Unit 2.5)Wave properties

Question 1

What is diffraction?

Answer 1

Diffraction is the spreading out of waves when they meet obstacles, such as edges or a slit.

Question 2

When does diffraction occur?

Answer 2

When waves encounter slits or obstacles.

Question 3

what is two source interference?

Answer 3

when two waves pass through the same medium at the same time, they combine together.

Question 4

How two source interference occurs in water waves, sound, microwaves and light ?

Answer 4

Using Water Waves
Two-source interference in can be demonstrated in water using ripple tanks
The diagram below shows diffracted circle shaped water waves from two point sources eg. dropping two pebbles near to each other in a pond
Water waves interference fringes, downloadable AS & A Level Physics revision notes

Water waves interference pattern from a ripple tank

The two waves interfere causing areas of constructive and destructive interference
The lines of maximum displacement occur when all the peaks and troughs line up with those on another wave

Using Sound Waves
Two source interference for sound waves looks very similar to water waves
Sound wave interference experiment, downloadable AS & A Level Physics revision notes

Sound wave interference from two speakers

Sound waves are longitudinal waves so are made up of compressions and rarefactions
Constructive interference occurs when two compressions or two rarefactions line up and the sound appears louder
Destructive interference occurs when a compression lines up with a rarefaction and vice versa. The sound is quieter
This is the technology used in noise-cancelling headphones

Using Microwaves
Two source interference for microwaves can be detected with a moveable microwave detector
Microwave interference experiment, downloadable AS & A Level Physics revision notes
Microwave interference experiment

Constructive interference: regions where the detector picks up a maximum amplitude
Destructive interference: regions where the detector picks up no signal

Using Light Waves
For light rays, such as a laser light through two slits, an interference pattern forms on the screen
Light interference experiment, downloadable AS & A Level Physics revision notes
Laser light interference experiment

Constructive interference is shown as bright fringes on the screen
The highest intensity is in the middle
Destructive interference is shown as the dark fringes on the screen
These have zero intensity

Question 5

Why is a laser useful in showing interference and diffraction?

Answer 5

It produces monochromatic light(same wavelength/colour) meaning diffraction and interference patterns are more defined

Question 6

What was Youngs double slit experiment?

Answer 6

A single light source is directed towards two slits, which each act as a coherent light source. The light interferes constructively and destructively to create an interference pattern.

Question 7

Describe the interference pattern created using white light?

Answer 7

A bright white central maximum flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest.

Question 8

Why does an interference pattern form when light is passed through a single slit?

Answer 8

The light diffracts as it passes through the slit. Where the waves are in phase, constructive interferences occurs making bright fringes. Where the waves are completely out of phase, destructive interferences occurs making a dark fringe.

Question 9

Increasing the slit width increases the width of the central diffraction maximum TRUE OR FALSE?Why?

Answer 9

False-the slit is further away from the wavelength in size so less diffraction occurs-the central maximum becomes narrower and more intense.

Question 10

What is a stationary wave?

Answer 10

A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant.

Question 11

What is a node?

Answer 11

A point on a stationary wave where the displacement is 0

Question 12

What is an antinode?

Answer 12

A point on a stationary wave with maximum displacement.

Question 13

what are the conditions for a stationary wave to be produced?

Answer 13

The waves must be the same frequency, wavelength and amplitude.
They must be travelling in opposite directions.
These conditions are often met when a wave is reflected back on itself.

Question 14

What is the principle of superposition?

Answer 14

If waves from two sources occupy the same region , then the total displacement at any one point is the vector sum of their individual displacements at that point.

Question 15

What are the two types of interferences that arise as a result of superposition?

Answer 15

constructive interference
destructive interference

Question 16

What is constructive interference?

Answer 16

If the two waves arrive at a point in phase, if their crests arrive at exactly the same time they will interfere constructively.

Question 17

What is destructive interference?

Answer 17

If the two waves arrive at a point in antiphase, that is, if the crests of one wave arrive at the same time as the troughs from the other, they will interfere destructively.

Question 18

How are stationary waves produced?
and What is the Internodal distance equation?

Answer 18

A stationary wave is formed from the superpositions of 2 progressive waves with equal amplitude and frequency
travelling in opposite directions.
-Where the waves meet in phase, constructive interference occurs so antinodes(regions of maximum amplitude)form.
-Where the waves meet completely out of phase, destructive interference occurs and nodes(regions of no displacement)form.

The internodal distance is wavelength/2.

Question 19

Describe the first harmonic for a stationary wave with two closed ends?

Answer 19

It consists of 2 nodes at either end an antinode in the middle.
Contexts could include waves on a string, sound
waves in air, microwaves. Candidates should know
that there are nodes at the ends of a string if these
are fixed.

Question 20

Describe the second harmonic for a stationary wave with one open end and one closed end?

Answer 20

It consists of two nodes and two antinodes, with one of the nodes at the closed end and one of the antinodes at the open end.

