Sun Glasses

Himanshu

This question was asked today in my physics examination.

Sun Glasses do not have power. Why?

I know that the main function of Sun Glasses is to reduce the intensity of light by polarisation. Therefore there no need for them to have power.

Or is it something else.

russ_watters

Sun glasses absorb sunlight, so they do dissipate heat.

Himanshu

So then how is heat dissipation related to the power of Sun Glasses.

Doc Al

What do you mean by "power"?

TVP45

Some sun glasses are polarized and this is done to reduce the intensity of reflected light, primarily from surfaces such as snow or water.

All sunglasses absorb some degree of light. This is especially important in the UV range. Generally the darkness of the lenses indicate this, although the color also affects this.

Sunglasses may also have magnification. This may result from a prescription or simply from a slight magnification that is added in for old eyes (as in my welding goggles).

There are also electrochromic glasses that darken when a voltage is applied, usually as the result of a light sensor signal.

Exactly what kind of power are you referring to?

mgb_phys

I think the OP means optical ,ie refractive, "power"
Where optical power is the inverse focal length, usually in dioptres.

Sunglasses don't have power because they don't, ideally, change the image seen by the eye.

D H

He is referring to corrective power, and of course off-the-shelf sun glasses are plano. They have to be, as most people do not need their vision corrected. My prescription sunglasses do have a non-zero corrective power.

stewartcs

That was my initial thought as well, but I'm not sure what the OP intended.

TVP45

But they do. Granted, the $2 ones from Wal-Mart have no optical power, but reading sunglasses, including bifocals, are widely sold starting at less than $20.

ZapperZ

Just because they use the same word, does not mean they have the same meaning. Do not fall into this trap.

The "power" as used in reading glasses have more to do with the focal length or how much focusing distance. Read mgb_phys post. This is not the same "power" as defined in mechanics.

Zz.

TVP45

Zz,

I miss your point. If, as mgb_phys said, the OP intended optical power (or diopter), I agree that has nothing to do with power as defined in mechanics. I was questioning the original question which appeared to start with a false premise that sun glasses have no optical power. I own sunglasses with optical power (by prescription), and many optical stores, and others such as birding stores, sell bifocal sunglasses with plano uppers and lowers having (typically) 1.25, 1.50, 1.75, 2.00, ...

mgb_phys

Wow - it seemed like such a simple question.
I think the answer the examiner was looking for is that - for people with normal eyesight the sunglasses shouldn't have any optical effect, other than to darken things!
To acheive this pefectly, the glasses would have to be a parallel spherical meniscus centred on each eyeball with a radius of curvature equal to the distance to the centre of the eyeball.

ZapperZ

Oh, sorry. I read it differently.

Zz.

russ_watters

Being an engineer, when I saw "power", I immediately thought the thermodynamic kind...

TVP45

Now, that is an interesting question. Perhaps he meant that? I think sunglasses might not be an issue there, but large tinted windows are said to sometimes build up heat (ancedotal and may also be the result of a goofy mixture with the metallic films).

A simple, back-of-the-envelope calculation says a pair of Elton John sunglasses on Miami Beach in July absorbs maybe 1W. That's not enough to worry about, so maybe that is the question?? If so, it's very clever; the actual numbers would require a lot of detail. hmm?

Danger

I took it to mean why they weren't powered by an external source. In that light, as TVP45 pointed out, some are. I'm only aware of welding helmets in that category, but it could certainly be applied to sunglasses if someone wanted to.