Why is a smooth clean piece of glass transparent but not invisible?
What is the difference between transparent and invisible?
Transparent objects allow light to pass through neither absorbing nor reflecting nor scattering it. Invisible objects are not perceivable by vision.
There is always some scattering, particularly from the edges. Indeed, if there is no scattering, absorption or reflection, the object will indeed be invisible. That is, however, the ideal case.
So the fact that glass scatters light makes it transparent but not invisible?
So what other optical properties or effects might a transparent object have that would be percievable by vision? Can you think of any other quantity that is often used to characterize transparent media?
Invisibility means electromagnetic waves are bent around the object. For some frequencies and small objects they've already been able to build cloaks. In transparent objects the waves just go through.
I thought about it but I can't come up with anything else unfortunately.
Refraction. It is the thing that makes lenses work and makes straight sticks look bent when they go from water to air.
But the medium of transmission is a smooth piece of glass.
Yes. Do you know what refraction is? You may want to read about it.
As DaleSpam points out, refraction makes even transparent objects "visible" because you can see the "bending" of light as it passes through the object. It is also true that the edges of any object, glass or whatever, are rough enough to scatter light, also making the object "visible"
But refraction occurs only when the light passes the boundary between media such as air and glass at an angle. It doesn't happen when the light hits the boundary in a direction perpendicular to the medium.
Isn't this relevant?
I wasn't sure at first because English is not my first language but I brushed up on my knowledge of it. However, as I replied to Chandra Prayaga refraction occurs only when the light passes the boundary between media such as air and glass at an angle. It doesn't happen when the light hits the boundary in a direction perpendicular to the medium as far as I know.
Isn't this also of relevance when answering the question?
Your eyes are not limited to viewing surfaces at right angles. Particularly with binocular vision, it would be unusual to find a transparent object positioned such that all of its surfaces were perpendicular to your vision.
Even if you are looking at a smooth, transparent surface at right angles, a surface that has a refractive index different from air will cause reflection. It will look shiny.
The youtube video above, is a demonstration of why the refractive index is important here, not just the transmission.
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