Interference in Physics: Constructive and Destructive Waves, Study notes of Physics

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Physics 102: Lecture 20^ • Today’s Lecture will covertextbook sections 25.1, 3-4Physics 102: Lecture 20, Slide 1

Interference

Physics 102: Lecture 20, Slide 2

Superposition

t +1 -

t +1 -

t

Constructive Interference+ +2 -

In Phase

Superposition ACT Physics 102: Lecture 20, Slide 4

Different f

1. Constructive^

2. Destructive^

3. Neither 1.5^1 0.5^0 -0.5-1 -1.5 1.5^1 0.5^0 -0.5-1 -1.

Interference Requirements• Need two (or more) waves• Must have same frequency• Must be coherent Physics 102: Lecture 20, Slide 5

(i.e. waves must have definite phase relation)

Interference for Light …• Can’t produce coherent light from separatesources. (f Physics 102: Lecture 20, Slide 7

14 ≈ 10 Hz) Single source

Two differentpaths Interferencepossible here

• Need two waves from single source takingtwo different paths– Two slits– Reflection (thin films)*– Diffraction^

ACT: Young’s Double SlitPhysics 102: Lecture 20, Slide 8

Screen a distanceL from slits Single source ofmonochromaticlight^ λ

d 2 slits-separatedby d

1. Constructive2) Destructive3) Depends on LL Light waves from a single source travel through 2 slitsbefore meeting on a screen. The interference will be:

Young’s Double Slit ConceptPhysics 102: Lecture 20, Slide 10

Screen a distanceL from slits Single source ofmonochromaticlight^ λ

d L 2 slits-separatedby d

At points where thedifference in pathlength is 0,^ λ,2λ

the screen is bright.(constructive)At points where thedifference in pathlength isthe screen is dark.(destructive)

53 λ λ λ^ ",,^222

Young’s Double Slit Key IdeaPhysics 102: Lecture 20, Slide 11

L

Two rays travel almost exactly the same distance.(screen must be

very far away: L >> d)

Bottom ray travels a little further.Key for interference is this small extra distance.

d Physics 102: Lecture 20, Slide 13

L

Preflight 20.3 θ

y θ θ When this Young’s double slit experiment is placedunder water. The separation y between minima andmaxima1) increases^

2. same^

3. decreases

Physics 102: Lecture 20, Slide 14

Preflight 20.

In the Young double slit experiment, is it possible to seeinterference maxima when the distance between slits issmaller than the wavelength of light?1) Yes

2. No

Reflection + Phase Shifts Physics 102: Lecture 20, Slide 16

n^1 n^2

Incident wave Upon reflection from a boundary between twotransparent materials, the phase of the reflected lightmay change.• If n > n 1 2 • If n < n 1 2

Reflected wave

Thin Film Summary Physics 102: Lecture 20, Slide 17

n = 1.0 (air)^ n^ (thin film)^1 n^2 Determine^ δ,^ number of extra wavelengths for each ray.^1 2 t Ray 1:^ δ= 0 or ½^1 If |(δ–^ δ)| = ½ , 1 ½, 2 ½ ….^2

(m + ½)^ destructive If |(δ–^ δ)| = 0, 1, 2, 3 ….^2

(m)^ constructive

Note: this iswavelength infilm! (λ=^ λ/n^ )film^ o^1 Reflection^ Distance+ 2 t/^ λfilm This isimportant!Ray 2:^ δ= 0 or ½^2

Physics 102: Lecture 20, Slide 19

Did you know?

You should never get immersion oilon microscope lenses that aren’tdesigned for it!The oil is a thin film that can create light anddark spots all over your image!(and it’s really hard to clean off, too!)

Physics 102: Lecture 20, Slide 20

ACT: Thin Film

n=1 (air) n =1.5glass^ n =1.8plastic^ (^1 2) t Blue light^ λ^ = 500 nmincident on a thin film(t = 167 nm) of glasson top of plastic. Theinterference is:(1) constructive(2) destructive(3) neither^ δ=^1 δ=^2 Phase shift =^ δ–^2

δ=^1