Why Does Light Slow Down in Glass?


I haven't found this explanation online but it seems to make sense so if I'm wrong please explain in the comments:   When a wave travels through an opening larger than it's wavelength it will travel in a straight line however when that aperture is smaller than the wavelength a wave will spread out in all directions and spread out and radiate from a new location (the aperture). Why waves do this I'm not so sure but it's true of all waves not just light. Since a solid's atoms are much closer together than any visible wavelength of light that light will then spread out from all the openings between atoms. It will do this at the speed of light however the distance is now greater because the light most of it is now bending (since it's radiating from many new locations) and it's average direction is roughly the same direction though it's altered slightly. You can see this in a drawing. It's net speed is slowed down and this slowing down pushes the wavelength together actually decreasing the wavelength. When you compress the wavelength the net direction is now different because the net radius is now a parabolic (or elliptical or hyperbolic) not a spherical. The wave front being a parabola the net direction is now different because the net direction is always perpendicular to the wave front. We only see an object clearly if it's wave-front is hitting our lens parallel to our lens. The wave front can then be focused by our lens which then focuses the light using these principles.



When the  net velocity of light slows down the net wavelength also changes however the actual velocity stays the same.
Note that the apparent distance through the glass seems further away.   How is that in a fish tank everything looks closer?   It's because the fish are in the water itself.  Actually I can do that experiment in Inkscape.


 The location of the object is captured because it radiates light in all directions and so it's direction is always perpendicular to the wave front. When it exits the glass however its new net direction is now exactly the same as the original direction but because it passed through the glass the glass changed it's (average) direction for a while the object now appears to be at a different location.  One of the things that makes glass and water so intriguing is that we process light with the lens in our eye thus we only see what is parallel to our lens while what is important is what is perpendicular to the wave-front.  If we had a different means of processing light perhaps we would see things differently.     A holographic plate accepts light from every direction.    I guess though a pinhole camera doesn't have a lens yet it would still work the same way.   Because the pinhole is larger than the wavelength of light the light doesn't spread out from the pinhole.   A pinhole is small enough to create high enough resolution yet large enough that light doesn't spread out in all directions since the pinhole is larger than the wavelength.  A lens does the same thing as a pinhole but changes the wave-front so that it focuses de-radiates the wavefront back to a copy of the object that radiated it in the first place.

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