The speed of light slows in various media. It is not a constant, but has a maximum value in a vaccum. The slowing in glass is part of the physics of refraction.
No, no, no..... Del.... light stops at night because of the abundance of dark matter that fills our universe...
Light itself doesn't travel, it really is this dark matter that travels.
Consider a light bulb, most people think it emits light whereas in reality it absorbs the dark matter making it appear light, this process consumes electricity, when the light bulb is switched off the dark matter resumes filling the void...
Remember dark matter abhors a vacuum.
John
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A little knowledge is a dangerous thing - Googling is far worse!
Now that's thinking outside the containment vessel.
I'm actually working on a process to bag dark matter. You'll "unburn" it in a proton pellet stove. It sucks the cold right out of your home. My only problem is disposal of all those used protons, they pile up after a while. I'm thinking you can spread them in your garden, reaserch continues.
Send money if you want a piece of this.
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I go into every human encounter expecting to be framed for a crime I didn't commit. Dilbert, 2013