Sunglasses which reflects like a mirror

1. Oct 10, 2003

Pandemonium

I hope this is the correct place to put this...

There are certain types of sunglasses which reflects like a mirror on one side, and normal shades on the other. I understand that this is due to a coating of some mercury salt on the lens, but what is the exact physical explanation of this phenomenon?

Last edited by a moderator: Feb 6, 2013
2. Oct 10, 2003

ahrkron

Staff Emeritus
When you put a translucent coating on a surface, you have set two reflecting surfaces together: light can bounce from the original surface and also from the surface of the coating.

Think of what happens at the surface of the coating: light bouncing from it is now accompanied by light coming back after bouncing off the glass.

Now you need to remember that light behaves like a wave, and notice that, if these two outgoing beams (the one reflected here, and the one coming back from reflection on the original, deeper, surface) are in phase (valleys with valleys), then they add up. On the other hand, if they are in opposite phases (highs on lows), they will cancel, hence producing no reflected beam.

So, all you have to do is get the coating to be just right (in terms of refraction index and thickness) to produce the desired effect.

3. Oct 10, 2003

krab

The explanation is simple. A reflective coating is applied to one side, but the coating is so thin that some light is still transmitted. I've done this myself when I was a grad student: i evaporated gold onto the outer surface of my prescription glasses.

How thin must the coating be? Remember that metals conduct electricity. Light waves consist of oscillating electric and magnetic fields. The metal effectively shorts out the electric field, but if the frequency is high enough, the electrons cannot move fast enough. This is expressed in a quantity known as skin depth - the depth of penetration of the light waves into a conducter. Skin depth, and hence the required thickness of metallic coating for sunglasses, is a few nanometres for light frequencies in the visible range.

4. Jan 15, 2010

wazinton

Re: Sunglasses

Hi...
Great explanations by you.
Can you tell how much thickness is to be considered for the coating, so that the coating doesn't conduct the electric field..
Thank you...

oakley fives sunglasses