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Comment by jbverschoor | Hacker News Reader

Can someone explain the difference between this and a mirror? A mirror reflects also something like 98%, right?

A mirror reflects with specular reflection. When parallel rays of light hit a flat mirror they remain parallel after being reflected:

https://en.wikipedia.org/wiki/Specular_reflection

Paint reflects primarily with diffuse reflection. The rays of light are scattered in all directions:

https://en.wikipedia.org/wiki/Diffuse_reflection

That's an awesome learning point; thank you.

What percent of the light does a mirror reflect in a specular way?

Like the design of paint, the design of a mirror can affect its reflectivity. For an extreme example:

https://www.semrock.com/maxmirrors.aspx

Mirrors that are a cheaper coating on glass or metal can be in the 95% range without too much effort. If you don't care about actually forming an image with a mirror, it can be quite cheap, e.g., plain aluminum (with whatever oxide forms on its surface) or aluminized mylar film.

I was going to ask this same question, thinking it would be a dumb question, glad this got answered! Thank you

I would expect it's twofold. This paint acts like a "diffuse mirror" so doesn't create any bright spots like specular reflection. This is great particularly in cities where every building would become a giant mirror. On the other hand a mirror finish is harder to achieve on rough surfaces that the paint would presumably be used on.

Adding:

Mirrors don't have to reflect very much, they have to reflect as a picture. That's why one-way mirrors work — people generally don't notice that the light mirrored is much less than 100%. White paint is free to reflect the photons in any direction, so that >98% is a harsher requirement on the share of incoming photons, but a laxer requirement on the photons' direction.

Some mirrors can reflect a lot more light even if in narrower wavelength ranges. But that only makes the matters worse from a practical human perspective, when trying to use them on large surfaces outside. It would feel like staring into the Sun far too often.

Which mirrors are that, and why are they made, what are they used for? Just giant telescopes on remote mountaintops, or is there more?

Lasers! :)

I'm talking about dielectric mirrors. Wikipedia suggests 99.999% or better reflectivity over a narrow range of wavelengths.

Of course. Duh.

Aluminium coating (most mirrors) only reflects between 80% to 90%. It reflects light in single direction. White paper reflects more, but scatters light in several directions.

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