Glass as Mirror: Reflection of Light | Online Community Discussion

[psgill93]

I was posed this question by a professor, and thought I would share with the online community! He asks, "if one were to use simple glass, how could they use it as a mirror, where 100% of the light is reflected?" The glass is transparent, but his hint was to look at a window! Whats your thoughts?

 

[jtbell]

Did he set any restrictions on whether the light has to come from outside or inside the glass?

 

[psgill93]

There were no restrictions in this aspect! I have a feeling he was intending natural light though!

 

[Integral]

From a well lit room look through a window into darkness. What do you see?

 

[psgill93]

That is correct however, he wants 100% reflection of the light just as a mirror would reflect! I'm afraid, the well lit setting wouldn't produce those results!

 

[Born2bwire]

Coat one side with silver.

 

[physiqueper4]

what about using silver on one side?

 

[jtbell]

Investigate how the prisms in binoculars work. They serve as mirrors to "fold" the light path so as to make the binoculars shorter. The reflecting surfaces are not silvered.

 

[psgill93]

We cannot use Silver lining guys, cause that would be the process of making a mirror! The task is to take a simple sheet of glass and make it reflect 100% light!

 

[Antiphon]

Tilt the window to a steep grazing angle. The fresnel reflection coefficients will approach one.

 

[peter.ell]

While Antiphon's answer is correct, here's a slightly more understandable explanation:

Since glass is a dielectric material, the amount of light that it reflects actually changes depending on the angle of the light striking the glass, unlike metals where the amount of light reflection is constant no matter what angle light strikes the surface. But this aspect of glass means that it can actually achieve perfect 100% reflection, whereas no mirror made with metal can achieve that because there is always some level of absorption (this is why most metals have a sort of greyish look to them).

So how do you get glass to act as a perfect mirror? It's called total internal reflection, and it's how fiber optics work. Look it up, it's got some pretty neat consequences and uses. Basically, when light exits glass into a medium like air with a lower index of refraction, there is a certain angle, called the critical angle, where the angle of the refracted light bends so far back toward the glass that it actually never even enters the air and instead just refracts right back into the glass. This is total internal reflection. Since it's 100%, fiber optics can send information encoded in light for long distances without any appreciable attenuation.

Here's an image to give an idea of what's going on: https://chemicalparadigms.wikispaces.com/file/view/tir.JPG/33653365/tir.JPG

The above image might lead you to believe that the light source must be inside the glass, but it actually doesn't have to, it just has to pass through the glass so that the angle it makes with the glass to air interface is greater than the critical angle.

 

[psgill93]

Hey thanks guys for all the help! I'm going to propose some of these ideas forward but credit u as well! Thanks for the brainstorming everyone and the simplification peter.ell! :)