A red laser emits photons with a wavelength of 635 nm. (a) If this laser has a p

Question

A red laser emits photons with a wavelength of 635 nm. (a) If this laser has a power of 23 W, how many photons does it emit in 1 s?
1 photons

(b) If the light emitted by the laser is absorbed by a black sail,what is the force on the sail?
2 N

(c) If the light from this laser is reflected from a mirror, whatis the force on the mirror?
3 N (a) If this laser has a power of 23 W, how many photons does it emit in 1 s?
1 photons

(b) If the light emitted by the laser is absorbed by a black sail,what is the force on the sail?
2 N

(c) If the light from this laser is reflected from a mirror, whatis the force on the mirror?
3 N

Explanation / Answer

a) the energy of 1 photon : Ep = hc/ = 3.132x10^-19 J => the number of photon are emitted in 1 s : N = E/Ep = p*t/Ep = 7.343x10^19photons b) black sail=> the momentum change : h/ to 0 => The force : F1 = P/t = N*t*(h/)/t =N*h/ = 7.67x10^-8 N c) mirror=> the momentum change : h/ to -h/ => force : F2 = P/t = N*t*(2h/)/t =2N*h/ = 1.54x10^-8 N

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