Balazs had a question regarding Diffraction and Refraction with light and why they occur......
As you know it occurs when light travels from one medium into another. The index of refraction of the two media are different and hence so are the speeds in the media. . Therefore the wavelengths of the waves in the 2 media are directly proportional to the velocities in the 2 media. When we have waves travelling from a fast to a slow medium or vice versa we get refraction of the light. Frequencies are unchanged in refraction whether it is monochromatic or white light.
For refraction in a dispersive media, the velocity of the light depends on its frequency. Since white light is made up of many different frequencies, when you shine white light in a prism each frequency will travel at a different speed so in the prism the different frequencies and respective wavelengths (colours) will refract by different amounts.
The original frequency components of the white light remain constant. Remember the only reason for the dispersion is that each of the differing frequency components of the white light will experience a different index of refraction due to that frequency undergoing a different velocity in the dispersive medium.
Note: In a non-dispersive medium all frequencies of light undergo the same refraction.
Diffraction occurs when light bends around an opening. There is no change in medium (as there is in refraction) so there is no change in speeds. Observable diffraction occurs when the wavelength divided by the opening width is greater than or equal to one . Note that since red light has a longer wavelength than violet light, it will diffract more. Since light has small wavelengths, observable diffraction only occurs around very small openings. Therefore you do not see observable diffraction with light in common everyday experiences. You could get observable diffraction with light only through very minute slits or what we call diffraction gratings.