Polarisation

[Corey Nicholls]

I was having a talk to an optometrist the other day about polarising filters and we got into the topic of which direction the filter actually runs in the lens.

Now, correct me if I am wrong, but I was taught that even though the axial marks on the side of a polarising filter before you cut it run at 180 degrees, the filter itself runs at 90 degrees thereby cutting out the plane reflected light at 180 degrees (eg like in one of those "Nu-Polar" polarising kits that has the fluro tube sitting above a flat piece of glass, and by putting the filter infront of your eyes you take out the reflected light travelling in the 180).

The Optometrist seemed to think that for some reason, the filter was set at 45 degrees. The problem I see with this theory, is that you could very easily have one set at 45, and the other set at 135, while still cutting the lens with the axial marks at 180 degrees.

So after all that, if anyone can understand my ramblings, am I correct?

[Darryl Meister]

Hi Corey,

Yes, the filter is oriented so that it absorbs light vibrating in the horizontal (180) plane. At a certain angle (Brewster's angle), light reflected off a horizontal surface becomes completely plane-polarized in the horizontal direction.

Best regards,
Darryl

[Corey Nicholls]

Darryl,

Can you explain Brewster's angle to me? If you can do it in the most basic of ways so I can pass it onto the Optometrist!

Thanks.

[Darryl Meister]

Hi Corey,

When light is reflected off a surface, such as water, the amount of light reflected depends upon both the angle of incidence and the particular plane that the light is vibrating in. Meaning, light vibrating in the horizontal direction/plane is reflected by a different amount than light vibrating in the vertical direction/plane for most angles. The greater the angle of incidence, the greater the amount of light reflected. If the angle of incidence is 0 deg (i.e., like your line of sight is normal to the surface), the amount of light reflected r for either the horizontal or vertical plane depends upon a simple relationship given by Snell:

r = [(n' - n)/(n' + n)]^2 = [(n - 1)/(n + 1)]^2 in air

For water, the amount of light reflected at an angle of 0 deg is around 2%. For hard resin spectacle lenses, it's around 4%.

If the angle of incidence is 90 deg, 100% of the light is reflected in both the horizontal and vertical planes. In between these two angles, however, light vibrating in the vertical plane is generally reflected less than light vibrating in the horizontal plane. And, at Brewster's angle, no light vibrating in the vertical plane is reflected for non-absorbing materials -- just light vibrating in the horizontal plane. Brewster's angle b is given by:

b = atan(n'/n) = atan(n') in air

For water, Brewster's angle is about 53 deg. At this angle, water reflects no light vibrating in the vertical plane and roughly 10% of the light vibrating in the horizontal plane. (Keep in mind that, since the light began vibrating equally in both the horizontal and vertical planes, 10% of the light in the horizontal plane represents = 10% of 50% = 5% of the overall quantity of light.)

Best regards,
Darryl

[David Wilson]

Hi Darryl and Corey,
I've been away for a few days and, as usual, missed out on an interesting post while I was away. A great explanation as usual, Darryl. I think, though, that you are giving Snell credit that is actually due to Fresnel. The formula you quote is known to me as the Fresnel reflection factor.
There is some justice in Snell getting credit for something a Fenchman has done, since, in France, Snell's law is known as Descartes' Law.
Corey, it's great that you have conversations like these with your optom (it's also nice to have a few wins).
Regards
David

[Darryl Meister]

Originally posted by David Wilson
I think, though, that you are giving Snell credit that is actually due to Fresnel. The formula you quote is known to me as the Fresnel reflection factor.

You are exactly right, David. That was a mistake made in haste on my part. How could we ever forget the man who invented those giant lighthouse lenses, anyway? ;)

Best regards,
Darryl

[Laurie]

Here is a nice article on Polarization from Vision Care Product News...

http://www.visioncareproducts.com/05/lenses_polarized.html

written by Randy Smith.

(also, I see that Diane has an article there!)

[Corey Nicholls]

Thanks to all, it was good to have a win in an optical discussion with one of the bosses! The information given was gladly passed on to her.

Thanks again.

[chip anderson]

Shouldn't this be spelled with a "z"?

[David Wilson]

Not in Australia, Chip. Corey has used the correct spelling for Australia (the British spelling).
Regards
David

[Steve Machol]

Originally posted by David Wilson
Not in Australia, Chip. Corey has used the correct spelling for Australia (the British spelling).
Regards
David

There's that "British' thing again. Are they ever going to learn how to speak and spell proper English?!

:D :bbg: :D

[David Wilson]

Funny that you should say that, Steve! Australia, it seems, is gradually becominbg 'Americanised', partly, I suspect, through the efforts of Mr Gates. For example, we now tend to spell program your way rather than programme, the English way. But, we still cling on to the u in colour, neighbour, labour etc...
Regards
David