# Determining the number of photons that reach the earth

## Homework Statement

Light is shining perpendicularly on the surface of the earth with an intensity of 910 W/m2. Assuming all the photons in the light have the same wavelength (in vacuum) of 668 nm, determine the number of photons per second per square meter that reach the earth.

## Homework Equations

The power per area is:
P/A = (# of photons /t /A)*(energy / photon)

E/photon = h nu = hc / lambda

photons /t /A = (P/A) * lambda / hc

## The Attempt at a Solution

photons /t /A = (P/A) * lambda / hc
photoms /t /A = (910)*(668x10^-9)/(6.63x10^-34)(3x10^-8) = 3.066x10^21

Redbelly98
Staff Emeritus
Homework Helper
Looks good!

Just a small typo, c is 3x10^+8. Looks like you used the correct value for the calculation though.

The program I enter my homework answers into doesn't let me have that many decimal places in my answer though (we have to write it out long, it doesnt let us put x10^21), so I was assuming I must've done something wrong.. :-/

Redbelly98
Staff Emeritus
Homework Helper
Looks like 3 significant figures are justified here, given the values of intensity and wavelength.

By the way, if you are entering numbers into a computer, you can usually write them as
6.63e-34
3e8​
The "e" is pretty standard notation for computer entry, and simply means "times 10 to the ___ power"

For example: