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Illustration 38.2: Application of Diffraction Gratings

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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.

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