Physics 102 Homework set 10 solutions Exercises: Chapter 29: 2) Because sound waves have a much longer wavelength than light waves. Light waves have a wavelength of less than 1 millionth of a meter. Sound waves have wavelengths ranging from 15 mm (at 20 kHz) to 15 m (at 20 Hz). 12) Red refracts through the largest angle, blue through the smallest. There are two good reasons: a) I looked through the diffraction glasses and that is what I saw and b) red light has a longer wavelength than blue light so that it diffracts more. 20) The iridescent colors are due to thin-film interference (due to a thin layer of a transparent material on the surface of the shell). The path length of the light travelling through the film (and thus the wavelengths that have constructive and destructive interference) depends on the angle through which you view the film. At shallower angles, the path length increases, so that the wavelength at which there is destructive interference increases. 26) Sound is a longitudinal wave. There is only one orientation of the pressure variation in the sound wave (back and forth). Light is a transverse wave. There are many possible orientations of the electric field including left-right and up-down. Polarized light has only one electric field orientation. Sound cannot be polarized because it only has one orientation. 30) 50% of unpolarized light passes through one perfect polarizing filter. If the second filter is aligned parallel to the first, then all of the light that passes through the first filter will pass through the second (ie: 50%). If the second filter is aligned perpendicular to the first, then none of the light that passes through the first filter will pass through the second (ie: 0%). Chapter 30: 4) The damage is done by individual photons. UV photons have enough energy to damage skin, visible light photons do not. 8) The red light from a laser is monochromatic (only one frequency). We saw that when I shined the laser through the diffraction grating and there was only one color spot on the wall. Red light from the neon lamp is composed of many colors. We saw that when we looked at the light through the diffraction glasses and saw lots of red, yellow and orange lines, all at different frequencies. 12) A scientist will see the same emission lines in the receding star, but they will be shifted to lower frequencies by the Doppler Shift. (Just like sound from a receding source is shifted to lower frequencies.) Chapter 31: 6) Each photon of red light has less energy than each photon of blue light. Therefore, to have the same total energy in the light beam, you would need more red photons. 8) This is the same as chapter 30, number 4. Photons of red light do not have enough energy to break apart the silver bromide molecules. Photons of blue light do have enough energy. Thus even really bright red light will not expose the film whereas even dim blue light will expose the film. 12) Light striking a metal ejects only electrons because the electrons are on the outside of the atom. The protons are deeply bound in the nucleus of the atom. They can be ejected, but it takes a photon with at least 1 million times more energy than visible light. 26) The slower electron has the longer wavelength. lambda = h/p = h/mv (since momentum = mass times velocity). The faster the velocity, the shorter the wavelength. Problem Chapter 31: 2) The de Broglie wavelength is lambda = h/p = h/mv. In this case m = electron mass = 9.1 * 10^(-31) kg and v = electron speed = 0.1 * c = 3 * 10^7 m/s h = planck's constant = 6.6 * 10^(-34) J-s (where a Joule = kg m^2 / s^2 so a J-s = kg m^2 / s ) lambda = 6.6 * 10^(-34) kg m^2/s -------------------------------------- 9.1 * 10^(-31) kg * 3 * 10^7 m/s = 2.4 * 10^(-11) m or about 1/10 the size of an atom ----------------------------------------- Estimation: If all of the pizza boxes from all of the pizza consumed by all of the students at ODU in 1 year are laid out evenly, how many football fields will they cover? There are about 2*10^4 students at ODU (actually about 18,000). Let's say that they each have 1/2 an 18-inch pizza twice a week. (I'm guessing here [But that's the point of these questions.]. I'm pretty sure that pizza consumption is somewhere between 1/4 and 1/2 pizza each time and somewhere between once and 4 times a week.) Since 18" = 1/2 yard, an 18"x18" pizza is 1/2x1/2 = 1/4 square yard. This means that the total consumption is 52 weeks/year * 2 pizza/(student-week) * 1/4 yd^2/pizza * 2*10^4 students = 5*10^5 yd^2 Now a football field is 100 yd * 50 yd = 5*10^3 yd^2 Therefore the total pizza consumed will cover either 1 football field to a depth of 100 boxes or 100 football fields. That's a lot of pizza!