Diffraction

Diffraction is the apparent ``bending'' of light waves around obstacles in its path.

  

Figure 22.7: Diffraction of waves through a slit

This bending is due to Huygen's principle, which states that all points along a wave front act as if they were point sources. Thus, when a wave comes against a barrier with a small opening, all but one of the effective point sources are blocked, and the light coming through the opening behaves as a single point source, so that the light emerges in all directions, instead of just passing straight through the slit.

Note:

• For sizeable diffraction effects to occur the width of the opening must be of the same order or less than the wavelength of the light used.
• Diffraction limits the resolving power of microscopes and other magnifying devices. If the object being viewed is smaller than the wavelength of light used, then the light diffracts around the object, and severely distorts the image. Thus microscopes using visible light have a resolving power of only about 600 nm $\approx$ 10^{- 6}m, but X-rays, whose wavelength is about 0.1 nm ( 10^{- 10} m) have a resolving power four orders of magnitude smaller.