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crybllrd
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Homework Statement
The laser used in a typical DVD player has a wavelength λ = 650 nm and power of
240 mW.
(a) Look up the speed of light in empty space and calculate the frequency f of this laser light.
(b) Light of f carries energy in bundles of hf, called “photons”, where h is Planck’s
constant. Calculate the energy of a photon using the value of f you found in
part (a).
(c) Calculate the number of photons striking the DVD disk every second.
(d) A photon also carries momentum h/λ. Momentum for an object with mass m is defined as mv, where v is the speed (so long as v is much, much less than c). In a collision between a photon and a stationary massive object, in which the photon bounces straight backwards, the momentum of the object after the collision is twice the photon’s momentum before the collision. Estimate the speed of a typical DVD of mass 16 g (grams) – if it were not held stationary by the DVD player – after one second of bombardment by the laser.
Assume the same momentum gain 2h/λ for each collision.
Homework Equations
Planck's constant = h = 9.80665m/s
Speed of light = 300,000km/s = 3e8m/s
The Attempt at a Solution
(A) v=λf
f=v/λ
f=(3e8m/s)/(6.7e-7m)
f=4e14s (is this Hz?)
(B) energy=hf
= (9.80665m/s)(4e14s)
= 4e15m/s2
(C) I know power is energy transmitted per unit time, so can I divide the energy by 1s? If so, then:
(4e15m/s2)/(1s)=4e15m/s
Where does the 240 mW come in?
(D)