm (λ true = 630 nm)

15 15.0 cm Green: Distance between minima: y = 11 = 1.36 cm. (On Wednesday, I counted incorrectly: I got 12.3 cm but I should have got 9 = 1.37 cm.) 12.3 cm 8 = 1.54 cm, So we get λ = y δ L = 1.36 cm 0.25 mm 612 cm = 560 nm (λ true = 543 nm)

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17 The next few slides are borrowed from the materials for Visual Studies 101. The Richard Feynman handout on color vision has similar coverage with lots of explanatory text.

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21 Test for color blindness

22 Color blind: 2 vs. normal 3 kinds of cone cells

23 Blind spot Your retina has no sensitivity where the optic nerve exits the eye. The wikipedia entry below shows you how to find your blind spot. Try it it really works!

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25 The trick for finding the image point graphically: draw the easy-to-draw rays ( principal rays ), and see where they meet. For thin converging lens, what comes in parallel to axis on LHS must pass through focus on RHS. What passes through focus on LHS must exit parallel to axis on RHS. What passes through the point at center of lens does not bend. (There are analogous tricks for mirrors, thin diverging lenses, but thin converging lens is most useful case to remember.)

26 Lens & mirror summary (light always enters from LHS): converging lens f > 0 d i > 0 is RHS 1 ( f = (n 1) 1 R R ( 2 ) 1 diverging lens f < 0 d i > 0 is RHS f = (n 1) 1 R R 2 converging mirror f > 0 d i > 0 is LHS f = R/2 diverging mirror f < 0 d i > 0 is LHS f = R/2 Horizontal locations of object, image (beware of sign conventions!): 1 d o + 1 d i = 1 f Magnification (image height / object height): m = d i d o Lenses: R 1,2 > 0 for outie (convex), < 0 for innie (concave). Real image: d i > 0. Virtual image: d i < 0. Real image means light really goes there. Virtual: rays converge where light doesn t go. )