help with a question (1 Viewer)

[vds700]

A 2 watt laser beam is emitting light of wavelength 600 nm. Find the no. of photons emitted per minute.

ANS: 6 x 10¹⁸

I dont remember how to do these power calculations, help appreciated

 

[scotte9291]

c = f landa
f = 5 x 10 ^ 14

E = hf = 2j/s
E = 3.313 x10^-19

# of photons = 2 / 3.313 x10^-19
= 6 x10^18

 

[vds700]

c = f landa
f = 5 x 10 ^ 14

E = hf = 2j/s
E = 3.313 x10^-19

# of photons = 2 / 3.313 x10^-19
= 6 x10^18

sorry im a bit confused, the question asks for the no. of photons per minute. Why did u do 2 j/s /3.313 x10^-19. Or is it 2 joules of energy per minute?

Thanks

 

[Trebla]

The E = hf equation is the energy of EACH photon.
The power is the total energy of the system. So divide the total energy by energy of each photon tells you how many photons there are.

 

[vds700]

The E = hf equation is the energy of EACH photon.
The power is the total energy of the system. So divide the total energy by energy of each photon tells you how many photons there are.

yeah i get that bit, but the 2 watts, is that the energy emitted per second or per minute? It would seem that its minutes, but I thought watts was joules/seecond (SI units). :S

 

[Trebla]

yeah i get that bit, but the 2 watts, is that the energy emitted per second or per minute? It would seem that its minutes, but I thought watts was joules/seecond (SI units). :S

Technically it should be joules per second, but since you want photons per minute the conversion factor is already done.
In other words if you did the thing in joules per second, you end up with number of photons emitted per second and then to get per minute just multiply by 60.

 

[vds700]

Technically it should be joules per second, but since you want photons per minute the conversion factor is already done.
In other words if you did the thing in joules per second, you end up with number of photons emitted per second and then to get per minute just multiply by 60.

Ah i get it.

Thanks