5.6 Wave Behavior - Diffraction and Young’s Slits

We like to think that waves travel in straight lines. This would explain why you have a shadow on a sidewalk, for instance, because your body would be blocking light waves traveling from the sun to the sidewalk. Why is it then that you can hear what someone says even if they are standing in another room, or around the corner from where you are? It is because of another simple phenomenon common to all waves: diffraction.

The basic principle behind this is the bending of waves as they go though gaps, or around corners (you can think of these as partial gaps). This occurs more noticeably when the size of the gap is close to that of the wavelength. Sound waves (up to several meters) have wavelengths much larger than light waves (in nanometers), which is why they diffract more noticeably around everyday objects. In diffraction, of course, the wavelength and frequency of the waves involved remain constant.

Here are two examples of waves being diffracted.

When the gap becomes larger, wavefronts only bend at the ends. This can be extended to objects, where the gap may be indefinitely large, but where bending of wavefronts (lines indicating the crests of a wave) is still noticed near the object.

A common experiment for observing the phenomenon of diffraction was first carried out by Thomas Young, a British physicist. This allows the wavelength of a wave, in this case, light, to be determined in a laboratory. What this experiment involves is setting up two coherent light sources (sources with a constant phase difference) placed behind two gaps so that they will both diffract quite well. The rounded wavefronts, a series of semicircles overlap at points, and due to superposition construct or destruct depending on their amplitudes. If a screen is placed a certain distance away, parallel to the line between the two sources, these points of construction and destruction can be noted. Points of maximum light intensity are where wave constructive interference is taking place, and those of minimum intensity are where destructive interference takes place.

In the diagram above, red dots indicate places where waves construct. The wavefronts drawn continue spreading and being superimposed so that when the screen is reached fringes of maximum (light) intensity occur. By finding the distance between the screen and the slits, D, the distance between the centers of the two slits, d, and the average distance between two fringes (shown in green), x, the wavelength of the waves used can be calculated: