Re: Why by using polarizer glasses, we can see a fish underwater ?

Hi Joni, Thanks for the question. The answer comes in 3 parts: why is it hard to see fish underwater, how does a polarizer work and how does it help? We also need to know a little about what light is.

First, why is it hard to see fish underwater? The answer is that the surface of water is very reflective and it also tends to scatter light in many directions. So if the sunlight is strong, a lot of light that hits the water will be scattered away in all directions. This means that the light entering your eyes is quite intense and the glare prevents you from seeing through to the water below.

How does a polarizer help? To understand this, you need to know a little about light. The nature of light is actually rather mysterious, but the way that we think of it is as being made of 2 vibrating fields, a magnetic field and an electric field, which vibrate at 90 degrees to one another. What this means is that if you could look along a beam of light heading straight for your eyes and if you could see the fields vibrating, you would see waves. If you imagine only the electric field, some of the waves would vibrate straight up and down as viewed by you, some would vibrate from side to side and others would be vibrating at all angles inbetween. This is rather hard to describe without a diagram, have a look at Light and Polarization to see what I mean.

The way a polarizer works is by only transmitting electric fields that vibrate in one plane. For instance, vertically polarized light contains only an electric field that vibrates up and down. Horizontally polarized light has a field that vibrates only from side to side. Try an experiment; cut out 2 pieces of polaroid from some sunglasses, look through them and rotate one around. At some point, no light passes through. The first piece transmits only one plane of light, when the second is at 90 degrees to this plane, all light is blocked.

Back to the fish. Now you know how polarization works, you'll see what's happening. The polarizer is allowing through only the reflected light from the water in one plane and the rest is blocked. This substantially reduces the amount of glare from the surface and you can now see through to the water below.

At the website that I mentioned above, there is a nice interactive animation about polarized light; click on the 'polarization' link near the bottom. There's also some more complex physics of light waves at Wave Nature of Light.