Why do we see 7 colors in a rainbow?

In 1966, Isaac Newton passed sunlight through a prism and it split up into its component colors. He then passed the spectrum back through another prism producing white light once again.

A Newton's color wheel is a disk with all colors of the spectrum. When the color wheel is spun round rapidly, the disk changes color and becomes practically white.


Dispersion of light, Color mixing
How is white light formed?
Newton Color Wheel


Material Needed

1. Newton's color wheel sticker - 1 No

2. Plastic container with 2 diametrically opposite holes
   & 1 hole at the bottom - 1 No

3. Lid which fits the container & has a hole at the center - 1 No

4. Cycle spoke with nut and bolt - 1 No

5. Thread - 1 Roll

Steps To Make

Cut the sticker along the circular disc as shown

Peel off the sticker

Stick the sticker on the lid of the bottle

Insert the rod through the hole in the lid and fix it using the nut

Pass the thread from the hole in the bottom of the bottle

Pass the rod through the two holes at the top of the bottle

Tie the thread to the rod as shown

Insert the rubber piece at the end of the rod as shown

Real Life Applications

How are rainbows formed?
Immediately after a rain shower, thousands of water droplets are left floating in the air. As sunlight passes through these spherical droplets, light is bent and scattered into a continuous band of colours. This light is reflected off the back of the drop and refracted again as it leaves the drop. Thus, the light coming out of several raindrops creates a rainbow.

What’s happening?

When the disk (which consists of seven colours: VIBGYOR) is rotated at high speed it seems as if the colours mix and we see a white disc.

When the thread is pulled, the disk unwinds and automatically winds-up due to inertia of motion. So the disk doesn’t stop immediately.