# Why does dispersion of light behave in the opposite way for diffraction compared to refraction?

When blue light is shined through a diffraction grating it bends less than a longer wavelength, let's say red light. The shorter the wavelength the less it is bent by diffraction. When the same procedure is performed by passing the light through a medium, let's say glass, the opposite is true. The blue light bends more than the red. Shorter wavelengths bend more. It's interesting to me that they are not just different, but that they are the exact opposite. Why is this? Thanks for addressing this question.

• Refraction of different colors depends on the dispersion of the medium, which can make blue bend more or less (depending on the material). Diffraction applies momentum perpendicular to the diffraction surface, so the first order will always deflect blue less than red by energy conservation. – Jon Custer Apr 27 '18 at 0:43
• Doesnâ€™t diffraction take place inside a medium like glass because it would deliver momentum perpendicular to the glass surface too. Many diffraction grating are even made of glass. It perplexes me that diffraction would not happen in the medium. – Lambda Apr 27 '18 at 2:12
• @JonCuster: That should be an answer. – user4552 Apr 27 '18 at 4:02
• As a concrete example, the index of refraction of silica glass is an increasing function of frequency within the visible portion of the EM spectrum, but it decreases from 3x10^15 Hz to 4x10^15 Hz (in the UV). – user4552 Apr 27 '18 at 4:05
• Not sure what you mean by that should be an answer. – Lambda Apr 27 '18 at 4:05