ATOC/ASEN 5235 Cumulative Exam
10 December 2009
1. (5 points). In round numbers, the human body temperature is about 100°F. What is the
wavelength of our maximum thermal emission?
Answer:
Wien’s law tells us that the wavelength of maximum emission is given by:
λ
max
~2897 / T
where T is in degrees K and the wavelength is in units of
μ
m.
Convert 100°F to K:
deg K = (100 – 32) / 1.8 + 273 = 311 K.
λ
max
= 2897/311 =
9.3
μ
m
2. (5 points). Rank the following transitions from lowest energy to highest energy:
rotation, vibration, electronic.
3. (5 points). Mark the following statement as true or false, and explain your answer:
If I take a photograph of a clear, blue sky about 10° above the northern horizon at sunrise
on 21 September, with a linear polarizer that only transmits vertically polarized light, the
sky will appear dimmer than if I were to use a linear polarizer that only transmits
horizontally polarized light.
Answer:
False. The Rayleigh scattered light from the sky at 90° from the sun, which is the
situation described, will be vertically polarized. This is because its original
horizontal component is in the direction of propagation from the horizon to your
camera, which means it must be extinguished when the radiation is scattered
toward your camera. Thus the radiation will be transmitted through the vertical
polarizer. If you were to use a horizontal polarizer, the sky would appear dimmer
because it would not transmit the vertically polarized scattered light.
4. (5 points).
(a) Draw a diagram that shows sunlight being scattered at a 30° scattering angle.
(b) Is this forward or backward scattering?
5. (10 points). Define “emissivity” and “absorptivity”, and give a statement of
Kirchhoff’s Law. Use words, not mathematical equations.
Answer:
Emissivity is the amount of radiation emitted by an object relative to the amount
emitted by a blackbody at the object’s temperature.
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 Fall '10
 Randell
 Light, Polarization, SCATTERING, Total internal reflection

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