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) A photon of red light has less energy  than a 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 bluelight 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