PhysicsLAB: 2003 B7

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?