about how many of photons are in one ray of our sunshine? Where do photons go after they have reflected off their surfaces?
In: Planetary Science
A “ray” isn’t a defined unit of light. If you want to count photons you need an aperture of a defined size, and a defined time. You also need to decide if infrared light counts, as about half of the solar energy that reaches the surface of the Earth is in the infrared. Photons are either absorbed and converted into heat, or reflected and fly off somewhere else.
It is impossible to answer because a ray of sunlight is not a well-defined unit. It is the sunlight that passes through a gap when what is around it is blocked. It is often through clouds but can be through any other coverage like leaves of a tree or a hole in a wall. The difference in area is enormous. Then the question is how long it is.
What you can calculate is the number of photons per cross-sectional area of sunlight per second. That will depend on the distance to the sun and if something blocks part of the light. Let’s pick at the earth’s surface because that is where we humans mostly are.
On Earth at the surface it is approximately 1370W/m^2, it varies around +-3% because the orbit is not circular. It alos depends on how high up in the sky the sun is, the lower the sun the more is scattered before it reaches you. The area should be measured perpendicular to the light.
Then there’s the question of energy per photon, which depends on wavelength. Look at [https://sunwindsolar.com/wp-content/uploads/2013/06/SunWind-WEB-Solar-Radiation-Spectrum-V2.png](https://sunwindsolar.com/wp-content/uploads/2013/06/SunWind-WEB-Solar-Radiation-Spectrum-V2.png) for spectral power distribution of sunlight. I will pick red light at 780nm= 0.78 µm. I chose a single wavelength because it is simpler that way. I pick that wavelength because is close to the middle in regards to power, 53% is IR, 42% visible light and 5% is UV. There is more wavlenght diffren on the right then to the left in the image so the photons with avrage wavelenth is likly in the IR specrum. That would increase the number of photons.
The energy of a photon is in electron volts E=1.2398/wavelength in µm. So we have have 1.2398/0.78 ~=1.6 eV
1 eV( Electon volt) = 1.6 *10^-19 joules. 1 W = 1joule /second so the flow is 1370/(1.6*1.6*10^-19) = 5.35*10^21 photons/( m^2 s) lets just call it 5*10^21
That means around 5 thousand billion billion photons per square meter of sunlight each second on the Earth’s surface.
Latest Answers