Diffraction, when referring to sound waves is the bending of sound waves. Imagine you are on one side of a street corner, you can still hear people who are around the corner, correct? If sound waves only travelled straight, the sound would cut off, but since it doesn't we can assume sound bends. Generally sound diffracts the best when the opening is smaller than the wavelength of the wave. If not the wave would have no reason to spread out, meaning less diffraction. Lower frequencies have longer wavelengths therefore more diffraction, make sense?
The scattering of sunligh by air molecules also causes the sky to look blue. This effect is caused by the selective absorption of light. As oscillators, air molecules have resonant frequencies (at which they scatter the most light) in the ultraviolet region. Consequently, when sunlight is scattered, the light in the blue end of the visible region is scattered more than in the red end. The sky would appear violet (it has an even shorter wavelength than blue), but violet light is not very abundant in sunlight, plus our eyes don't pick up violet as well. You may notice that the sky looks a deeper blue directly overhead than on the horizon (try this on a clear day), this is because the air molecules are more dense on the horizon. This causes multiple scatterings and the recombination of scattered light makes it appear white. A good example of that is milk. Take a glass of water and add a few drops of milk to it. If you shine a flashlight through it, the light on the other end will be bluish. If you do the same thing with pure milk, the light will be white because of multiple scatterings. I'm a physics major, any other questions?
