Light is a Wave

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Light is a Wave
Blue light 4500Å
Red light 6000Å
1 Nanometer 10-9Meters
1 Angstrom 10-10 Meters
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Waves
wavelength
crest
Frequency 1/Wavelength Measured in Hertz (Hz)
1/Seconds
amplitude
trough
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Compression Displacement
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Interference

• In phase addition of two waves gives a third wave
  with an amplitude equal to the sum of the first
  two waves
• Out of phase addition of two waves gives a third
  wave with an amplitude equal to the difference of
  the first two waves
• Green and Red are the first two waves, black dots
  are the result after addition
• For two equal waves
• In phase produces third wave of twice the
  amplitude
• Out of phase produces third wave with zero
  amplitude

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Is Light a Particle or a Wave?

• It's both!
• In certain experiments light acts as a stream of
  particles (photons) bombarding a surface
• In other experiments light must be considered as
  a wave traveling through space
• Amplitude corrisponds to brightness
• Light is a displacement wave although it
  requires no medium to travel through
• Wavelength determines the color of the light, ?f
  c

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Diffraction Circular Aperture
A 2.44?/D A is the angular diameter of the Airy
Disk, ? is the wavelength of the light, D is the
diameter of the pinhole The wavelength of the
light were using is 675 nm (6.75 x 10-5cm)
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Linear Size of the Airy Disc

• We know A 2.44?/D
• If s is the diameter of the disk and l is the
  distance from the pinhole to the image
• A s/l from the small angle approximation
• s 2.44?l/D

s
l
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Diffraction Terms

• Constructive Interference leads to bright spots
  on the screen where the amplitude is increased
• Destructive Interference leads to dark spots on
  the screen where the amplitude is decreased
• As the aperture decreases in size the central
  bright point (Airy Disc) increases in size and
  decreases in power it looks bigger and fainter
• This leads to smaller telescopes, i.e. smaller
  apertures, having poorer resolution than larger
  telescopes
• Resolution defines how close objects can be
  together and still appear as separate objects

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Diffraction Slit Aperture
s nl?/2D l is the distance from the slit to the
image D is now the width of the slit n is an
integer If n is odd, it is the distance to a
bright fringe If n is even, it is the distance to
a dark fringe NOTE n1 corresponds to the
central fringe
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Diffraction Double Pinhole
D
s/3
Screen
l
s l?/D D is the distance between the two
pinholes s is the spacing between two adjacent
fringes l is the distance from the pinholes to
the image