Diffraction is a modification that light undergoes as it passes by the edges of opaque bodies or through narrow slits. The rays appear to be deflected: they ?bend? around an obstacle, producing fringes of parallel bright and dark bands as they fall on the screen. If the obstacle has a linear shape (a slit, a thread, or a screen's edge), a set of parallel diffraction fringes appears on the screen.

French physicist Fresnel developed the theory of diffraction in 1818 on the basis of the wave theory of light. Fresnel complemented Huygens' principle by introducing the concept of interference coming from the secondary wavelets. Fresnel was the first to calculate the diffraction patterns that arise from interaction of light with simple obstacles.

The diffraction theory is based on the notion of Fresnel zones. For round obstacles, the radii r_{m} of circular Fresnel zones equal to

r_{m} = mλL^{1/2} (m=1, 2, 3 ...)

where λ is the wavelength, and L is the distance between the obstacle and the screen. The result of diffraction greatly depends on the number m of the Fresnel zones (where r_{m} = R):

m = R^{2}/(λL) .

When a circular aperture in an opaque screen causes diffraction and the number of Fresnel zones (m) is odd, a bright spot is visible at the center of the diffraction rings. If the number m is even, the center spot is a dark one.

In diffraction on a round disk or a ball, the spot in the center of the pattern is always a bright one (the so-called Poisson bright spot).

In diffraction that is caused by a single slit of the width d, the number m of the band-like Fresnel zones determines the character of diffraction pattern. This number is determined by the following formula:

m = (d/2) ^{2}/(λL) .

The notion of Fresnel zones determining the diffraction pattern allows one to simulate diffraction phenomena using waves of different spectrum band, and to choose an appropriate size of the generating apparatus. For example, diffraction of radio waves can be simulated by light waves.