Partial Mirrors?

[crackmonkey]

Lasers use a lasing source with a mirror on one end, and a partial mirror on the other. How can a mirror reflect some photons yet transmit other identical photons? The photons are coherent, remember... is this some sort of quantum tunneling at work, or something else?

[a_unique_person]

Originally posted by crackmonkey
Lasers use a lasing source with a mirror on one end, and a partial mirror on the other. How can a mirror reflect some photons yet transmit other identical photons? The photons are coherent, remember... is this some sort of quantum tunneling at work, or something else?

Tunneling is the best way to get under the wire. Either that, or hide in the garbage truck.

[OdderMensch]

I thought the mirror at the other end only let photons go thru at a certain angle?

[Jethro]

Look out your window at night. A partial mirror!

It is a property of waves that when a wave goes from one medium to another part of the wave is transmitted and part of the wave is reflected. This is regardless of any coherence or frequency or whatever. There's no quantum weirdness here.

Tunneling is when a wave is able to pass through a barrier that it "should not have been able to pass through." Strictly speaking tunneling is not quantum weirdness unless something like an electron does it. Otherwise it's just a certain behavior of waves.

[repcon]

How can a mirror reflect some photons yet transmit other identical photons?

Try looking through a small mirror at a bright light or the sun (be carefull), you should be able to see some light getting through, the metal film isn't usually thick enought to be completely opaque.

The mirrors in a laser are dielectric (nonmetalic) and are designed to allow only a small amount of light to pass through, typically <1% on a HeNe laser (output end), and are only reflactive at a very narrow wavelength.

I thought the mirror at the other end only let photons go thru at a certain angle?

In a laser tube (external mirrors), the ends of the tube may have windows at Brewsters angle, this only allows light with a certain polorization to pass straight through, the other plane(s) being reflected away from the mirrors, resulting in a polorized laser beam.

Regards,
Chris