Module 4 - Section 2 - Two-Source Inteference

Question 1

Why so it easy to demonstrate two-source interference with water and sound waves ?

Answer 1

This is because they have got wavelengths of a handy size that you can measure

Question 2

How can you demonstrate two source interference with water waves ?

Answer 2

You need coherent sources as then the interference patterns are easier to see - coherent means that the frequency and wavelengths have to be the same - then the trick is to use the same oscillator to drive both sources meaning both waves - as then the waves will be coherent - for water omen vibrator drives two dippers

Question 3

How can you demonstrate two-source interference for EM radiation ?

Answer 3

You can use two coherent light sources or use a single laser and shine it through two slits ( at the same time ) - so the laser has to be big enough to shine through both slits at the same time - this is called youngs double slit experiment

Question 4

How can you do the Youngs double slit experiment ?

Answer 4

• the laser light is coherent and monochromatic - meaning that there is only one wavelength present
• The slits have to be the same size as the wavelength of the laser light so that the laser light gets diffracted - then the light diffracting from the slits acts like two coherent point sources
• you get a pattern of light and dark fringes, depending on whether constructive or destructive interference happens

The constructive interference will be the light fringes and the destructive inference will be pie/2 and will be dark fringes and the light fringes will be pie

Question 5

How can you figure out the wavelength with the double slit experiment ?

Answer 5

You have to measure the fringe spacing - the distance from one centre of the light fringe to the centre of the light fringe next to it - or centre of dark to centre of dark fringe

The fringes are so tiny that its very hard to get an accurate value for their spacing (x) - its easier to measure across several fringes and then just divide by the number of rod fringe spacing between them

Question 6

What do you need to change to do the double slit experiment but just with microwaves instead ?

Answer 6

Your have to replace the laser with 2 microwave transmitter cones attached to the same signal generator and also replace the screen with a microwave receiver probe - then you just move the probe up and down one line in front and away of the microwave generators - and then you should get readings of strong and weak signals just like the dark and light fringes seen in the double slit experiment

Question 7

How can you work out the wavelength with the Youngs double slit formula ?

Answer 7

X = wavelength x D / a

X = fringe spacing
Landa = wavelength
D = distance from slits to screen
A = spacing between slits

Question 8

How was the Youngs double slit experiment evidence for light being a wave ?

Answer 8

Youngs double slit experiment showed that light could diffract through the narrow slits and interfere to form the interference pattern on screen