Unit 13: EM Radiation and Waves

The Wave Nature of Light

Unit 13 Outline

• Waves Versus Particles

•EM Waves and EM Spectrum

•Scattering of Light by the Atmosphere

• The Visible Spectrum and Dispersion,

Rainbows

•Interference by Thin Films

•Polarization

•

Do Now

Double Slit Interference:

A pair of narrow, parallel slits separated by

0.250mm are illuminated by the green

component from a mercury vapor lamp (λ =

546.1nm). The interference pattern is

observed on a screen 1.20 m from the plane

of the parallel slits. Calculate the distance

from the central maximum to the first bright

fringe.

Given:

L=1.20m

m=1

Find: y-?

Solution:

θ=0.125°

md 310250.0

m910546

tanLybright

00218.010250.0

105461sin

3

9

dm

mLybright 00262.0)0021.0(20.1tan

Thin Film Interference

Wave Nature of Light

• In the early nineteenth century, Thomas

Young showed that the interference of

light passing through two slits produces an

interference pattern when projected on a

screen.

• Now we will investigate another example

of interference that provides further

evidence in support of the wavelike

behavior of light.

Thin Film Interference • The bright color reflected from soap

bubbles or streaks of color in a thin film of

oil resting upon a water puddle or concrete

driveway - thin film interference.

• Thin film interference provides another

evidence for the wave behavior of light.

• Thin film interference present only if the

film thickness is greater than a few

wavelengths.

Interference by Thin Films

• Fast ->slow phase

change

• Slow->fast no phase

change

• Reflected rays 1 and

2 are out of phase.

11 n

5.12 n

13n

Я R

Interference by Thin Films

• Reflected rays 1 and 2

are out of phase.

• Path length difference is

multiple of wavelength –

destructive interference.

• t- thickness of the film

• m=1,2,3…

• - wavelength in the

film

11 n

5.12 n

13n

Я R

t

nmt 2

n

Interference by Thin Films

- wavelength in air

- wavelength in

soap water

n – index of refraction

f – frequency (is fixed)

Destructive

interference (dark)

Constrictive

interference (bright)

f

cair

nair

f

vn

v

cn

n

airn

nmt air

2

nmt air

)2

1(2

24.8 Interference by Thin Films

The wavelength of the

light will be different

in the oil and the air,

and the reflections at

points A and B may or

may not involve

reflection.

24.8 Interference by Thin Films

A similar effect takes place when a shallowly

curved piece of glass is placed on a flat one.

When viewed from above, concentric circles

appear that are called Newton’s rings.

24.8 Interference by Thin Films

One can also create a thin film of air by creating

a wedge-shaped gap between two pieces of

glass.

24.8 Interference by Thin Films

Problem Solving: Interference

1. Interference occurs when two or more waves

arrive simultaneously at the same point in

space.

2. Constructive interference occurs when the

waves are in phase.

3. Destructive interference occurs when the

waves are out of phase.

4. An extra half-wavelength shift occurs when

light reflects from a medium with higher

refractive index.

Example

A soap bubble appears green (λ=540nm) at

the point of its front surface nearest the

viewer. What is the smallest thickness the

soap bubble could have? Assume n=1.35.

Solution: Phase change at only one surface

is equivalent to a path difference (m=0). 2

1

nmt air

)2

1(2 nm

nm

nt air 100

)35.1)(4(

540

4