M4 Waves - Part 2

Question 1

What is the principle of superposition of waves

Answer 1

when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements

Question 2

what is constructive interference

Answer 2

when 2 waves meet, if their displacements are in the same direction, they combine to give a bigger displacement
the points are known as maxima

Question 3

what is destructive interference

Answer 3

if a wave with a positive displacement meets a wave with a negative displacement, they cancel each other out
when it’s equal and opposite displacements they completely cancel out (total destructive interference)
the points are known as minima

Question 4

what is coherence

Answer 4

waves are emitted from 2 sources with a constant phase difference

Question 5

what does it mean if waves are coherent

Answer 5

a constant phase difference/relationship

same frequency and wavelength

Question 6

what do the waves have to be for interference occur

Answer 6

when two waves are coherent

Question 7

what are examples of interference

Answer 7

water waves
sound waves ( 2 speakers connected to same signal generator)

Question 8

what is the equation for path difference in constructive interference

Answer 8

path difference =nλ

Question 9

what is the equation for path difference in destructive interference

Answer 9

path difference =(n+0.5)λ

Question 10

in interference, what is it called when the path difference is 0

Answer 10

central maxima

Question 11

in interference, when there is phase difference of 2π, what is the path difference

Answer 11

1 wavelength

Question 12

What is observed in Young’s Slits Experiment

Answer 12

Light from a single source directed to a double slit arrangement
Where the light from these slits overlaps, interference occurs and a pattern of bright and dark fringes is observed

Question 13

In young slits experiment, why are the two light sources derived from a single laser

Answer 13

To ensure the two interfering sources of light are coherent

Question 14

In Young’s slits experiment, how do we ensure that maximum diffraction occurs at the slits

Answer 14

Make the size of the slit in the same order of magnitude as the wavelength of the wave
Light is 10^-7 m

Question 15

In Young’s slits experiment, explain why there is a bright fringe at the centre of the pattern

Answer 15

The light waves are in phase with each other as they leave the source
The waves travel the same distance so constructive interference occurs and a bright fringe results

Question 16

In the equation
λ = ax / D
What does it all mean

Answer 16

In Young’s double slit experiment

Wavelength = ( separation of the slits X fringe separation) / slit to screen distance

Question 17

In Young’s double slit experiment, what happens to the fringe spacing as
A) wavelength increases
B) separation between slits increases
C) distance between slit and screen increases

Answer 17

A) increases
B) decreases
C) increases

Question 18

How does the fringe pattern change as you cover the first slit

Answer 18

And interference pattern is formed

Question 19

How does the fringe pattern change if there was red light at the first slit and blue light at the second slit

Answer 19

The frequencies aren’t the same and therefore the sources aren’t coherent as wavelength is different
Therefore no interference pattern occurs

Question 20

What is the equation to find the critical angle from one substance to the next

Answer 20

sinC = n2 / n1

N2 is the less dense substance
N1 is the more dense substance

Question 21

what is diffraction

Answer 21

when waves spread around obstacles or through gaps

Question 22

what do you observe when a light from a laser is directed to a single slit of suitable width

Answer 22

a bright central band that extended well into the shadow region of the slit - central maximum
the bright band is bordered by regions of darkness - minima
moving out past the dark regions, the bright regions reduced in intensity - subsidiary maxima

Question 23

describe interference with sound waves

Answer 23

there is a loud sound at the centre of the interference pattern because the waves are in phase with each other
they travel exactly the same distance to the point at the pattern’s centre and so are still in phase with each other
constructive interference occurs so a loud sound is detected

Question 24

explain the meaning of interference

Answer 24

when 2 (or more) waves meet and there is a change in overall displacement

Question 25

what does the equation n=c/v mean

Answer 25

refractive index of the material = speed of light / speed of light in the material

Question 26

what’s the refractive index of air

Answer 26

1

Question 27

what’s snell’s law

apply it to nsin(θ) = constant

Answer 27

the angle that incoming light makes to the normal is called the angle of incidence
the angle the refracted ray makes with the normal is the angle of refraction

so
n1sin(θ1) = n2sin(θ2)

Question 28

what vital evidence did Young’s double slit experiment provide about diffraction and interference?

Answer 28

light could both diffract (through two narrow slits) and interfere ( to form the interference pattern on the screen)

Question 29

define path difference

Answer 29

the amount by which the path travelled by one wave is longer than the path travelled by the other wave

Question 30

Two coherent sources of sound waves each with a wavelength of 1.5m are set up so that they produce interference fringes. When the path difference is 3.75m, would you expect constructive or destructive interference?

Answer 30

find out how many times the wavelength goes into the path difference:
3.75 / 1.5 = 2.5

so the path difference is 2.5 X λ so the interference is destructive

Question 31

how can you demonstrate the diffraction of light

Answer 31

shining a monochromatic, coherent light through a very narrow slit onto a screen

Question 32

define monochromatic light

Answer 32

light of a single wavelength (so a single colour)

Question 33

if you narrow the slit, what happens to the diffraction pattern

Answer 33

becomes wider

Question 34

define diffraction grating

Answer 34

lots of equally spaced slits very close together

Question 35

what happens when a monochromatic light passes through a diffraction grating? why?

Answer 35

the interference pattern is really sharp because there are so many beams reinforcing the pattern

Question 36

in a diffraction grating pattern, what is the line of maximum brightness called
what are the lines called next to it

Answer 36

zero order line

the lines either side are called first order lines, then second order lines and so on

Question 37

how can you calculate the wavelength of light (using a diffraction grating)

Answer 37

• pass the light through the grating
• use the zero order and first order lines
• measure the distance between the lines and the distance from the grating to the screen
• work out the angle between them by using tan^-1(x / D)
• work out the spacing between the slits in the diffraction grating
• use the equation dsin(angle) = n X wavelength (n=1 and d is the slit spacing)

Question 38

in diffraction gratings what does it mean if:

a) the wavelength increases
b) split spacing increases

Answer 38

a) wavelength increasing means the angle gets bigger so the pattern will spread out
b) d increasing means the angle is smaller so the pattern comes closer together

Question 39

what happens when you diffract white light

Answer 39

zero order maximum is white

all the other orders form spectrums with red on the outside and violet on the inside

Question 40

what other factor do coherent waves have to have for interference to be noticeable during an experiment

Answer 40

the waves have to have approximately the same amplitude