# Force exerted by a laser

1. Apr 19, 2017

### Kara386

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

3. 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!

2. Apr 19, 2017

### Staff: Mentor

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

3. Apr 19, 2017

### Kara386

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

4. Apr 19, 2017

### haruspex

Yes.

5. Apr 20, 2017

### Kara386

Brilliant, thanks! :)