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I had read that violet bends the least. But most of the webpages state that red bends the least. I would like to know why it is so.

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    $\begingroup$ It depends. Through a prism, say, red bends the least. But through a diffraction grating it bends the most. $\endgroup$
    – lemon
    Commented Feb 23, 2016 at 10:02
  • $\begingroup$ @lemon and it depends on ? $\endgroup$
    – Faiz Iqbal
    Commented Feb 23, 2016 at 10:08
  • $\begingroup$ "Blue bends best" by refraction, such as a prism. If you follow the "marching soldiers" into the mud model you quickly see that shorter steps lead to a sharper bend, while longer steps lead to shallower bends; blue is shorter than red when it comes to wavelengths. $\endgroup$
    – Peter Diehr
    Commented Feb 23, 2016 at 11:20
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    $\begingroup$ @PeterDiehr that's inaccurate and misleading. There are plenty of negative dispersion materials out there. $\endgroup$
    – Carl Witthoft
    Commented Feb 23, 2016 at 12:19
  • $\begingroup$ Natural materials exhibit negative dispersion only near strong absorption bands, and you thus "see" nothing in the optical range - and the OP was discussing visible light, red through violet. Very recently materials have been designed and fabricated which have negative index of refraction; of course these will give the opposite result, but a comment is not a dissertation, and I did consider mentioning all of this - and would have if I had posted an actual answer. $\endgroup$
    – Peter Diehr
    Commented Feb 23, 2016 at 13:50

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You need to distinguish between two different optical phenomena of diffraction and refraction.

In diffraction the magnitude of the effect depends on the size of the object causing the diffraction relative to the wavelength of the light. As the object gets bigger than the wavelength the amount of diffraction gets small. This means as we reduce the wavelength of the light, i.e. go from red to violet, the diffraction decreases - violet light diffracts less than red light.

In refraction the light interacts with the electrons in the glass or whatever medium the light is passing through, and the amount of refraction depends on how strong the interaction is. In most transparent materials the interaction goes up with the energy of the photons in the light. Since shorter wavelengths have higher energies refraction increases as we go from red to violet, so violet light refracts more than red light.

If you're interested to know more about why refraction (usually) increases at shorter wavelengths there is an excellent but rather technical discussion in Why do prisms work (why is refraction frequency dependent)? and a less technical explanation in Why does the refractive index depend on wavelength?. There is a detailed discussion of the mechanism of diffraction in Why does wavelength affect diffraction?.

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  • $\begingroup$ Does the rainbow appear to bend because of the spherical shape of the raindrop or is there another science behind it? $\endgroup$
    – Anagha
    Commented Feb 26, 2016 at 6:16
  • $\begingroup$ @Anagha: the system generating the rainbow is axially symmetric, and the rainbow is a ring centred on this axis. There is lots about rainbows already on this site. $\endgroup$
    – John Rennie
    Commented Feb 26, 2016 at 6:29

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