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# photons and intensity watch

1. Hi, am new here and I need some help. I don't know how to do this question, can some please explain it to me? Its part (c) that i don't get:

A 60W filament lamp hangs from the ceiling in a room. Assume that 5% of the electrical energy is converted into visible radiation.
(a) Calculate an approx. value for the intensity in Wm-² of the visible radiation on a table which is 1.5m below the lamp. State any assumption you made.
(b) the average wavelength of the visible radiation is 550nm. What is the average energy of each photon?
(c) At what rate are photons striking an A4 sheet of paper which is placed on the table?

Ans:
(a)0.10Wm-²
(b) 3.6 x 10-9J
(c)1.8 x 10^16 s-1

Can someone please explain how you do part (c)?
2. (Original post by diesirae)
Hi, am new here and I need some help. I don't know how to do this question, can some please explain it to me? Its part (c) that i don't get:

A 60W filament lamp hangs from the ceiling in a room. Assume that 5% of the electrical energy is converted into visible radiation.
(a) Calculate an approx. value for the intensity in Wm-² of the visible radiation on a table which is 1.5m below the lamp. State any assumption you made.
(b) the average wavelength of the visible radiation is 550nm. What is the average energy of each photon?
(c) At what rate are photons striking an A4 sheet of paper which is placed on the table?

Ans:
(a)0.10Wm-²
(b) 3.6 x 10-9J
(c)1.8 x 10^16 s-1

Can someone please explain how you do part (c)?
Its assuming you know the area of an A4 sheet of paper. In which case you can find the energy per second by multiplying by the intensity. Then divide that energy by the energy of 1 photon and that is the number hitting the paper per second
3. aaaahhhhh....i feel enlightened thanks a lot
4. (Original post by diesirae)
Hi, am new here and I need some help. I don't know how to do this question, can some please explain it to me? Its part (c) that i don't get:

A 60W filament lamp hangs from the ceiling in a room. Assume that 5% of the electrical energy is converted into visible radiation.
(a) Calculate an approx. value for the intensity in Wm-² of the visible radiation on a table which is 1.5m below the lamp. State any assumption you made.
(b) the average wavelength of the visible radiation is 550nm. What is the average energy of each photon?
(c) At what rate are photons striking an A4 sheet of paper which is placed on the table?

Ans:
(a)0.10Wm-²
(b) 3.6 x 10-9J
(c)1.8 x 10^16 s-1

Can someone please explain how you do part (c)?
im gettin 3.6 x 10-19J
5. (Original post by mackin boi)
im gettin 3.6 x 10-19J
that must be right. the answer he has is huge so he must have missed out a 1.
6. (Original post by mackin boi)
im gettin 3.6 x 10-19J
i'm not getting that - houw'd u do it?

dont you have to use E=hf?
7. u gotta use E= hc/ (lamda) cos u dont kno frequency.
8. thats the same as using v=f(lamda) to find frequency and then e=hf, i got 3.6*10^-19
9. (Original post by James K)
thats the same as using v=f(lamda) to find frequency and then e=hf, i got 3.6*10^-19
i must have been day dreaming - i invented my own eqtn

λ=1/f ; too bad it does not exist

pk

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