Lecture 30

Friday, April 17Review two slits, gratings

Thin Film Interference

Two slit interference

• Bright fringes occur at angles

• On the observing screen,

sin mm d

tanm my L

Workbook Problems

• Graphical representation• WB 17: 1-9• 1 and 2:• 3: Shadow when d>>λ• 4: Two slit diffraction to compare with 3• 5: Graphical picture of two slit diffraction

paths.

0n n

Grating Problems

• **6: Graphical path differences for four slits• 7. What happens when wavelength is

doubled?• 8: What happens when slit width is doubled?• 9. Δr = 2λ and ½λ

The Intensity Pattern Due to a Diffraction Grating

Slide 17-14

The Fringes Become Very Narrow as the Number of Slits is Increased

Slide 17-15

All waves spread out after passing through a small enough gap in a barrier. This phenomenon is

known as

Ant

irefle

ction

dou

ble-sli

t in...

Refr

actio

n

diff

ract

ion

25% 25%25%25%1. Antireflection

2. double-slit interference

3. Refraction

4. diffraction

Problems due Today

• 17: 1, 2, 4, 7, 9, 10, 12• 2: speed in glass• 4: wavelength in glass• 7: two slit interference• 9: ditto, given d and y5 – y1, find λ• 10: two slits given fringe spacing, λ• 12: Δr

Thin-Film Interference

Slide 17-17

Phase Changes Due to Reflection

Slide 17-18

Workbook Problem 10-11

• Wave reflected at various interfaces

A film of oil (noil = 1.2) floats on top of an unknown fluid “X” (with unknown index of refraction nx). The thickness of the oil film is known to be very small, on the order of 10 nm. A beam of white light illuminates the oil from the top, and you

observe that there is very little reflected light, much less reflection than at an interface between air and X. What can

you say about the index of refraction of X?

nx >

1.2

nx <

1.2

nx =

1.2

Ther

e is i

nsuf..

.

25% 25%25%25%

1. nx > 1.2

2. nx < 1.2

3. nx = 1.2

4. There is insufficient information to tell.

Assignment for Monday

• Enjoy your weekend