Describe and explain how the spacing of fringes in an interference pattern is affected by:

Do now:Friday, April 21, 2023

1) The wavelength of a wave If λ increases spacing increases

2) The distance from the screenIf L increases spacing increases

3) The slit spacingIf d increases spacing decreases

Click if you need a hint...dx

nL

n L

xd

Check withPhet!

Today’s Task:

Activity 4B, Questions 1, 2, 3, 4

Reminder:

1) Homework: Interference questions,

- due Tues 3rd June

2) End-of-topic test: Fri 6th June

Diffraction Gratings

A diffraction grating is a series of many (eg. 6000 per cm) very fine parallel slits, closely spaced, on a piece of glass or plastic

Diffraction Gratings - demo

The interference pattern produced is similar to the double slit pattern

The differences are:There are lots of slits so fringes are brighterSlits are closer together so fringes are widely

spreadSlits are narrow so the light is diffracted through

a wider angle (almost 180°)

Diffraction Gratings

Use the same formulaHowever, often N, the number of slits per m (or

slits per cm) is given.Slit spacing d is related to N by:

Nd

1

Check with

N=1,2,3…

Light from a laser passes through a diffraction grating of 2000 lines per cm.

The diagram shows the measurements made.

Calculate the wavelength of the light.

Example 1:

Diffraction Gratings

A CD surface acts like a diffraction grating because it has many finely spaced groves on it’s surface.

Light is reflected off the disc, but produces spectra (interference patterns) in the same way

Diffraction gratings are a useful tool for determining the chemical composition of substances – measure θ calculate d

Diffraction Gratings

Diffraction gratings are a useful tool for determining the chemical composition of substances.

This is done by analysing the light produced when the atoms are excited by heat or electricity.

This is how astronomers can tell what stars are made of.

Tasks:

1)Activity 4B

2)Activity 4C

Instead of shining monochromatic light (e.g. a laser) through a diffraction grating, what would happen if we shone a polychromatic light source (e.g. white

light)?

Diffraction Gratings

If we shine white light onto a grating, we produce a series of spectra each side of the central fringe

This is because white light is made of many frequencies (colours) which all diffract at slightly different angles

White n=0

violet

violet

red

red

n=1

n=1

n=2

n=2

Diffraction Grating

Diffraction Gratings

The spectrum produced by a grating is more widely spread that that produced by a prism

It is also the other way around (i.e. red is diffracted the most) – why?

Wavelength of red light is nearest

to slit spacing