Question

Why do we see white colour in Newton's disc when we rotate it very fast?

Answer 1

The Newton's Disc is made up of seven colors.
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When it is rotated fast the colors blend to form white color due to persistence of vision.
White light is a combination of mainly 7 colors.So, Newton wanted to prove this by carrying out this experiment.

Answer 2

We see white colour in Newton's disc because of the fact that white light is made up of 7 rainbow colours.
i.e VIBGYOR

And, a Newton disc have parts of different colours. As we rotate our sees the light coming from that colours as combined and it appers white.
Dispersion of light was itself proven by Sir Issac Newton.