Part A The intensity of sunlight hitting the surface of the earth on a cloudy da
ID: 2103384 • Letter: P
Question
Part A
The intensity of sunlight hitting the surface of the earth on a cloudy day is about 0.50 kW/m^2 Assuming your pupil can close down to a diameter of 2.0 mm and that the average wavelength of visible light is 550 nm, how many photons per second of visible light enter your eye if you look up at the sky on a cloudy day?
N=?
Part B
A red LED (light emitting diode) is connected to a battery; it carries a current. As electrons move through the diode, they jump between states, emitting photons in the process. Assume that each electron that travels through the diode causes the emission of a single 630 nm photon.
What current is necessary to produce 7.4 mW of emitted light?
Explanation / Answer
energy of each photon = hc/lambda = 3.57x10^-19
energy entering the eye = 500 x pi x r^2 = 0.00157 J/s
number of photons = 4.39 x 10^15
energy of each photon = 3.11 x 10^-19
no of photons req = 2.37 x 10^16
no of e = 2.37 x 10^16
current = 4.5 mA