Chemist is misleading that graphene can't be seen.
Every atom (and molecule) absorbs and emits photons whose energies correspond to the energy gaps between available electron states. It can be the case that none of the frequencies which are absorbed or emitted are within the visible spectrum of human eyes. When this is the case, the object is transparent. However, even transparent objects can be "seen" when they have a different index of refraction than their surroundings.This one-atom-thick crystal can be seen with the naked eye because it absorbs approximately 2.3% of white light.
-Wikipedia "graphene"
Index of Refraction is the ratio of the speed of light in vacuum divided by speed of light within a material.
IoR = c_vacuum/c_material
Note that the speed of light in vacuum is the fastest possible speed in the universe. This means the denominator is always less than or equal to the numerator, so the index of refraction is always a number greater than or equal to 1.
IoR for air is ~1.0003
Ior for glass is ~1.5 (with a lot of variety in glass types)
Whenever a photon passes through from one IoR to another, it does a couple of things - reflection and refraction. Usually, it does them both, partially... meaning, based on probability, some otherwise identical photons will reflect and some will refract.
This is why sometimes when you look at a window, you see 2 images... one reflected image, and one that passed through the glass without reflecting from the other side. If the sides of the glass are not parallel, then you have a lens, in which the refraction effect is accentuated and a prism can produce a spectrum from a white beam of light.
Note that "normal" windows have parallel planes and they do also refract the light. The refraction is just less noticeable, because it results in a very subtle shift of the image to one side (depending on the angle which you view the window). Simplified: The light bends when it crosses into the window, and unbends in the same way when it crosses out of the window. The crossing in to the window resulted in a slight change in its direction of travel which was changed equally in the opposite way when it left the window. So objects are not distorted in size and shape, but the image as a whole is shifted slightly. The shift is more noticeable with thicker glass.
So even perfect glass, which is absolutely transparent in the visible spectrum, can still be seen under certain circumstances... namely, some ability to detect reflection and/or refraction.
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