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# Light is a Wave - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about ' Light is a Wave' - neith

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Presentation Transcript

Blue light = 4500Å

Red light = 6000Å

1 Nanometer = 10-9Meters

1 Angstrom = 10-10 Meters

wavelength

crest

Frequency =

1/Wavelength

Measured in Hertz (Hz) =

1/Seconds

amplitude

trough

• 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

• 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

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)

• 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

• 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

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

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