Questions and Answers
What is the wavelength of light emitted by the common He-Ne laser?
What is the maximum value that sin θ can have?
What is the equation that describes constructive interference?
What is the highest-order constructive interference possible with the given system?
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What is the relationship between the angle for constructive interference and the wavelength?
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What is the purpose of using interference patterns?
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What is the value of m that corresponds to the maximum diffraction angle?
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What is the condition for constructive interference?
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What is the reason why interference patterns do not have an infinite number of lines?
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What is the relationship between the number of fringes and the wavelength?
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Study Notes
Young's Double Slit Experiment
- In 1801, Thomas Young performed the double slit experiment, which showed that light is a wave.
- Young's experiment involved passing pure-wavelength light through a pair of vertical slits, creating a pattern of numerous vertical lines spread out horizontally on a screen.
- The acceptance of the wave character of light came as a result of Young's experiment.
- The wave character of light is not ordinarily observed due to the need for light to interact with something small, such as closely spaced slits, to show pronounced wave effects.
- Young first passed light from a single source through a single slit to make the light coherent (in phase or having a definite phase relationship).
- Then, he passed the light through a double slit, providing two coherent light sources that interfered constructively or destructively.
Interference Patterns
- Constructive interference occurs when identical waves are in phase, resulting in bright lines on the screen.
- Destructive interference occurs when identical waves are exactly out of phase or shifted by half a wavelength, resulting in dark regions on the screen.
- The light must fall on a screen and be scattered into our eyes for us to see the pattern.
- The angles at which constructive and destructive interference occur depend on the wavelength and the distance between the slits.
Path Length Difference and Interference
- The path length difference between the two waves from the slits to a common point on the screen is given by d sin θ, where d is the distance between the slits and θ is the angle from the original direction of the beam.
- For constructive interference, the path length difference must be an integral multiple of the wavelength, or d sin θ = mλ, for m = 0, 1, −1, 2, −2,...
- For destructive interference, the path length difference must be a half-integral multiple of the wavelength, or d sin θ = (m+1/2)λ, for m = 0, 1, −1, 2, −2,...
- The order of the interference is given by m.
Double Slit Interference Pattern
- The intensity of the bright fringes falls off on either side, being brightest at the center.
- The closer the slits are, the more the bright fringes spread out.
- Small d gives large θ, hence a large wave effect.
Applications of Double Slit Interference
- Interference patterns can be used to measure wavelengths.
- The technique is still widely used to measure electromagnetic spectra.
- For a given order, the angle for constructive interference increases with λ, so that spectra can be obtained.
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Description
Learn about the famous experiment that proved light is a wave, overcoming Newton's objections to Christiaan Huygens' wave theory of light.