Question

A pulsed ruby laser emits light at 694.3 nm. For a 15.0-ps pulse containing 2.80 J of energy, find the following.

(a) the physical length of the pulse as it travels through space
mm

(b) the number of photons in it
photons

(c) If the beam has a circular cross section 0.600 cm in diameter, find the number of photons per cubic millimeter.
photons/mm³

Answer 1

distance d = speed*time = 3*10^8*15*10^-12 = 0.0045 m = 4.5 mm <<<=======ANSWER

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(b)

wavelength L = 694.3 nm

number of photons N = d/L = (4.5*10^-3)/(694.3*10^-9) = 6481

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(c)

volume of the beam V = A*d

A = =area of cross section = pi*D^2/4

D = diameter = 0.006 m

number of photons per cubic millimeter = N/V = 6481/(pi*0.006^2/4*0.0045)

number of photons per cubic millimeter = 50937441292.769073 photons/m^3

number of photons per cubic millimeter = 50937441292.769073*10^-9 photons/mm^3 = 51 photons/mm^3