What is the force exerted by a laser pulse on a target?

[Kara386]

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.

Homework Equations

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!

 

[DrClaude]

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]

By dividing the total momentum by the duration of the pulse?

Yes.

 

[Kara386]

Yes.

Brilliant, thanks! :)