why is rainbow semi circular?

Answer 1

"A rainbow is produced when sunlight suffers one refraction while entering water droplets hanging in the air, a total internal reflection at their back surface and a second refraction while coming out of the droplets. Most of the light of the rainbow emerges close to the “angle of minimum deviation”. This angle is 42 degrees with respect to the direction of the incidence of sunlight. Therefore, we will see a rainbow at this angle with respect to a line connecting the Sun with the back of our head.

This can be true only for a circular arc at an angle of 42 degrees. This circle intersects the Earth and standing on the Earth, the largest sized arc we can see is a semicircle; this occurs when the Sun is close to the horizon.

If however, we were up in an airplane, the Sun directly overhead and raindrops hanging below us, then we will be able to see a rainbow that is fully circular.

One can give a more general argument that does not require any detailed physics. In the phenomenon of the rainbow there is only one preferred direction, namely that of the Sun.

After it rains, water droplets are everywhere. Should there be anything like a patch of colored light coming in our direction, it can be only democratically distributed, equidistant in angle with respect to the only special direction. Hence the locus must be circular. "

Answer 2

Rainbows are not semi circular. They are always circular. That is because the rainbows are formed when light from the Sun is refracted through the water droplets present in atmosphere. Since the source of the light is spherical ( circular in two dimensional view), the rainbow is also circular. It appears to be semi circular because we cannot see below the horizon standing on the Earth. If we can reach sufficient height we will see a circular rainbow.

Answer 3

Rainbows are caused by refraction and reflection of sunlight within raindrops,
which are generally spherical. The refraction effect, coupled with the
non-planar drop shape, account for the separation of light into colors. The
reflection effect bounces the light once within the drop via a phenomenon known
as total internal reflection. This direction reversal accounts for the fact
that rainbows are only observed on the opposite side of the sky from the sun.
When an observer on the ground looks at rain in the sky which is illuminated by
the sun over his/her shoulder, only certain raindrops are located where the
refraction/reflection splits the sunlight hitting them to return a visible color
to the observer. Because the drops are spherically symetric, the drops that
return a particular color are all located at the same angle to the beam of
sunlight illuminating them -- so that they appear to come from a circular region
in the sky. Because the sun can never go below the horizon and still illuminate
rain seen by an observer, rainbows always terminate on the ground. On the other
hand, however, if you make artifical rain (with a hose and a sprayer) and stand
on a ladder or the second floor of a house or are very tall (I'm a little over 2
m in height, so I count for this), you can see a complete rainbow if you're very
lucky and patient.

[Moderator note: If you are in a high flying airplane and sitting on the side
opposite the sun you will often see a circular rainbow surrounding the shadow of
the airplane on the clouds below.]

If you're really lucky, you can see a second rainbow in addition to the first.
This corresponds to more reflection and refraction events, and leads to a dimmer
rainbow. And if you're truly lucky, once in a very very long time you can see
the faint hint of a third rainbow. I've only seen this once, while driving in
California near the town of Livermore during an unexpected evening shower on a
very clear day. These multiple bows are displaced from the others by the
reflection process and are generally a little broader because of the refraction
effects.

Answer 4

Rainbows are caused by refraction and reflection of sunlight within raindrops,
which are generally spherical. The refraction effect, coupled with the
non-planar drop shape, account for the separation of light into colors. The
reflection effect bounces the light once within the drop via a phenomenon known
as total internal reflection. This direction reversal accounts for the fact
that rainbows are only observed on the opposite side of the sky from the sun.
When an observer on the ground looks at rain in the sky which is illuminated by
the sun over his/her shoulder, only certain raindrops are located where the
refraction/reflection splits the sunlight hitting them to return a visible color
to the observer. Because the drops are spherically symetric, the drops that
return a particular color are all located at the same angle to the beam of
sunlight illuminating them -- so that they appear to come from a circular region
in the sky. Because the sun can never go below the horizon and still illuminate
rain seen by an observer, rainbows always terminate on the ground. On the other
hand, however, if you make artifical rain (with a hose and a sprayer) and stand
on a ladder or the second floor of a house or are very tall (I'm a little over 2
m in height, so I count for this), you can see a complete rainbow if you're very
lucky and patient.

[Moderator note: If you are in a high flying airplane and sitting on the side
opposite the sun you will often see a circular rainbow surrounding the shadow of
the airplane on the clouds below.]

