Problem — Momentum of a laser pulse

Problem

A laser pulse carries energy $U=0{,}1$ J. Calculate the momentum delivered to a perfect absorber. How much is delivered in the case of a mirror?

Solution

Perfect absorber. The radiation carries momentum $p=U/c$; if it is entirely absorbed, all of it is delivered:

$p=\frac{U}{c}=\frac{0{,}1}{3{,}00\cdot 10^8}\approx 3{,}3\cdot 10^{-10}\ \text{kg}\cdot\text{m/s}$

$\ev{p\approx 3{,}3\cdot 10^{-10}\ \text{kg}\cdot\text{m/s}}$

Mirror. Reflected light reverses the direction of its own momentum: the change (and hence the impulse transferred) doubles,

$2p\approx 6{,}7\cdot 10^{-10}\ \text{kg}\cdot\text{m/s}$

Collegamenti

Argomenti: Onde elettromagnetiche Concetti: Pressione di radiazione · Fotone