Energy Quantization
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A monochromatic light source is incident on a neutral density filter which decreases the light source's intensity by a factor of 3 but does not change its color. What happens to the energy of a photon in the less intense beam?
None of these
The photon energy decreases by a factor of 3.
The photon energy increases by a factor of 3.
The photon energy decreases by a factor of 9.
The photon energy decreases only slightly.
Explanation
The photon energy does not change since the color of light (and therefore the frequency of the light) does not change.
A laser outputs light of a single color with photon energy 
Explanation
Halving the wavelength is equivalent to doubling the frequency since for a light wave the wavelength and frequency are related by 
