## A Layman’s Guide to Photonic Crystals

Featured image credit: Flickr, Steven & Courtney Johnson & Horwitz (Picture)

## The Underwater Optical Man-hole

###### By Anupum Pant

Agreed, sometimes, when you find yourself being interrogated in a room covered with one-way mirrors, you can’t see the people who are observing you; Instead, you see yourself in the mirrors. Otherwise, If you can see something, it seems normal to assume that the thing can see you too.

A trout’s window to the outside world is something similar to what a person in the interrogation room experiences. However, unlike the person, a fish can actually see things that are out of the water, but the view is very limited.

### The Snell’s Window

When a fish looks up from water, it sees only a circular window of light, from under the water surface. Everything that lies outside of this circle is darker. This darker area of vision is replaced by the reflection of the sea/lake bed (where there is no source of light to illuminate it). This effect isn’t due to any limitations of a fish’s eye. In fact, even human divers see only a circle of light when they are under water. This circle is called the Snell’s Window or the optical man-hole.

Irrespective of the fish’s visual acuity, some physical properties of water and air get together and have a great effects on what a fish can see. It sees a circle with diameter calculated by the Snell’s equation.
In short, the window is about 2.3 times as wide as the fish’s depth. So, a fish can see more if it goes deeper. At a depth of 1 meter, it can clearly see things on a circle that is 2.3 m wide on the surface of water.

So, even if you can see a fish in water, it will be foolish to assume that the fish can see you too. Some times it can’t. It looks something like this from under water:

In Wikipedia’s words:

Snell’s window is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees.

## Why does it happen?

It happens due to a simple optical phenomenon called the total internal reflection.

## Sun’s Green Flash

###### By Anupum Pant

More often while setting than rising, if the conditions are right, a part of the sun (on the top) can appear green. This happens for very short interval lasting for about 2-3 seconds and is considered a rare phenomenon. Since it is green and lasts for a very small interval, it is also called the green flash, emerald flash or green ray. If you have ever captured it or plan to do it in the future, do share your results with me through mail/twitter. [See the animation] [Real GIF]

### What does it look like?

Sometimes the sun’s rim can appear green (in optically zoomed images). Otherwise, when the sun is set, for a brief moment, it appears as if a part of sun has separated from the main body and has turned green. It is usually seen as a horizontal line, like in the video below. But, a few lucky ones have captured complete green auras too.

### Why does it happen?

The sun gives out a white light, which contains all the colors – Green is one among  them. Normally, our eye isn’t able to resolve the separate colors and sees them as a mixture which is white. When the sun sets, our atmosphere acts like a prism and bends the colors. A few colors get bent more than others. For example, green bends more than red. As a result the two colors get separated enough to be resolved by our eye. But the right amount of bending happens only if the atmospheric conditions are right.

In extremely rare cases, blue or violet flashes have been reported. [image]

For a detailed explanation you can go through this – [Geometric Optics of Green Flashes]

At poles where the sun moves in a different manner, probably the green ray can last much longer. Admiral Richard Byrd has claimed to have seen this green flash for 35 minutes while on an expedition to Antarctica.