The human eye can barely detect a star whose intensity at the earth's surface is 1.6×10−11W/m2. physics help?

May 2021 3 132 Report

The human eye can barely detect a star whose intensity at the earth's surface is 1.6×10−11W/m2. physics help?

If the dark adapted eye has a pupil diameter of 6.2 mm , how many photons per second enter the eye from the star? Assume the starlight has a wavelength of 550 nm

Science & Mathematics / Physics

oldprof

So the energy E = 1.6E-11 Joules striking the retina and each photon has an average e = hc/L = 1240/550 = 2.25 MeV of energy. So N = E/e = 99863312//2.25E6 = 44 photons. ANS.

NOTE: E = = 99863312 eV is the Joules equivalent. Also note, we don't need all that about the lens radius etc because that energy is assumed at the retina, already passed on to the back of the eyeball. Finally note hc = 1240 MeV.nm is a useful number to remember in this context.

Randy P

You have 1.6 x 10^-11 Watts per square meter.

First, figure out how many Watts are hitting the pupil: it's a circle of diameter 6.2 mm (so radius is half of that), adn the area of a circle is pi*r^2.

Multiply 1.6 x 10^-11 Watts per square meter by the number of square meters in the circle to get how many Watts are hitting the eye.

A Watt is a Joule per second, so that tells you how many Joules of energy are entering the eye each second. Divide by the number of Joules in a photon to find out how many photons.

What is the formula for energy of a photon, in Joules? It's in your textbook.