A laser pointer directs a narrow beam of lightthat can be used by a speaker to i

Question

A laser pointer directs a narrow beam of lightthat can be used by a speaker to illuminate locations on aprojected image during a lecture. If a 3.7 mW laser pointerilluminates a circular area with a radius of 1.2 mm on aprojection screen with an albedo of 0.86, find the pressure thelaser exerts on the illuminated portion of the screen.

Explanation / Answer

There are 2 contributions to the pressure; one by reflected photonsand one by absorbed photons. let n = number of photons hitting the surface each second. nE= 3.7 mW >>> E = energy of each photon reflected photons: 86% of the photons are reflected and each contributes a change inmomentum = p = p - (-p) = 2p >>> p is the momentum of eachphoton (p=E/c) force = 2p*(.86n) = 2(E/c)(.86n)=(2)(.86)(nE/c) absorbed photons: 14% of the photons are absorbed and each contribute a change inmomentum = p = 0-(-p)=p force = (.14)(nE/c) total force = (nE/c)(.14+2*.86)=(3.7*10^-3)(.14+2*.86)/(3*10^8)=2.3 x 10^-11 N pressure = total force/area = 5.07 * 10-6N/m2

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