- 795

- 0

Hey, I'm having a few problems with the questions below, now I think alot of it is to do with not knowing how to approach the question. Please forgive me if there is not alot of working out, but I may just need to directing in the right direction.

The average wavelength of light emitted from an incandescent torch bulb with a metal filament is 120nm. Calculate the number of photons emitted by a 20W torch bulb in one hour.

Photon energy is proportional to the frequency of the wave.

[tex]v=f\lambda[/tex]

[tex]3\times10^8 = f\times 120nm[/tex]

[tex]\frac{3\times10^8}{120\times10^{-9} = f[/tex]

[tex]f=2.5\times10^{15}[/tex]

I have the frequency now, but how do I get from here to finding how much is emitted by a 20W torch bulb in 1 hour?

A photon has a momentum given E/c where E is the enerrgy of the photon and c is the speed of light. If the torch bulb emits parallel beam light, then calculate the force on the torch.

I have no idea, at all. I am not asking for the answer, but could someone please direct me in the direction of a method of some sort, even if it is only the intial stages.

Calculate the initial acceleration of the toch if it was in empty space, and it had a mass of 200g.

Again, not idea. I know I have the mass, but that is the only value I have. It could be possible that I need values from previous questions above.

I apologise for th elack of working, but the whole thing has me stumped. I know it is against PF regulations to just dish out the answers, but I am willing to work through it, all I need is a gentle push in the right direction!

Any help is much appreciated.

_Mayday_

**Question 1**The average wavelength of light emitted from an incandescent torch bulb with a metal filament is 120nm. Calculate the number of photons emitted by a 20W torch bulb in one hour.

**Answer 1**Photon energy is proportional to the frequency of the wave.

[tex]v=f\lambda[/tex]

[tex]3\times10^8 = f\times 120nm[/tex]

[tex]\frac{3\times10^8}{120\times10^{-9} = f[/tex]

[tex]f=2.5\times10^{15}[/tex]

I have the frequency now, but how do I get from here to finding how much is emitted by a 20W torch bulb in 1 hour?

**Question 2**A photon has a momentum given E/c where E is the enerrgy of the photon and c is the speed of light. If the torch bulb emits parallel beam light, then calculate the force on the torch.

**Answer 2**I have no idea, at all. I am not asking for the answer, but could someone please direct me in the direction of a method of some sort, even if it is only the intial stages.

**Question 3**Calculate the initial acceleration of the toch if it was in empty space, and it had a mass of 200g.

**Answer 3**Again, not idea. I know I have the mass, but that is the only value I have. It could be possible that I need values from previous questions above.

I apologise for th elack of working, but the whole thing has me stumped. I know it is against PF regulations to just dish out the answers, but I am willing to work through it, all I need is a gentle push in the right direction!

Any help is much appreciated.

_Mayday_

Last edited: