Question

why does a diamond flashes colour when observed under white light? ​

Answer 1

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A diamond flashes colour when observed under white light because of the refraction caused by a diamond's faceting, normal white light can be split into many spectral colors as it leaves the diamond.
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Answer 2

Answer:

• Diamond flashes colour when observed under white light because of the way the stone is cut as it refracts or bends, which splits the white light into its full spectrum of colors i.e., from red to violet which is also called the VIBGYOR also seen in the colors of the rainbow.

• This is called dispersion and this occurs due to the difference between the refractive index (RI) and the various wavelengths of light energy that compose the light. Diamond has a high refractive index of $2.417$ and therefore a small critical angle of $24.5^\circ$.

• The outside of the diamond reflects rainbow of colors onto other surfaces, while the inside of the diamond sparkles white and grey.

Hence outside of the diamond reflects rainbow of colors onto other surfaces, while the inside of the diamond sparkles white and grey.