7.7.1: Illustrations

Illustration 1: Single Slit Diffraction

This applet calculates seven frames and then runs continuously. For a large number of sources, or for very small wavelengths, this calculation can take some time, so let the applet finish calculating all seven frames.

To model diffraction from a single slit, we can think of the light entering the slit as point sources for the light exiting (this is effectively Huygen's principle; see Illustration 34.3). The light from these point sources interfere with each other, and diffraction is due to this interference. Restart.

So, in the animation small slit you see five point sources generating the light passing through the slit and the interference pattern from the point sources (diffraction is due to the interference of the waves). Notice that the waves spread out from the slit and that the width of the light (waves) leaving the slit is wider than the slit. It looks as if light "bends" around the corner. Without diffraction, the light exiting the slit would be of the same width as the slit itself.

• Now look at light passing through a slightly wider slit. What is the difference in the effect of the small slit and the wider slit on the light passing through?
• If you change the wavelength of the source, the diffraction pattern also changes. Look at a source with a longer wavelength (represented by red color). Then observe a shorter wavelength (represented by blue color). What is the effect of the longer wavelength on the width of the light leaving the slit? In diffraction, as waves pass through a slit, the size of the slit and the wavelength determine how much the waves appear to "bend" around the slit.

For an example of the effect of wavelength and slit size on diffraction, think of the door to your room as a slit. If the wavelength is much smaller than the slit (visible light passing through a door, for example), there is no noticeable diffraction (you see a straight shadow of the door frame). But if the wavelength is much larger than the slit (like a sound wave), there is noticeable diffraction (sound from down the hall bends into your room. It also reflects into your room, so it is hard to separate the effects of diffraction from reflection.). If the wavelength of light were the size of the door, you would see a fuzzy shadow of the door frame.

Illustration authored by Anne J. Cox.

Illustration 2: Application of Diffraction Gratings

This animation models a diffraction grating that is a series of parallel slits. As you change the wavelength, notice where the bright spots are. These are the spots where light traveling through different paths interferes constructively. This is due to the diffraction of the light from the different slits, which creates path differences. Restart.

The central line is from the light rays that constructively interfere at the center. The lines above and below the central lines are spots of constructive interference where light from one slit has traveled one complete wavelength farther than light from an adjacent slit. These points are called the first-order maxima. Similarly, the rays at the top and bottom of the screen, second-order maxima, are rays where the light from one slit has traveled two complete wavelengths of light farther than light from its neighbor. This model is a bit misleading because the light is dimmer for higher-order diffraction peaks, and here the brightness is the same for the two orders.

Diffraction gratings are used to study the spectrum of light from different elements. When an atom gets extra energy (is excited from its ground state), it releases energy in the form of an electromagnetic wave. The wavelength of the light released depends on the energy levels inside the atom. Each atom has its own unique, discrete light spectrum (different wavelengths of light emitted when the atom releases its extra energy). So, if the light from excited atoms goes through a diffraction grating, you can see the spectrum for that element. Look at what light from excited hydrogen atoms would look like with this diffraction grating. This is one way to determine what elements are in an unknown substance. White light sources have a continuous spectrum (like the white light spectrum). However, as the light from the interior of the Sun or other stars passes through the gas in the outer atmosphere of the star, that gas absorbs light at its own unique spectral wavelength. Hydrogen, would, for example absorb light at the wavelengths in its discrete spectrum. Looking at the light from the sun and other stars through a diffraction grating, astronomers can determine what elements are in the sun and stars by the spectral lines that are missing from the white light spectra.

Illustration authored by Anne J. Cox.
Script authored by Anne J. Cox and Morten Brydensholt.

Physlets were developed at Davidson College and converted from Java to JavaScript using the SwingJS system developed at St. Olaf College.