# Force exerted by a laser

## Homework Statement

A target is hit by a laser pulse. The frequency of the pulse is in the visible spectrum, it has duration ##5\times 10^{−15}## s and it contains approximately ##10^{19}## photons. Estimate the force exerted by the pulse on the target, given the pulse is completely absorbed by the target.

## The Attempt at a Solution

I think a reasonable wavelength in the visible spectrum would be ##550##nm. I can calculate the total energy of the photons that hit the target by using ##E = \frac{hc}{\lambda}## and then multiplying by ##10^{19}##. I get ##3.6\times 10^{-19}##J, and don't know where to go from there.

Alternatively I suppose I could use force = change in momentum / change in time. Then I'd use ##p = \hbar k## where ##k = \frac{2\pi}{\lambda}##. If that's the correct approach, to get the total change in momentum I just multiply the momentum expression by ##10^{19}##, right?

Thanks for any help!

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DrClaude
Mentor
That will give you the total momentum. You'll then have to convert it to a force.

• Kara386
That will give you the total momentum. You'll then have to convert it to a force.
By dividing the total momentum by the duration of the pulse?

haruspex
Homework Helper
Gold Member
By dividing the total momentum by the duration of the pulse?
Yes.

• Kara386
Yes.
Brilliant, thanks! :)