1B24 Waves Optics and Acoustics Lecture 27 of 33 (UCL)

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Interference - the oil film

air oil

h

water

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C D=?>A@ <BAE The phase change in the reflection in air from the upper surface of the oil is π; the phase change on reflection at the oil-water interface is 0.

F=?>A@ <B takes G= Iplace H Constructive interference when the total phase difference between a Jray reflected from the surface of the oil "and K a ray which LB Nhas M passed into the oil and been reflected from the water is an integer multiple of 2π. Given that the phase changes on reflection differ Tby S π for the two rays, this requires the path length in oil to be an odd half-integer multiple of the wavelength in the oil.

The film is observed at near-normal incidence, so we may take the geometrical path length in the oil to be 2h.We therefore seek a B Z a ]_b B [1Z \ .]_^`U λ WVYX >ASLU. WVYX > c \ ]_^ wavelength λ in air which satisfies 2 h p> 2 n oil Oilfilm.mcd 1 21/03/99 16:35 dV

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Interference - a film at non-normal incidence

d n1

n2

A θ2

θ1

D

B C

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Interference Fringes - Newton's Rings Side view Radius R Air film

Lens

P

t

Glass plate

Top view

Lens P

r

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1

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