Question 21

What is phase difference and what is it measured in?

Answer 21

How much a particle/wave lags behind another particle/wave. It is measured in radians, degrees or fractions of a cycle.

Question 22

Define coherence?

Answer 22

Coherent waves have a fixed phase difference and the same frequency and wavelength.

Question 23

State 2 applications of diffraction gratings?

Answer 23

1.Splitting up light from stars to make line absorption spectra-used to identify elements present in stars.
2.X-ray crystallography: a crystal sheet acts as the diffraction grating and the x-rays pass through-used to find the spacing between atoms.

Question 24

When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?

Answer 24

The wavelength of light has increased so it will diffract more meaning the orders will become further apart.

Question 25

What is diffraction?

Answer 25

The spreading out of waves when they pass through or around a gap.

Question 26

How did young’s double split experiment provide evidence for the wave nature of light?

Answer 26

Diffraction and interference are wave properties hence the interference pattern of light shows that light has wave properties.

Question 27

What is path difference?

Answer 27

The difference in distance travelled by 2 waves.

Question 28

How could you investigate stationary sound waves?

Answer 28

Place a speaker at one end of a closed glass tube, lay powder across the bottom of the tube, it will be shaken from the antinodes and settle at the nodes. The distance between each node is half a wavelength.

Question 29

What is the frequency of the first harmonic of a string length 2m, mass 0.03kg with a mass of 2kg hanging off it?

Answer 29

f=1/2L (√T/μ)
T= tension = 2x9.81 = 19.62N
μ=mass/unit length = 0.03/2 = 0.015 kg/m
f=(1/4)x(sq.rt: 19.62/0.015)
f=9hz

Question 30

Only light can produce interference patterns, True or False?

Answer 30

False, interference patterns can be formed by sound waves and all EM waves too.

Question 31

What formula is associated with Young’s double slit experiment?

Answer 31

W=λD/s
w-fringe spacing
λ- wavelength of light used
D- distance between the slits and the screen where interference pattern is observed.
S- slit separation(distance between the two slits)
or
λ=SW/D

Question 32

What is the path difference?

Answer 32

The difference in the distance travelled by two waves arriving at a particular point on the screen.

Question 33

How can constructive interference occur in path difference?

Answer 33

If the path difference is a whole number of wavelengths then the waves arrive at P in phase which means that constructive interference occurs.

Question 34

How can destructive interference occur in path difference?

Answer 34

If the path difference is an odd number of half wavelengths then the waves arrive at P in antiphase which means that destructive interference occurs.

Question 35

What is a diffraction grating?

Answer 35

an opaque material with many narrow, parallel, equally spaced slits.

Question 36

if monochromatic light is incident normally(perpindicular) on a diffraction grating what happens?

Answer 36

a pattern of narrow, bright fringes is produced.
(These fringes are also referred to as orders)
These fringes are caused by constructive interference.

Question 37

What is the central bright fringe on a diffraction grating referred to as?

Answer 37

The zeroth order maximum.

Question 38

What is the diffraction grating equation?

Answer 38

From the triangle ABC it can be seen that:

sin⁡θ=
AB
BC
​

But AB = slit separation, d, and BC=path difference

Therefore:

sin⁡θ=
d
BC
​

Re-arranging:

path difference, BC=d sin⁡θ

As we have seen, the condition for constructive interference is:

path difference=nλ

where n is an integer (whole number).

Therefore:

d sin⁡θ=nλ

This is the diffraction grating equation.

Question 39

A diffraction grating will often be described as having a certain number of lines per mm or lines per cm and you will need to calculate d.

For example, if the diffraction grating has 600 lines/mm, then:

Answer 39

d = 1/600 = 1.67x10^-3 mm = 1.67x10^-6m

Question 40

When is there little diffraction?

Answer 40

When the wavelength is smaller then the dimensions of the of the obstacle or slit little diffraction occurs.

Question 41

What happens if wavelength is equal to or greater then the width of a slit?

Answer 41

Waves spread as roughly semi-circular wavefronts.

Question 42

What if wavelength is less then the slit width?

Answer 42

the main beam spreads through less then 180 degrees.

Question 43

Historical importance of Young’s experiment?

Answer 43

Earliest demonstration of the wave-like properties of light.

Question 44

For a diffraction grating a very small d makes beams(orders)____ then in young’s experiment?

Answer 44

A very small d makes beams(orders) much further apart then in young’s experiment.

Question 45

Large number of slits makes the bright beams much___?

Answer 45

sharper.

Question 46

coherent sources?

Answer 46

coherent sources are
monochromatic with wavefronts continuous
across the width of the beam and, (when
comparing more than one source) with a
constant phase relationship.

Question 47

Examples of coherent and incoherent sources?

Answer 47

Coherent-laser
incoherent-tungsten filament lamp, led bulbs.

Question 48

For Two source interference to be observed sources must?

Answer 48

have zero or constant phase difference and have oscillations in the same direction.