A sodium lamp emits light at the power P = 70.0 W and at the wavelength = 597 nm

Question

A sodium lamp emits light at the power P = 70.0 W and at the wavelength = 597 nm, and the emission is uniformly in all directions. (a) At what rate are photons emitted by the lamp? (b) At what distance from the lamp will a totally absorbing screen absorb photons at the rate of 1.00 photon /cm2s? (c) What is the rate per square meter at which photons are intercepted by a screen at a distance of 2.50 m from the lamp?

I got a , but I need b and c ( units like here)

a = 2.1e20 (s^-1)

b= ??? (m)

c= ??? (s^-1*m^-2)

Explanation / Answer

P = 70 W
E = 70*t J
number of photons = n
n*h*c/lambda = 70*t
so n/t = 2.104*10^20 photons per second

Let the distance be r
so 1 = 2.104*10^20/4*pi*r^2
r = 40930.94238 km

rate = 2.104*10^20/4*pi*2.5^2 = 2.68*10^18

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