Why are gold mirrors yellow?

Why are golden mirrors yellow?

Do they add a yellow component to the spectrum or absorb non-yellow components?

If they absorb, then why are they used in telescopes being imperfect?

If they add a yellow component, then where do they take energy for it from?

JWST mirrors are coated with gold

Do they add some corrections in the on-board computer to compensate for the color of gold?

• Since the Webb telescope operates in the IR, the color in the visible is pretty irrelevant... Aug 17 '15 at 16:19
• On the color of metals: physics.stackexchange.com/q/72368 Aug 17 '15 at 16:23
• Just as a fun fact - visible light is actually a pretty crap approximation to other forms of light. Like you may think "close enough" and hope but wavelengths matter, for example infrared lenses are opaque, they're made out of ... uhh... a metal? Lens: i.imgur.com/ypYhXdb.jpg radiowaves are weirder still! i.imgur.com/rfVfT42.jpg (this is 22.5cm tall for reference) Aug 17 '15 at 23:34
• @Johannes that's something I addressed in my (first!) answer here. Aug 18 '15 at 2:00
• I'm surprised this question isn't merely: "Why is gold yellow?" since most gold things are yellow (plus metallic lustre). The answer is, of course, relativity. physics.stackexchange.com/questions/72368/… Aug 18 '15 at 19:31

I've made this into an answer because it's too long for a comment, and I really want to show the pictures.

It is tempting to think of visible light as "close enough" to (near by wavelengths) and to conclude that "yes, actually, the yellow does affect it. I want a mirror without an obvious tint"

However you are wrong, Physics will slap you down.

Exhibit A

(There's a book called Optics by Eugine Hecht that has a picture of some such lenses, but I couldn't find that picture. This is the best I found with quite a bit of googling)

This lens is made from "silicon or germanium" according to this link (where I found the picture) - I could have swore it was steel, anyway!

This lens is completely opaque to us, but to actual infrared cameras (the 1-$\mu$ sort of wavelengths) this is probably a good lens!

Let us go further.

Exhibit B

For reference this is 22.5cm tall

This is a lens for radiowaves. As you can just about see, it doesn't even "refract" our visible light, that light just goes right through the MASSIVE GAPS in the mesh. However to radiowaves, this is a lens!

Exhibit C

Finally, we come to visible light, I took this picture (bit of shameless "I'm proud of this", I know):

As you can probably tell from the soft background and such it was taken with a lens with a very large aperture (50mm f/1.4), and as you can (hopefully not) see the different wavelengths of light have actually gone different paths through the lens. Notice:

As you can see, "glass" isn't even very good at treating the visible wavelengths the same way. A picture is worth a thousand words.

I used a good lens, which is why the effect is so small, but this shows the principle behind it.

Exhibit D

This is a microwave door - it is opaque to microwaves, but as you can see, lets visible light through. (See Faraday Cage)

Exhibit E

WiFi. It can pass through walls and doors.

It should be clear now that light doesn't exactly behave like what our brain calls "light"

Finally

I hope this helps. As you can see - WAAAY to long for a comment.

• Great synopsis of the many varied interactions between electromagnetic radiation and matter. A bit off the original topic, but still. Incidentally, chromatic aberration on modern digital cameras is often mitigated with a lookup/scaling: the pixel maps for R,G and B are magnified by different factors and turned into the composite. For the price of half a pixel's worth of blurring, the aberration is almost completely eliminated. Aug 18 '15 at 12:33
• @Rafael I had hoped to find a picture of some of the lenses used for the massive radiowave telescopes, as they're literally just poles of different lengths! Unfortunately my Google-fu is weak. I am really glad that you and 5 other people at least found this fun. I was hoping to go for "cool related facts" Aug 18 '15 at 13:11
• I liked it enough that I leafed through my Hecht and Zajak to see if I could find the image you remembered. But it does not exist in the 1980 printing. I know, I am dating myself... Aug 18 '15 at 15:29
• @Floris just for reference, I read the 4th edition book (2001 edition) - it was a library copy because there's no way I could afford that! It is (sadly) a very good book though. I'd love an alternative if anyone has any books in mind! Also thanks. I think I'll like this SE site. Aug 18 '15 at 15:49
• Most confusing but "oh wow!" moment of college was when my chem professor taught me that radio waves are light. Then I realized you can shine that light through a house. Mind blown. Mar 4 '16 at 5:45

If you look at the reflectivity of gold (vs silver or aluminum) you can see a plateau at wavelengths below 500 nm source:

If blue wavelengths are not reflected as well as other colors, the resulting image will look "more yellow" - which is what you see.

At longer wavelengths, gold is a very good reflector (better than the other two above 600 nm). It also doesn't tarnish, so its reflectivity is less affected by atmospheric contamination.

If you need anything approaching accurate measurement, you have to calibrate your system at any rate - beside the mirrors and lenses, you need to consider the response of the detector, effects of the atmosphere, and pretty much everything in (or near) your optical path. Serious photometry needs serious calibration, as Chris White pointed out in the comment.