Is diffraction related to wavelength?
Refraction occurs when light bends as it crosses a boundary between two different mediums, each with a different index of refraction.
In contrast, diffraction occurs when light bends in the same medium. The bending is the result of light waves "squeezing" through small openings or "curving" around sharp edges.
Since light waves are small (on the order of 400 to 700 nanometers), diffraction only occurs through small openings or over small grooves. Moreover, waves diffract best when the size of the diffraction opening (or grting or groove) corresponds to the size of the wavelength. Hence, light diffracts more through small openings than through larger openings.
The formula for diffraction shows a direct relationship between the angle of diffraction (theta) and wavelength:
d (sin theta) = m (wavelength) --> for constructive interference
(A similar formula for destructive interference exists.)
From either formula, however, it's clear that as the wavelength increases, the angle of diffraction increases, since these variables are on opposite sides of the equal sign. Conversely, as the wavelength decreases, the angle of diffraction decreases.
In short, the angle of diffraction is directly proportional to the size of the wavelength.
Hence red light (long wavelength) diffracts more than blue light (short wavelength). And radio waves (really long wavelength) diffract more than X-rays (really short wavelengths).
Really detailed explanation, that includes the student's point of view on this website: http://physics.stackexchange.com/questions/95126/relationship-between-slit-size-and-wavelength-in-diffraction