PhysicsLAB AP Free Response Question
2003 B7

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Energy-level diagrams for atoms that comprise a helium-neon laser are given below. As indicated on the left, the helium atom is excited by an electrical discharge and an electron jumps from energy level E0 to energy level E2.
The helium atom (atomic mass 4) then collides inelastically with a neon atom (atomic mass 20), and the helium atom falls to the ground state, giving the neon atom enough energy to raise an electron from E0' to E2'. The laser emits light when an electron in the neon atom falls from energy level E2' to energy level E1'.
 (a) Calculate the minimum speed the helium atom must have in order to raise the neon electron from E0 ' to E2 '.

 (b) Calculate the DeBroglie wavelength of the helium atom when it has the speed determined in (a).

 (c) The excited neon electron then falls from E2 ' to E1 ' and emits a photon of laser light. Calculate the wavelength of this light.

 (d) This laser light is now used to repair a detached retina in a patient’s eye. The laser puts out pulses of length 20 x 10-3 seconds that average 0.50 W output per pulse. How many photons does each pulse contain?

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