# how is glass reflective and transparent?

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How is glass both able to be reflective and see through. Obviously it is but my brain can’t understand how the light can both pass through to the eyes of the person on the inside as well as reflect off the material and be seen by someone on the outside. Please help me understand. Thanks 🙂

In: Physics

It reflects some fraction of the light on the surface, absorbs some and lets some pass through. Typically it will let most light pass through, reflect some and absorb very little, but it depends on the glass and its use.

You notice the reflection more when your side is much brighter, e.g. in the evening with the lights on in the room. The glass still lets most of the light from your room pass through, but the small fraction that gets reflected can be more light than the bit of outdoor light that gets in.

Only a little bit is reflected. Same way a nice coat of black paint can be shiny and still black.

When light waves enter glass, the electrons in the glass move as a response. This produces a new wave, which ‘adds’ to the original wave moving forward through the glass and slightly weakens it as the waves are opposite. The new wave also travels back, though, as a reflection, which is what we see bouncing off.

Basically, because only 4% gets reflected, while 96% goes through. But if you adjust the brightness on each side of the glass, you can get different effects.

Say that I’m standing on one side of a piece of window, and a friend is standing on the other. I am standing in a bright room, and my friend is outside in the dark night.

The following will use some arbitrary numbers, just to give a sense of what’s happening. The actual numbers of photons involved would be much higher.

From the bright side of the window, there are 1,000,000 photons hitting the window. That means 40,000 photons get reflected, and 960,000 pass through.

From the dark side of the window, there’s only 10,000 photons hitting the window. This means 400 photons are reflected, and 9,600 pass through.

So when I look at the window, I see the 9,600 photons that passed through from the dark side, and the 40,000 photons that bounced back from my side. That makes the reflection I see 4x brighter than the light that came through the window from the dark side.

For my friend, they are seeing the 960,000 photons that passed through from the bright room, and only 400 photons that reflected back. The light passing through from the bright room is so much brighter than the reflection that the reflection may as well not exist.