The Physics
Hypertextbook
Opus in profectus

# Diffraction and Interference (Light)

## Summary

• Diffraction
• is the bending or spreading of a wave around an obstacle or through an opening.
• is most apparent when the size of the obstacle or opening (a) and the wavelength of the wave (λ) are of the same order (a ~ λ).

• A shadow is a region behind an obstacle into which a wave does not easily diffract.
• Umbra: a region of total shadow; the source of the wave is completely obscured.
• Penumbra: a region of partial shadow; the source of the wave is partially obscured.
• Sound vs. Light
• Sound waves are millions of times longer than light waves.
• Diffraction is more easily observed for sound than light in everyday experience.
• You can hear around corners, but you can't see around them.
• Light sources cast noticeable shadows.
• Interference
• When waves occupy the same place at the same time they interfere or superpose.
• The resulting disturbance is the sum of the individual disturbances at every point in space and time.
• This is known as the principle of linear superposition.
• Diffraction and interference occur together in many situations.
• Double slit interference or double source interference
• This was the original experiment that showed that light is a wave.
• It was first performed by the English scientist Thomas Young at the end of the 18th century and is often called Young's double slit experiment.
• Description
• Monochromatic light is incident on an opaque barrier with two parallel, narrow openings (often called slits).
• 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.
• Single slit interference or single source interference or self interference
• Description
• Monochromatic light is incident upon a single opening or obstacle.
• Light diffracts through the opening or around the obstacle.
• An interference pattern arises from the interference of light waves from extreme ends of the opening or opposite sides of the obstacle.
• Multiple slit interference or multiple source interference
• Description
• Monochromatic light is incident upon a series of equally small, equally spaced, equally shaped openings or obstacles.
• Light diffracts through the openings or around the obstacles.
• An interference pattern arises where the bright regions are distinct and widely separated.
• Everyday examples
• grooves in phonograph records
• CDs, DVDs
• scratches in a car windshield caused by rough wiper blades
• A diffraction grating is…
• an optical device consisting of a series of tightly spaced, narrow grooves etched onto a plastic, glass, or metal surface.
• a device for illustrating multiple source interference in the classroom
• a device for producing a spectrum

Widely spaced interference fringes of red laser light through a diffraction grating

• Spectra come in two basic types depending on the nature of the source of light.
• Continuous spectra
• contain every frequency or wavelength of light without interruption
• Mathematically, there is a nonzero value of the intensity for every value of frequency or wavelength.
• Imagine a smooth curve on a graph where frequency or wavelength is on the x-axis and intensity is on the y-axis.
• are produced by "hot" objects
• This is an informal way of saying that the characteristics of the spectrum are affected by the temperature of the source.

daylight

• Discrete spectra
• are made up of a collection of separate and individual frequencies or wavelengths
• Mathematically, there is a finite set of wavelengths or frequencies, each with its own intensity.
• Imagine a data table with frequency or wavelength in the first column and intensity in the second column that has a limited number of unique entries.
• are produced when bound electrons experience a drop in energy level
• Since different elements have different energy level structures, discrete spectra are often described as the "fingerprints" of the elements.
• are often photographed using narrow sources or narrow openings
• This is why discrete spectra are often said to be made up of spectral lines.
• excited mercury

excited helium

fluorescent light