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- Light diffracts through each opening. The overlapping light waves combine to produce interference fringes (parallel bright and dark bands) when projected onto a flat surface (often called a screen). Interference can be produced for other shaped openings, so in general an interference pattern is produced.
(a) Light spreads out (diffracts) from each slit, because the slits are narrow. The waves overlap and interfere constructively (bright lines) and destructively (dark regions). You can only see the effect if the light falls onto a screen and is scattered into your eyes.
The effects of diffraction are often seen in everyday life. The most striking examples of diffraction are those that involve light; for example, the closely spaced tracks on a CD or DVD act as a diffraction grating to form the familiar rainbow pattern seen when looking at a disc.
Diffraction effects are the deviations from rectilinear propagation that occurs when light encounters edges, screens and apertures. These effects were explained in 1816 by French physicist Augustin-Jean Fresnel.
Explain behaviors of waves, including reflection, refraction, diffraction, interference, and coherence, and describe applications based on these behaviors. Perform calculations related to applications based on wave properties of light.
- Overview
- Doppler effect
Fresnel presented much of his work on diffraction as an entry to a competition on the subject sponsored by the French Academy of Sciences. The committee of judges included a number of prominent advocates of Newton’s corpuscular model of light, one of whom, Siméon-Denis Poisson, pointed out that Fresnel’s model predicted a seemingly absurd result: I...
In 1842 Austrian physicist Christian Doppler established that the apparent frequency of sound waves from an approaching source is greater than the frequency emitted by the source and that the apparent frequency of a receding source is lower. The Doppler effect, which is easily noticed with approaching or receding police sirens, also applies to light waves. The light from an approaching source is shifted up in frequency, or blueshifted, while light from a receding source is shifted down in frequency, or redshifted. The frequency shift depends on the velocity of the source relative to the observer; for velocities much less than the speed of light, the shift is proportional to the velocity.
The observation of Doppler shifts in atomic spectral lines is a powerful tool to measure relative motion in astronomy. Most notably, redshifted light from distant galaxies is the primary evidence for the general expansion of the universe. There are a host of other astronomical applications, including the determination of binary star orbits and the rotation rates of galaxies. The most common terrestrial application of the Doppler effect occurs in radar systems. Electromagnetic waves reflected from a moving object undergo Doppler shifts that can then be used to determine the object’s speed. In these applications, ranging from monitoring automobile speeds to monitoring wind speeds in the atmosphere, radio waves or microwaves are used instead of visible light.
How Does it Work? Light travels in a wave formation. When the light hits an obstacle, it is able to move through openings. The smaller, the opening, the wider the light spreads out. Part of this is that the light interacts with the edge of objects. If the obstacle doesn't absorb all the light, some is diffracted.
Diffraction gratings work both for transmission of light, as in Figure \(\PageIndex{3}\), and for reflection of light, as on butterfly wings and the Australian opal in Figure \(\PageIndex{4a}\). Natural diffraction gratings also occur in the feathers of certain birds such as the hummingbird.
