why do rainbow always appears in semi circular shape and not in any other pattern ?

Answer 1

Actually, it's possible to see a rainbow as a full circle from an aeroplane.

Answer 2

Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind a person at a low altitude or angle (on the ground). The most spectacular rainbow displays happens when half of the sky is still dark with draining clouds and the observer is at a spot with clear sky in the direction of the Sun. The result is a luminous rainbow that contrasts with the darkened background.

The rainbow effect is also commonly seen near waterfalls or fountains. Rainbow fringes can sometimes be seen at the edges of backlit clouds and as vertical bands in distant rain or virga. The effect can also be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or night-time rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are most often perceived to be white.[1]

The rainbow's appearance is caused by dispersion of sunlight as it goes through raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of 40°–42°. The angle is independent of the size of the drop, but does depend on its refractive index. Seawater has a higher refractive index than rain water, so the radius of a 'rain'bow in sea spray is smaller than a true rainbow. This is visible to the naked eye by a misalignment of these bows.[2]

The amount by which light is refracted depends upon its wavelength, and hence its colour. Blue light (shorter wavelength) is refracted at a greater angle than red light, but because the area of the back of the droplet has a focal point inside the droplet, the spectrum crosses itself, and therefore the red light appears higher in the sky, and forms the outer colour of the rainbow. Contrary to popular belief, the light at the back of the raindrop does not undergo total internal reflection and some light does emerge from the back. However, light coming out the back of the raindrop does not create a rainbow between the observer and the sun because spectra emitted from the back of the raindrop do not have a maximum of intensity, as the other visible rainbows do, and thus the colours blend together rather than forming a rainbow

For lots more info. Great rainbow pic's. and to also learn that rainbows can be in different shapes go here. // http://en.wikipedia.org/wiki/Rainbow#Scientific_explanation

Answer 3

Rainbows are caused by sunlight refracting through water droplets or ice crystals.
Since the sun is at a fixed point, the angle has to right for you to see them.
That means, from where you're standing, the distance must be the same.
Since you can only be in one place, the rainbow forms a circle.
You can only see the part which is above the horizon.
Everyone else sees a slightly different rainbow.

Answer 4

A rainbow is a good example of a macromolecular phenomenon. It is the angle the two hydrogen atoms make with the oxygen atom in H2O that determines the arc, the curvature, of the rainbow.

Answer 5

The rainbow actually fores a full circle far overhead but being on the surface of the Earth you see only half of it.

Answer 6

Good question, but this is how it should be asked in English;

Why do rainbows appear in a semi-circular shape and not in any other shapes?

In fact you can leave off "...and not in any other shapes?"

I used to know why in grade school but i forgot. Good English though, really, you are almost there. It's that little stuff that is gonna kill you in your communication.