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

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Light is a Wave

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Light is a wave

Light is a Wave

Blue light = 4500Å

Red light = 6000Å

1 Nanometer = 10-9Meters

1 Angstrom = 10-10 Meters


Waves

Waves

wavelength

crest

Frequency =

1/Wavelength

Measured in Hertz (Hz) =

1/Seconds

amplitude

trough


Compression displacement

Compression & Displacement


Interference

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


Is light a particle or a wave

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, λf = c


Diffraction circular aperture

Diffraction: Circular Aperture

A = 2.44λ/D

A is the angular diameter of the Airy Disk, λ is the wavelength of the light, D is the diameter of the pinhole

The wavelength of the light we’re using is 675 nm (6.75 x 10-5cm)


Linear size of the airy disc

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


Diffraction terms

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


Diffraction slit aperture

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: n=1 corresponds to the central fringe


Diffraction double pinhole

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


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