What is a Rainbow?
Rainbow is one of the well-known optical effects that are related to weather, and one of nature's most glorious masterpieces which result from the refraction of sunlight from falling water droplets plus the reflection of the light from the back of the droplet.
It's an excellent demonstration of the dispersion of light and proof that visible light is composed of a spectrum of wavelengths, each associated with a distinguishing color.
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You will be surprised to know that you can even see it on sunny days when you are near the waterfalls or fountains.
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Rainbow in waterfalls Rainbow in fountains
What Are the Rainbow Colors?
We know that rainbows are made by the dispersion of white light by the water droplets.
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Rainbow has seven primary colors in the order of ROYGBIV, i.e., red, orange, yellow, green, blue, indigo, and violet.
What Makes a Rainbow?
We can see anything because the light falling on our eyes, enables us to view them.
Similarly, the red light coming from the upper part of the rainbow reaches our eyes enabling us to see it.
When we look at this color, we would wonder where this light is coming from?
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Let’s consider a single droplet in the red color region, a ray of white light incidents on the surface of it, will reflect some light in the boundary and refract some of it.
The reflected light is white again, and it goes away from our eyes, so we ignore this as this light doesn’t help in forming rainbows.
We know that the white light of the sun is composed of seven colors which are VIBGYOR.
When this white light is passed through the prism, it breaks this white light into its constituent seven colors.
The water droplets during rainfall behave exactly like a prism.
They disperse the white sunlight at the entry point, then this dispersed light falls onto the rare side of the droplet and each light color is again refracted by different angles.
If you observe, this light gets refracted in the same medium.
Here, you can see the light didn't pass from the water droplet to the air.
Well, this happens because of an optical phenomenon known as the total internal reflection.
Generally, when the light passes from denser to rarer medium, it bends away from the normal.
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However, in the case of total internal reflection, there is an angle called critical angle, ic, the angle at which the refracted ray becomes perpendicular to the normal.
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If you further increase the angle of incidence, the refracted ray remains in an incident medium.
Here, you can notice that a ray of light gets reflected in the same medium.
The same thing happens with the water droplet as well, this angle of incidence at this surface for each colored light is such that they all bend in the same medium, so these different colored light rays are now at the air-water interface again for the third time. However, at this time, the total internal reflection doesn’t occur and these rays bend in the air but now these rays are separated by some amount.
This is because the three refractions created a larger gap between these colors, so all the water droplets in the red region of the rainbow behave in the same manner.
You can see the red color only while all other light colors got refracted.
This is because only the red color from this region reaches our eyes while all other colors are refracted at such a large angle that they don’t reach our eyes on the ground. Similarly, it happens for the region of other colors as well.
This is how rainbows form in the sky.
How to Make a Rainbow?
Things required to make rainbows are:
Granulated sugar
Coloring tablets or food coloring water
Straw
Six glasses
Measuring spoon
Instructions
Fill each glass with water and add different amounts of sugar in each glass numbered from 1 to 5.
Add 0 tablespoon of sugar and red color to it.
Add one tablespoon of sugar and orange food color to it.
Add two tablespoons of sugar and yellow food color to it.
Add three tablespoons of sugar and green food color to it.
Add four tablespoons of sugar and blue food color to it.
Add five tablespoons of sugar and food color to it.
Stir these glasses until the sugar is dissolved.
Now, transfer about half of the blue into the empty glass, slowly add green water of the same amount on the top of the blue water, and do the same for the next colors.
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You can see a beautiful rainbow.
How to Create a Rainbow?
Materials Required
Water
Sunlight
Tools
Prism
Whiteboard
Method
Place a whiteboard on the ground under the sunlight.
Place the prism above the whiteboard.
Now, rotate the prism at certain angles until you get a rainbow.
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FAQs on Rainbow
1. What is the scientific explanation for a rainbow?
A rainbow is an optical phenomenon caused by the dispersion and total internal reflection of sunlight by spherical water droplets, such as rain or mist. When sunlight passes through these droplets, they act like tiny prisms, splitting the white light into its constituent spectrum of colours. The collective effect of millions of droplets creates the colourful arc we see in the sky.
2. What are the seven colours of a rainbow in the correct order?
The seven colours of a rainbow always appear in the same order, which can be remembered using the acronym VIBGYOR. From the inner edge of the arc to the outer edge, the colours are:
- Violet
- Indigo
- Blue
- Green
- Yellow
- Orange
- Red
3. What are the essential conditions for a rainbow to be visible?
To observe a rainbow, two primary conditions must be met:
- The Sun's Position: The sun must be behind you, and relatively low in the sky (less than 42 degrees above the horizon). This is why rainbows are typically seen in the early morning or late afternoon.
- Water Droplets: There must be water droplets in the air in front of you, such as during or after a rain shower, or in the mist from a waterfall or sprinkler.
4. How does a single water droplet separate sunlight into different colours?
A single water droplet separates light through a three-step process:
- Refraction and Dispersion: As sunlight enters the droplet, it slows down and bends, or refracts. Because the refractive index of water is slightly different for each colour, the white light splits into its spectral colours, a process called dispersion.
- Total Internal Reflection: The dispersed light travels to the back of the droplet and is reflected internally.
- Final Refraction: The coloured light then exits the front of the droplet, refracting again as it travels into the air towards the observer's eye.
5. Why is a rainbow always shaped like an arc?
A rainbow is actually a full 360-degree circle. We only see a semi-circular arc because the ground, or the horizon, gets in the way and obstructs the lower half of the circle. The circular shape is formed because a rainbow is the set of all points (water droplets) that reflect sunlight to the observer's eye at a specific angle, which is approximately 40-42 degrees relative to the antisolar point (the point directly opposite the sun).
6. What is the difference between a primary and a secondary rainbow?
A primary rainbow is the brightest and most commonly seen bow. Sometimes, a fainter secondary rainbow can be seen above it. The key differences are:
- Formation: A primary rainbow is formed by one total internal reflection inside the water droplets. A secondary rainbow is formed by two internal reflections.
- Colour Order: The primary rainbow has red on the outer edge and violet on the inner edge. The secondary rainbow has its colours reversed, with violet on the outside and red on the inside.
- Appearance: The secondary rainbow is always fainter and appears higher in the sky than the primary one. The sky between the two bows often appears darker, an area known as Alexander's dark band.
7. Why is it often difficult to distinguish all seven colours in a rainbow?
While we learn about seven distinct colours, a rainbow is actually a continuous spectrum with no sharp boundaries between colours. It is difficult to see all of them clearly for a few reasons:
- Eye Sensitivity: The human eye is less sensitive to frequencies at the edges of the spectrum, like indigo and violet.
- Atmospheric Conditions: The size of the raindrops and atmospheric scattering can affect the brightness and clarity of different colours.
- Colour Overlap: The colours blend into one another, making it hard to pinpoint exactly where one ends and the next begins. Indigo, being a very narrow band between blue and violet, is particularly difficult to distinguish.
8. Can you see a rainbow at noon?
It is nearly impossible to see a ground-based rainbow at noon. Rainbows are visible only when the sun is low in the sky, at an angle of less than 42 degrees. At noon, the sun is typically too high overhead. For the 42-degree arc to be visible, the antisolar point would be more than 42 degrees below the horizon, meaning the entire rainbow would form below the horizon, out of your sight.