If you're really lucky, you can see a second rainbow in addition to the first.
This corresponds to more reflection and refraction events, and leads to a dimmer
rainbow. And if you're truly lucky, once in a very very long time you can see
the faint hint of a third rainbow. I've only seen this once, while driving in
California near the town of Livermore during an unexpected evening shower on a
very clear day. These multiple bows are displaced from the others by the
reflection process and are generally a little broader because of the refraction
effects.

The best way to investigate rainbows for yourself is to get a good garden hose
with a fine atomizer nozzle, wait for a really sunny day, climb atop some steps
(being very careful not to fall if they're slippery), and start spraying --
being sure to put the sun at your back and looking exactly opposite to the
direction of the sun. It helps to wear a bathing suit, but if it's as hot as it
often is here in Orlando, Florida, USA, in the summer, it feels pretty good to
be out in regular clothes as well.

Hope this helps!!

Answer 5

A rainbow does not actually exist at a location in the sky, but rather is an optical phenomenon whose apparent position depends on the observer's location. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer's eye. These raindrops are perceived to constitute the rainbow by that observer. The position of a rainbow in the sky is always in the opposite direction of the Sun with respect to the observer, and the interior is always slightly brighter than the exterior. The bow is centred on the shadow of the observer's head, or more exactly at the antisolar point (which is below the horizon during the daytime), appearing at an angle of approximately 40°–42° to the line between the observer's head and its shadow. As a result, if the Sun is higher than 42°, then the rainbow is below the horizon and cannot be seen unless the observer is at the top of a mountain or a similar vantage point. Similarly, it is difficult to photograph the complete arc of a rainbow, which would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less would be required, whilst most photographers are only likely to have a 28 mm wide-angle lens. ______________________________________..... From an aeroplane, one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre. This phenomenon can be confused with the glory, but a glory is usually much smaller, covering only 5°–20°, as opposed to over 80° for a full circle rainbow. ______________________________________.....

Answer 6

The rain bow is not semicircle. It is only arc of a circle. The center of the circle is below the horizon.
When we draw a circle with a compass, we keep the angle between the two legs constant. Fixing the pointer of one leg at the proposed center we rotate the other leg keeping the angle always constant.

A ray from the top most point on the rainbow and another line from the center of the circular rainbow to our eye make an angle. Theory says this angle is a constant.

Imagine the ray as one leg of a long compass and the other line as another leg which points the center. Keeping the angle constant when the compass is rotated an arc is formed.
The center of the circle is not visible to our eye. It is below the horizon.
However, if we go high up we can see the center and the whole circle.

Answer 7

Rainbows are formed by light reflecting from rain drops. Often we see only one rainbow, called the primary bow, but sometimes we can see a second, outer rainbow, called the secondary bow.



Primary Bow
When sunlight strikes a raindrop, some of the light is refracted at the first surface, internally reflected from the back surface, and refracted again on exiting the rain drop.



For each colour there is a maximum angle of deviation of the light. So, in rain drops there is a minimum angle, for each colour, between the incident and exit rays. For red, this is 42° and for violet it is 40°. The nature of internal reflection is such that many rays will emerge near this minimum angle.

Hence the sunlight will appear to be concentrated over a small region of the sky in a narrow range of angles from 40° to 42° and separated into colours, from blue to red.

Secondary Bow
Some of the sunlight is also refracted at the first surface, but is reflected twice internally from the back surface, and refracted again on exiting the rain drop.



Again there is the same maximum angle of deviation for each colour, but now the double internal reflection leads to a minimum angle between the incident and exit rays of 50.5° for red and 54° for violet. The bow is at a higher angle, and so will appear higher in the sky. And the colours are reversed, going from red to blue.

The amount of rainbow you see depends on the angles between you, the sun, and the raindrops. As the sun rises, less and less of the rainbow is visible. As you climb a mountain, you see more a

Answer 8

We see the rain bow from ground, the water droplets that produce rainbow with total internal reflection are above us. Hence we recieve only half the rainbow.
If we manage to be at an altitude wherein sunrays from behind can pass all around us (either morning or evening) and there are rain droplets ahead of us receiving sunshine there will be fully circular rainbow. (I have never managed to see such one)
More than one rainbow is possible more generally one full and another partial are usual.

Answer 9

Because you only see half of it from the ground. I've seen quite a few fully circular rainbows from aircraft at 60 to 70 thousand feet.


Doug

Answer 10

A rainbow can be a full circle, since we are on the ground the water is in between us and the light source we see half a circle most of the time. a rainbow is just water breaking white light (sunlight) down into its visible colors!!!