how many photons lead to an observable flash?

Question

The human eye can respond to as little as 10-18 J of light energy. For a wavelength of 400 nm, how many photons lead to an observable flash?

I have no idea what to do.. >_<

Answer 1

You might start by not necessarily believing everything given to you as fact in a homework assignment. See the reference and others like it. The eye itself can detect a single photon, and it doesn't take many for the brain to perceive it. Sure, do the homework problem as assigned, but add a notation (with references) that the problem statement might be incorrect.

Answer 2

the energy of a photon is based on its frequency.

freq = c/wavelength

where c is the speed of light.

freq = (3e8)/(400e-9) = 7.5e14 Hz

E = hf = (6.626e-34)(7.5e14) = 4.97e-19 J

so if you're saying 10J is the smallest amount the eye can respond to then.

10/4.97e-19 = 2e19 photons

Answer 3

Let Ep = energy per photon.
λ = 400nm
Ep = hν = hc/λ

I leave the result for you to calculate.

Use the minimum, 10J.
Sorry, no subscripts, but
Let Et = total energy = 10J

Et = n x Ep
n = 10J / Ep