When an electromagnetic wave passes by an obstacle in space, the wave is bent around the object. This phenomenon is known as diffraction. The effects of diffraction are usually very small, so we seldom notice it.
However, you can easily see the effect of diffraction for yourself. All you need is a source of light, such as a fluorescent or incandescent light bulb. Hold two fingers about 10 cm in front of one eye and make the space between your fingers very small, about 1 mm. Now look through the space between your fingers at the light source. With a little adjustment of the spacing, you will see a series of dark and light lines. These are caused by constructive and destructive interference of light diffracting around your fingers.
The reason diffraction occurs is not exactly obvious. Christian Huygens in the mid-1600s offered an explanation that, strange though it may seem, still nicely explains the observations.
You may recall the inverse-square law of electromagnetic propagation from basic physics. As electromagnetic radiation leaves its source, it spreads out, traveling in straight lines, as if it were covering the surface of an ever expanding sphere. This area increases proportionally to the square of the distance the radiation has traveled.
Electromagnetic energy may be considered to propagate from a point source in plane waves. The inverse square law applies not only to the source of the energy but also to any point on a plane wave. That is, from any point on the plane wave, the energy is propagated as if the point were the source of the energy. Thus, waves may be considered to be continuously created from every point on the plane and propagated in every direction.
