2.4 Superposition, Interference and Diffraction

Question 1

Principle of superposition

Answer 1

When 2 waves meetThe total resultant displacement of the medium at any point in space is given by the vector sum of the individual displacements

Question 2

What is a standing wave

Answer 2

Wave in which no energy is transferred

Question 3

Conditions required to produce standing waves

Answer 3

2 coherent waves
Travelling in opposite directions
Superimpose

Question 4

Where is a node / anti node located

Answer 4

Node = point where s=0
Anti node = point where s=max

Question 5

State the relationship between f and lambda

Answer 5

Inversely proportional

Question 6

What is the fundamental mode of vibration

Answer 6

The lowest possible frequency that gives a pattern involving 2 nodes and an anti node

Question 7

What is the length of the string in the first mode of vibration in terms of lambda

Answer 7

L = &/2

Question 8

What does L = in the first mode of vibration for standing waves in air columns in terms of lambda
Same for 2nd and 3rd
Frequency

Answer 8

1/4 & f0
3/4 & 3f0
5/4 & 5f0

Question 9

Conditions for observable interference

Answer 9

Coherent
Constant phase difference
Similar /same amplitudes

Question 10

What is monochromatic light

Answer 10

Light of a single wavelength

Question 11

Describe how to determine the speed of sound in air using a resonance tube

Answer 11

Tuning fork of 250Hz is made to vibrate and then placed above closed end glass tube.
The tube is raised until the sound becomes noticeably higher. This is the fundamental mode of vibration
Measure the distance the tube was raised.
Repeat
Use v = 4Lf to find the speed of sound in air

&=4L
fo = 1/&o

Question 12

If 2 waves are in phase what is their path difference
Is there constructive or destructive interference

Answer 12

n&
Constructive

Question 13

If 2 waves are out of phase what is their path difference

Answer 13

(n+0.5)&

Question 14

Describe Young’s slits experiment to measure the & of monochromatic light (3)

Answer 14

A light source is filtered to give monochromatic light
The single slit diffracts light to make light hitting double slit coherent
The double slit acts as 2 coherent sources of light

Question 15

When are the best results acquired during young’s slits exp
What values should the slit separation be between
What should the distance between the double slits and the screen be

Answer 15

Low light conditions
0.1-0.5mm
1-4m

Question 16

Precautions to take when using a laser (2)

Answer 16

Don’t look directly at it
Project onto a screen

Question 17

Equation for calculating & during young’s double slit exp

Answer 17

& = ay/d

a = slit separation (0.1-0.5mm)
y = interference fringe separation (distance between 2 light patches on screen)
d = distance from double slits to screen (1-4m)

Question 18

What is diffraction

Answer 18

Spreading of waves when they pass through an aperture/round an obstacle

Question 19

What 2 conditions make diffraction more profound

Answer 19

& is bigger
Gap width is narrower

Question 20

When is diffraction at its greatest

Answer 20

& = gap width

Question 21

What is a diffraction grating

Answer 21

Glass /Perspex material with many parallel equally spaced opaque lines

Question 22

Describe the use of a diffraction grating and laser to measure & (recall equation)

Answer 22

Set laser behind grating
Turn on laser so diffraction pattern is projected on to screen
Use n& = d sin(theta) to calculate & where;
n = number of orders
*d= slit separation/m (=1/number of slits per m)
Theta = angle from 0 order (middle) to order in question

Question 23

What should you remember to do when calculating n in the diffraction grating equation (2 things)

Answer 23

Take theta to be 90
Round down

Question 24

When drawing the single slit diffraction pattern remember…(2)

Answer 24

The centre is by far the largest and is 2x wider than the others
Each outer fringe is the same width

Question 25

For d in the diffraction grating equation, what do you do if you’re given the no. of slits in 1m

Answer 25

1/ number of slits per meter

Question 26

Path difference

Answer 26

Difference in path distance travelled by 2 waves which meet at the same point in terms of &

Question 27

What does collimated mean

Answer 27

Light rays are parallel

Question 28

What is the time base setting of a CRO

Answer 28

Time /cm or division

Question 29

What does CRO stand for

Answer 29

Cathode ray oscilloscope

Question 30

Coherent

Answer 30

Same wavelength, frequency and constant phase difference