system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What distinguishes light that reflects on a window and light that travels through?

If you go to a window, you can obviously see out of it and also see your own reflection to an extent. My question is: someone on the other side of the window can also see you in the window. So, if a photon of light is reflected from a point on your body, what decides whether that photon is going reflect on the glass or go through? 

Follow up, no widows have patches of “dark spots” as you can see your entire reflection as well as the entire of what’s on the outside. So is this a completely random selection process as a photon from the same point will sometimes reflect, sometimes not? 

I’m gonna be surprised if I get an answer to this, but I hope I do!
gpt
First, ELI18, then ELI5.

ELI18: When any electromagnetic wave meets a discontinuity in a dielectric medium, part of the wave is reflected and part of the wave is transmitted. The amount reflected and transmitted depends on the dielectric constant (or index of refraction) of the medium and the frequency of the electromagnetic wave (it happens for radio waves as well as visible light waves). It also depends on the angle of the light to the interface, and if you are traveling from a medium with higher index of refraction to lower index, the wave will be totally reflected if the angle if incidence is below the angle of total internal reflection*. It also depends on the polarization of the incoming wave, with the reflection coefficient depending on both the angle and the polarization state.

ELI5:  When light enters a material other than air, some of it goes through, and some of it bounces off. How much goes through and how much bounces off depends on the material, the color of the light, the angle the light hits the material, and from which side you are shining the light.  For glass, if you shine the light straight at the glass, or look at your reflection, about 4% (or 1/25) of the light bounces back, and 96% (24/25) goes through. That amount changes if you tilt the glass. But what if you shined the light from "inside" the glass? In that case you can get 100% to bounce off the inside surface of the glass. For example, think about a fish tank. If you shine a light into the water, part will go through and part will bounce off. If, however, you shine a light through the side the aquarium at a shallow angle and off the surface, you can get all of the light (100%) to bounce off the surface and back out the other side. Similarly, if you shine, for example, a laser pointer into the edge of a sheet of glass so that it skims the edge from the inside, then at a low enough angle 100% of the light bounces back in. That is how optical fibers guide light - by shining the light *inside* the material at a low angle to the sides, the light just keeps bounding along the edges (or surface if the fiber is round) until it comes out the end nearly as bright at it started.

*edited based on zuppenhuppen's comment - thanks!