Diffraction

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Diffraction
the spreading of a wave around the edge of a
barrier or through an opening
Monochromatic light consists of only one color
(wavelength).
Polychromatic light consists of more than one
color (wavelength).
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Huygens Principle is used to explain diffraction.
Each point on a wavefront acts as a new
source of disturbance.
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Click here to read about Christiaan Huygens.
Click here and here to view simulations of
Huygens Principle. Huygens Principle can be
used to explain reflection and refraction, as
shown here.
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Youngs Double Slit Interference
x
d
l

L
x
d
x
L
l is the wavelength of light
d is the distance between slits
x is the distance from the central bright band
to the first order bright line
L is the distance from slits to screen
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Click here and here to view simulations of double
slit interference. Notice changes in the
interference pattern as each variable changes.
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Click here, here, here, and here to view single
slit interference simulations. Notice how the
pattern changes as each variable is manipulated.
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Characteristics

• central bright band
• pattern spreads out as wavelength increases
• pattern spreads out as distance to screen
  increases
• pattern spreads out as distance between slits
  decreases

Similar results occur with a narrow single slit.
Single Slit Interference Characteristics

• central bright band
• narrower slit produces wider central band
• larger wavelength gives wider pattern

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Diffraction Gratings
series of many slits etched on film
Diffraction gratings (or replica gratings) will
separate light into its component colors through
diffraction and interference.
They are often used to identify the component
colors of polychromatic light.
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Click here and here to view simulations of
diffraction gratings. Notice the changes in
the diffraction pattern as the variables are
manipulated.
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Thin Film Interference
The color spectrum seen in a soap bubble, or any
thin film, results from the interference of the
reflections of light from the front and back
surfaces of the film.
The colors seen depend on the thickness of
the film. The light most strongly reflected has
a wavelength such that the film thickness is
an odd multiple of l/4. Other wavelengths
will suffer partial or total destructive
interference.