The properties and behaviour of light, and how it interacts with droplets of water, give rise to one of nature's most colourful meteorological events - the rainbow.
It's all in the geometry...
Rainbows are formed when sunlight is scattered from raindrops into the eyes of an observer.
Most raindrops are spherical rather than the often depicted 'teardrop' shape and it is this spherical shape that provides the conditions for a rainbow to be seen.
The position of the sun and the raindrops in relation to the observer need to be just right for a rainbow to form:
- The sun needs to be behind the viewer
- The sun needs to be low in the sky, at an angle of less than 42° above the horizon. The lower the sun in the sky the more of an arc of a rainbow the viewer will see
- Rain, fog or some other source of water droplets must be in front of the viewer
The size of the raindrops does not directly affect the geometry of a rainbow, but mist or fog tends to disperse the effect more (see fogbows).
Rainbows only appear semi-circular over level ground at sunrise or sunset, when the sun is exactly on the horizon, the majority of the time a smaller segment of an arc is seen.
The effect of light hitting a raindrop
Because water is denser than air, light passing from air to a raindrop at an angle slows and changes direction, in a process called refraction.
Sunlight is made up of light of many different wavelengths that slow by different amounts causing the white light to split, or disperse, with the shorter blue and violet wavelengths going through a slightly increased change of direction to the longer wavelengths of the red light.
If the angles at which the light enters the droplet are correct, some of the light that enters the droplet will be internally reflected from the inside edge of the drop and will exit the drop, undergoing refraction again as it passes back from water to air.
Seeing the rainbow
An observer standing in the right place will see the dispersed sunlight reflected back towards them. Light scattered by many drops reaching the observers eye will appear as a colourful rainbow. Different colours exit the droplets at angles varying by around two degrees from red to violet. The red light seen by an observer comes from drops slightly higher in the atmosphere than the drops that scatter violet light towards the observer.
The colours seen are not a pure spectrum, there is some mixing and blurring of the colours. The human eye can pick out many hues, a popular idea is to think of a rainbow as having seven colours, red, orange, yellow, green, blue, indigo and violet (ROYGBIV). You can find out more about the colours of the rainbow What are the colours of the rainbow?.
Other rainbow phenomena
There are many different variations on conventional rainbows that form in certain conditions. You can find more information on them by clicking the links below:-
· What is a double rainbow?
· What is a fogbow?
· Full circle rainbow
· Moonbow
FAQs
Rainbows are formed when sunlight is scattered from raindrops into the eyes of an observer. Most raindrops are spherical rather than the often depicted 'teardrop' shape and it is this spherical shape that provides the conditions for a rainbow to be seen.
How does a rainbow form? ›
When light meets a water droplet, it is refracted at the boundary of air and water, and enters the droplet, where the light is dispersed into the seven colors. The rainbow effect occurs because the light is then reflected inside the droplet and finally refracted out again into the air.
What makes a rainbow answer? ›
The Short Answer:
A rainbow is caused by sunlight and atmospheric conditions. Light enters a water droplet, slowing down and bending as it goes from air to denser water. The light reflects off the inside of the droplet, separating into its component wavelengths--or colors.
What two things cause a rainbow to appear? ›
So, the main ingredients to see a rainbow are the following:
- You need to be standing with the sun to your back and the rain in front of you.
- The sun needs to be less than 42o above the horizon.
- The sun's rays must be hitting the raindrops to create the rainbow.
A rainbow is always formed in a direction opposite to that of the Sun so that the sunlight incident on the water droplets (acting as prism) can be easily refracted and dispersed thus forming a rainbow.
Can I touch a rainbow? ›
Rainbow is formed just because of dispersion of white light due to raindrops. Technically different colours are light waves of different wavelengths. Since we can not touch light, so we can not even touch a rainbow.
What makes a rainbow 🌈? ›
Rainbows are formed when light from the sun is scattered by water droplets (e.g. raindrops or fog) through a process called refraction. Refraction occurs when the light from the sun changes direction when passing through a medium denser than air, such as a raindrop.
How do rainbows form at night? ›
But have you ever seen a moonbow? This rare phenomenon, also known as a lunar rainbow, occurs at night when light from the Moon illuminates falling water drops in the atmosphere. Sometimes the drops fall as rain, while in other cases the mist from a waterfall provides the necessary water.
What gives rainbow its shape? ›
Note: The rainbow that is formed is actually a full circle but we are able to see only an arc because the horizon cuts off the lower half. The reason for the circular shape is the spherical shape of the droplets which reflect light at an angle making a cone of light.
What is rainbow one word answer? ›
A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky.
When a sunbeam is being refracted twice and reflected once by the droplet, a primary rainbow will form. If the beam is being refracted twice and reflected twice, a secondary rainbow will form. As the secondary rainbow is formed by one more reflection than the primary rainbow, it is much fainter and rare to see.
What are the colors of the rainbow 🌈? ›
He also noted that the sequence of the colours of a rainbow never changed, always running in the same order. He coined the idea that there are seven colours in a spectrum: red, orange, yellow, green, blue, indigo and violet (ROYGBIV).
How is a rainbow formed step by step? ›
A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally, refract it again when it comes out of the raindrop.
What are 5 facts about rainbows? ›
Five Cool Rainbow Facts for Kids (And How to Make Your Own Suncatcher)
- Rainbows Are Refracted Sunlight. ...
- They're Actually Full Circles. ...
- 'Sun Dogs' Happen When Sunlight Refracts off Ice Crystals. ...
- Rainbows from Sea Spray Are Smaller in Diameter. ...
- Rainbows Are Visible at Home When a Material Refracts the Light.
How long can a rainbow remain visible in the sky? As long as the sun shines upon the water droplets that are sending your eye the color refractions. On the Colorado plains I have seen rainbows that last for an hour, but only rarely - usually the droplets move out of position as the rain clouds that produced them moves.
What are the three steps of rainbow formation? ›
They refract and disperse the incident sunlight, then reflect it internally, and finally, refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colors reach the observer's eye.
How is the rainbow activity formed? ›
When shining a light in a glass of water, the white light passes from the air and into the water, causing the different wavelengths to bend, or refract. Some wavelengths bend more than others, separating the light into its different wavelengths and showing rainbow colors. You can also create this effect with a prism!
Why do we see rainbows in the sky only after rainfall? ›
The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally and finally refract it again when it comes out of the raindrop and hence due to dispersion of light and internal reflection, rainbow formation takes place during rainfall.
