How Light Travels?

Diffraction and Interference of Light
When light passes through a slit with a size that is close to the light’s wavelength, the light will diffract, or spread out in waves. When light passes through two slits, the waves from one slit will interefere with the waves from the other. Constructive interference occurs when a wavefront, or crest, from one wave coincides with a wavefront from another, forming a wave with a larger crest. Destructive interference occurs when a wavefront of one wave coincides with a trough of another, cancelling each other to produce a smaller wave or no wave at all.

The first successful theory of light wave motion in three dimensions was proposed by Dutch scientist Christiaan Huygens in 1678. Huygens suggested that light wave peaks form surfaces like the layers of an onion. In a vacuum, or a uniform material, the surfaces are spherical. These wave surfaces advance, or spread out, through space at the speed of light. Huygens also suggested that each point on a wave surface can act like a new source of smaller spherical waves, which may be called wavelets, that are in step with the wave at that point. The envelope of all the wavelets is a wave surface. An envelope is a curve or surface that touches a whole family of other curves or surfaces like the wavelets. This construction explains how light seems to spread away from a pinhole rather than going in one straight line through the hole. The same effect blurs the edges of shadows. Huygens’s principle, with minor modifications, accurately describes all forms of wave motion.