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Light is a Wave

Blue light = 4500Å

Red light = 6000Å

1 Nanometer = 10-9Meters

1 Angstrom = 10-10 Meters

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?

- 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

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

- 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 = 2.44l/D

s

l

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

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

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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