[iacopo.russo (OP)]

Rajesh wrote to us to ask:

Why do a High Index material and a Low Index material need to be used alternatively in high-performance optical mirrors?

What do you think?

[Origin]

Why do a High Index material and a Low Index material need to be used alternatively in high-performance optical mirrors?

The refractive index of the glass used in telescope mirrors is not a concern at all since the reflection is off of the surface of the mirror.  The important property of the mirror glass is that it has a low coefficient of thermal expansion (CTE).  The lower the CTE the lower the distortion due to temperature differences in the mirror.

[Bored chemist]

What do you think?

I think it's a homework question.

[evan_au]

Some of the highest performing mirrors are used in LIGO, the gravitational wave observatory. These use the technique of many layers of high/low refractive index.

It is claimed that these mirrors absorb something like 1 photon in 3.5 million. This is much better than the aluminium + aluminium oxide coating on astronomical mirrors, which only reflect about 90% of photons (and there are typically 2-3 mirrors in a large optical telescope, so the overall efficiency is lower again).

However, when you look at the LIGO mirrors, they are completely transparent. This is because LIGO uses an infra-red laser, so the mirror is reflective at only a very narrow range of wavelengths in the infra-red.
See: https://www.ligo.caltech.edu/page/optics