Diffraction is the bending of waves around corners.

Diffraction is easily seen in water waves, for example, when ocean waves pass through an opening in a breakwater. Diffraction also accounts for the fact that we can hear sound coming from a source that is hidden around a corner.

An aerial photograph of waves diffracting between two rocks. Credit: Fjellanger WiderÃ¸e A.S.

Diffraction can also be seen in light.

The idea that light is a wave was advanced in the 1600s, in contradiction to the then-popular theory that light was carried by "corpuscles," or particles. By the mid-1700s, some scientists were using the phenomenon of the diffraction of light to support the wave theory. By the 1850s, the idea that light can be considered a wave became widely accepted — thanks to these diffraction experiments.

Circular waves and plane waves

Let's go back to water waves and look at the difference between circular and plane waves.

Consider the waves generated by a stone dropped into a lake. The waves expand with ever-increasing radius from the point where the stone fell. Circular waves like these are generated by a point source (an isolated disturbance in space).

These circular waves from a point source contrast with plane waves, such as wind-driven waves in an ocean. In plane waves, the crests line up in successive straight lines.

Diffraction occurs when plane waves encounter a corner or obstacle that is close to the same size as the wavelength of the wave. In that case, the plane waves that get around the corner or through an opening emerge with more circular forms, as though generated by a point source.

See an animation of plane waves of a given wavelength passing through a gap.The user controls the size of the gap. Try different sizes to see the effect on the diffracted wave pattern.
Now that you have seen plane waves passing through a slit, it will be easier to understand this pattern of wave diffraction around a corner.

The diffraction or spreading of water waves passing through a slit that is comparable in width to the wavelength of the plane waves.

Remember:

Waves passing around a corner or coming through a slit that is comparable in width to the wavelength experience the greatest degree of diffraction. The diffracted waves look like they are coming from a point source located in the slit or corner — they have become circular waves.
The amount of bending increases as the gap or slit width gets smaller relative to the wavelength.

For a given slit width, if you make the wavelength of the plane wave smaller, the slit will look wider to the wave, and the diffraction pattern will be less bent. If the wavelength is increased, for the same slit width, the slit will seem smaller to the wave and the diffraction or bending will be more pronounced.

Diffraction works the same way for light waves.

If a screen is set up opposite a slit, the diffracted waves arriving at the slit will be seen to create a pattern of light and dark bands.

This site has a great animation of the diffraction of light through a slit. On the right is a pattern of dark and light bands, showing what would appear on a screen facing the slit. The bright bands are called maxima and the dark bands minima. Sometimes the maxima and minima are called fringes, and the pattern is called a fringe pattern.

To the right of this fringe pattern is a graph of the intensity of light as a function of distance along the screen. You can see that there is a broad, central bright maximum, and the successive narrow maxima get dimmer as you move away from the center of the screen. The fringes are often referred to by order number, where the 0th order fringe is the bright central band, and the first order fringes are the two maxima on either side of the central maximum, etc.

Young’s Double-Slit Experiment

Historically, the experiment that eventually convinced scientists that light can be considered to be a wave was performed by Thomas Young in Cambridge, England, in the late 1790s and published in 1801. This famous experiment has no particular name and is always referred to as “Young’s Double-Slit Experiment.” There is no way to explain Young’s experiment unless light has wave-like properties.

Thomas Young, medical doctor and physicist, was called a "truly original genius."

The basic physics of Young’s double-slit experiment is the interference of light passing through two small slits. Because Young could not, in practice, make his slits really small compared to the wavelength of light, his experiment also showed the effects of diffraction. So Young’s experiment combines the interference and diffraction of light.

Thomas Young's sketch of two-slit diffraction, based on his observations of water waves. Credit: Wikipedia

Young made a pinhole or slit for sunlight in a screen (call it screen number 1, and the slit $S_{0}$ .). In a second screen he then made two slits, $S_{1}$ and $S_{2}$ . (Traditionally the slit separation, or distance between $S_{1}$ and $S_{2}$ , is labeled d.) The light from $S_{0}$ diffracted and entered the second set of slits. Because the light entering the second set of slits came from a single source ( $S_{0}$ ), the waves entering the second set of slits had identical properties. In particular, they had the same phase. When two waves maintain a particular phase difference (or a phase difference of zero, in the case of waves of the same phase) we say they are coherent. Light then spread out by diffraction from $S_{1}$ and $S_{2}$ . The waves in the two diffraction patterns passed through each other and experienced constructive and destructive interference.

Young then set up screen number 3 to receive the light from the interfering waves and display the fringes. Traditionally, the distance from screen 2 to screen 3 is called D. Generally, D — the distance from the two slits in screen 2 to screen 3 — is much greater than the slit separation d: d << D

At any given point on the third screen, the waves from the two slits

S_{1}

and

S_{2}

have traveled a slightly different distance. Recall that the waves at

S_{1}

and

S_{2}

started out coherent, and in phase with each other. Because of the difference in distance traveled, or path length, they now have different phases.

Diffraction and interference of waves emanating from two pinholes or slits. Credit: Wikipedia

Depending on the exact difference in distance, they may be out of phase by any amount. If they happen to be 180 degrees out of phase, they cancel and there is a dark band or minimum on the screen. If they are 90 degrees out of phase, they only partially cancel and the amount of light on the screen is intermediate between bright and dark. Of course, they may also happen to be separated in phase by 360 degrees, or some multiple of 360, which is equivalent to 0 degrees, so they will form a maximum on the screen.

Because Young was using sunlight, and sunlight is made up of many colors or wavelengths, his fringes were not as well-defined as they are if one uses monochromatic (single-color, or single-wavelength) light. If there are multiple wavelengths present, they each form an interference pattern on the screen, and each pattern has its own fringes of different spacing, so overall the fringes get blurred together.

Young could not make his slits small in comparison to the wavelength of the light, so the fringe pattern also shows the effect of diffraction. The bright bands are not uniform in brightness because of the effect of diffraction.

See this site for a comparison of the diffraction and interference patterns of a single and double slit. You can choose to view the interference pattern only (idealized case), or view the combination of interference and diffraction. In both the single-slit and the two-slit cases, the "interference only" pattern has fringes of equal brightness. When the diffraction pattern is superimposed, the central maximum is the brightest and maxima on either side become progressively dimmer.

The main thing to remember about Young's double-slit experiment is that it convinced 19th century scientists that light must be a wave. A stream of particles that passed through $S_{0}$

and then either $S_{1}$ or $S_{2}$ would hit the third screen to create just two bright lines. The interference pattern indicated that waves were passing through $S_{1}$ and $S_{2}$

Today, our view of light is more nuanced: light has the properties of both waves and particles (photons). Visit this site for a good basic description of the wave-particle debate (not part of the California content standards).