Physics 102
Homework set 10
Solutions
L. Weinstein

Exercises:
----------

29.16: They will produce a light spot.  The beams will add
constructively because the peaks will add to peaks and the troughs
will add to troughs.  This is because if travel exactly two
wavelengths along a wave from a peak, there is another peak.


29.26: The colors of peacocks and hummingbirds result from
interference.  The blue is due to the destructive interference of the
other colors and the constructive interference of the blue.


29.32: Each color is due to a specific  difference in the path length
of the two light rays.  Therefore, each ring of color corresponds  to
a particular thickness of the gasoline slick on the water.


29.36: You might not be able to read an LED display through polarized
sunglasses since the glasses might not let the light through if the
polarization differs.  This is exactly like looking at the computer
display in class through the polarized filters.  Some colors are
polarized one way and other colors are polarized in a different
direction.


30.4: One photon of ultraviolet light can knock an electron out of a
molecule  and damage the molecule.  Photons of visible light cannot do
this.


30.18: We get a continuous spectrum from a tungsten filament rather
than the discrete colors from a tungsten gas because the electron
levels in a solid are very different from the electron levels in
isolated atoms.  The electrons in a solid metal have a continuous
energy distribution and therefore can emit all colors.


30.24: When ultraviolet (UV) light falls on certain dyes, a single UV
photon can excite one electron past two or more energy levels.  When
the electron deexcites, it can emit lower energy (eg: visible) light.

Infrared (I) light cannot do this because  IR photons are lower energy
than visible photons.


31.6: White light appears white to us because  it contains light of
many wavelengths.  There is no such thing as a photon of white light.
You need many different photons of different wavelengths for light to
appear white.


31.8: The beam of red light will contain more photons.  Red light has
a lower frequency than blue light so a red photon contains less energy
than a blue photon.  Therefore, to have the same total energy, you
need more red photons.


31.12: This is the same as 30.4.  This exercise has been removed from
the homework assignment.


31.26: A photon behaves like a wave as it travels.  A photon behaves
like a particle when it is absorbed or emitted.


Problems:
----------------------

31.2: 
p = h/mv = 6.6*10^(-34) J-s / (9.1*10^(-31) kg * 0.1 * 3*10^8 m/s)
  = 2.4*10^(-11) m

That's rather tiny!


Estimation:
---------------------
    
I'll assume that each student eats  one large (16") pizza each week.  (This
is  OK because they certainly eat less than 10 pizza/week and more
than 0.1 pizza/week.)  A box   for a  16" pizza is 16" x 16" or 0.4 m
x 0.4 m = 0.16 m^2.

There are 20,000 students at ODU and they are here for about 30 weeks
per year so that gives a total area of

A = (0.16 m^2 / student-week) * 2*10^4 students * 30 weeks = 10^5 m^2

Foreman Field is the same size as a football field which is 100 yd x
50 yd or about 100 m x 50 m = 5*10^3 m.

Thus, 10^5 m^2 of pizza boxes will cover a field that is 5*10^3 m to a
depth of

# layers = 10^5 m^2 / 5*10^3 m = 20 

Answers should be between 2 and 200 layers.