Laser Cooling Lasers can cool a group of atoms by slowing them down, because the
ID: 1650029 • Letter: L
Question
Laser Cooling Lasers can cool a group of atoms by slowing them down, because the slower the atoms, the lower their temperature. A rubidium atom of mass 1.42×1025kg and speed 229 m/s undergoes laser cooling when it absorbs a photon of wavelength 781 nm that is traveling in a direction opposite to the motion of the atom. This occurs a total of 7700 times in rapid succession. Part A What is the atom’s new speed after the 7700 absorption events? WRITE CORRECT UNITS Part B How many such absorption events are required to bring the rubidium atom to rest from its initial speed of 229 m/s? Express your answer to three significant figures.
Explanation / Answer
Given,
Mass of Rubidium atom, m = 1.42 x 10-25 kg
Speed, v = 229 m/s
Wavelength, ? = 781 x 10-9 m
A) Photon momentum: p = h/?
Rubidium momentum: p = mv
Each absorbtion reduces the momentum of the rubidium atom by h/?. After 7700 absorbtions:
mv2 = mv1 ? 7700 h/?
v2 = v1 ? 7700h/m?
v2 = 183 m/s
B) m x 0 m/s = mv1 ? nh/?
mv1 = nh/?
n = mv1?/h (where h refers to Planck's constant 6.626 x 10-34)
n = 3.83 × 104 absorptions
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