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Light is a Wave

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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  1. Light is a Wave Blue light = 4500Å Red light = 6000Å 1 Nanometer = 10-9Meters 1 Angstrom = 10-10 Meters

  2. Waves wavelength crest Frequency = 1/Wavelength Measured in Hertz (Hz) = 1/Seconds amplitude trough

  3. Compression & Displacement

  4. 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

  5. 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

  6. 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)

  7. 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

  8. 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

  9. 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

  10. 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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