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6-10 A laser can...
6-10 A laser can be made by placing a plasma tube in an optical resornant cavity formed by two highly reflecting flat mirrors, which act like rigid walls for light waves (see figure). The purpose of the plasma tube is to produce light by exciting normal modes of the cavity. Plasma tube ㄈㄧ Vo frequency (a) What are the normal mode frequencies of the resonant cavity? (Express your answer in terms of the distance L between the mirrors and the speed of light c.) (b) Suppose that the plasma tube emits light centered at fre- quency vo = 5 × 1014 Hz with a spectral width ay, as shown in the sketch. The value of△v is such that all normal modes of the cavity whose frequency is within ±1.0 × 109 Hz of vo will be excited by the plasma tube. (1) How many modes will be excited if L = 1.5 m? (2) What is the largest value of L such that only one normal mode will be excited (so that the laser will have only one output frequency)? (c = 3 × 108 m/sec.)