Visibility:- 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 happen 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. 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 often perceived to be white. You can create your own rainbow by facing 180 degrees from the sun and spray mist from a garden hose in front of you in a circular motion, outlining a 360 degree “rainbow”.

It is difficult to photograph the complete arc of a rainbow, as this 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.

Scientific explanation:- 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. Then it is 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  rainbow in sea spray is smaller than a true rainbow.  The amount by which light is refracted depends upon its wavelength, and hence its colour. Blue light having 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. Thus the colours blend together rather than forming a rainbow.

A rainbow does not actually exist at a particular location in the sky. It is an optical illusion whose apparent position depends on the observer’s location and the position of the sun. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer’s eye. This light is what constitutes the rainbow for 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 centered 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 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 as there are not usually sufficient raindrops between the horizon that is: eye height and the ground, to contribute. Exceptions occur when the observer is high above the ground, for example in an aeroplane, on top of a mountain, or above a waterfall.