Question

An owl has good night vision because its eyes can detect a light intensity as small as 5.0 × 10-13 W/m2. What is the minimum number of photons per second that an owl eye can detect if its pupil has a diameter of 8.8 mm and the light has a wavelength of 572 nm?

Answer 1

Given the diameter we can find the area, A=d²/4 = 3.14*(8.8*10^-3)²/ 4

A= 6.08*10^-5 m²

hence power, P= 5*10^-13*6.08*10^-5 = 3.04*10^-17 W

Now, We know that Power is Energy per unit time,

P=E/t

also, e= hc/ {for one photon}

Let there be N photons, then'

E =Ne = Nhc/

hence,

P = Nhc/(t) = nhc/

where, n= N/t , and is the number of photons per second,

n = P/(hc)

n = 3.04*10^-17*572*10^-9/(6.67*10^-34*3*10⁸)

n=86.9