Refraction,Diffraction,interference

Question 1

what causes interference

Answer 1

when two waves are superimposed on each other. they must be coherent

Question 2

define coherent

Answer 2

when the two waves have identical frequency and constant phase difference

Question 3

what is the path difference

Answer 3

the difference in length the waves have travelled to reach a certain point

Question 4

define monochromatic

Answer 4

light of the same wavelength

Question 5

how can you create two coherent sources of light

Answer 5

shine a laser through a slit with a double slit behind it. the light diffracts when passing the first slit and then goes through the second two and since the laser produces monochromatic light the slits act like two coherent sources.

Question 6

why is an average fringe spacing found

Answer 6

to ensure the value is accurate because the fringe spacing is usually very small

Question 7

what is a light fringe

Answer 7

light or dark bands caused by interference or diffraction of light

Question 8

what happens when white light is used instead of a laser in the double-slit experiment

Answer 8

the middle fringe will be bright white light, the fringes will be more spread out and side fringes will be a spectrum of visible colours (blue closest to the centre cause it diffracts less than red light)

Question 9

risk of lasers

Answer 9

eye injury. the natural aversion reflex (the eye will automatically look away from the light source) is too slow to prevent retina damage

Question 10

how to be safe with lasers

Answer 10

don’t point it in peoples eye,
wear protective eyewear,
stick to institutional safety requirements

Question 11

theories of light

Answer 11

Newton- said that light was made of corpuscles (pockets of light) in the 1600s
Huygen- gave the wavefront theory in the 1600s
Young- the double-slit experiment gave evidence for Huygen’s theory
Einstein- provided the photoelectric effect as evidence for newtons theory
Compton scattering (a photon scattering of an electron) gave evidence for the quantisation of light in the 20th century

Question 12

how can interference be observed from sound waves

Answer 12

place two sources facing each other. the sound needs to be coherent. (same sound needs to be played at the same time from each source). then move a microphone from one source to the other. constructive or destructive interference is measured by whether the noise is loud or quite

Question 13

why do the sound waves have to be coherent when measuring interference

Answer 13

the get a constant interference pattern

Question 14

what causes dark/light fringes from a single slit experiment

Answer 14

the light fringes are caused by constructive interference. the dark ones are caused by destructive interference

Question 15

how does the slit width affect diffraction

Answer 15

an increase in the slit width will decrease the central maximum because diffraction effects will decrease. the intensity of the central maximum will increase because the photons are less spread out

Question 16

how does the wavelength affect diffraction

Answer 16

increasing incident light wavelength will increase the width of the central maximum because diffraction effects will increase. the intensity of the central maximum will decrease because the photons are more spread out.

Question 17

what happens when light is shone through a diffraction grating

Answer 17

if the slit widths are the same as the wavelength the light is diffracted into a pattern of bright and dark lines.

Question 18

what is an order

Answer 18

the position of the maxima in a diffraction grating pattern. the central point is the ‘zero-order’. its the brightest of all the lines. either side of it is the first-order lines. the further from the central line the dimmer the lines and the more visible the orders

Question 19

what happens if there are more slits in the diffraction grating

Answer 19

it causes a sharper line pattern on the screen. this makes them more easily measurable

Question 20

how does changing the spacing between slits and the wavelength of light change Sin(θ) in a diffraction grating

Answer 20

increasing wavelength increases θ so Sin(θ) also increases.

increasing the slit spacing decreases θ so Sin(θ) also decreases

Question 21

when does the first-order maximum occur

Answer 21

it happens at an angle when the path difference between the two sources is one λ

Question 22

how do you derive the grating equation

Answer 22

1. produce coherent light sources
2. 1st order maxima happens at an angle that the path difference = λ (angle θ)
3. creat the triangle from the diagram of the grating (the hyp is the slit spacing and the opp is λ)
4. trig shows that d*Sin(θ) = λ
5. generalise for all. nλ=d*Sin(θ) where n = a whole number

Question 23

why can crystals be used as a diffraction grating

Answer 23

there are tiny spaces between atoms in a crystal for light to pass through. the spacing can be found by looking at how x-rays diffract through

Question 24

how can a diffraction grating be used to analyse light

Answer 24

a diffraction grating can be used to separate wavelength of light from different substances. the present wavelengths can be used to learn about the substance

Question 25

define refraction

Answer 25

the change in direction of light as it changes speed when passing from one medium to another

Question 26

what is a refractive index

Answer 26

the ration of the speed of light in a vacuum to the speed of light in that material

Question 27

whats the refractive index of air

Answer 27

its so similar to the speed of light in a vacuum that the refractive index of air is approximately 1. c↓air ≈ c

Question 28

what does snell’s law do

Answer 28

relates the angle of incidence and refraction to the refraction indices of the mediums

Question 29

what is optical density

Answer 29

the more optically dense a material is the slower light travels through it. When light travels from a less optically dense medium to a more optically dense medium, it bends towards the normal.
When light travels from a more optically dense medium to a less optically dense medium, it bends away from the normal.

Question 30

what happens to light if the incidence angle is greater than the critical angle

Answer 30

when the angle of incidence is greater than the critical angle, no light is refracted. instead, it is reflected (this is called total internal reflection)

Question 31

what is internal reflection

Answer 31

when light enters a less optically dense medium it speeds up. some light is refracted but some is reflected

Question 32

what happens when the angle of incidence is exactly equal to the critical angle

Answer 32

the ray of light will travel along the boundary of the two mediums. the angle of refraction is at 90 degrees to the normal

Question 33

how do optical fibres work

Answer 33

they use total internal reflection to transmit light long distances

Question 34

how is an optical fibre designed

Answer 34

each fibre contains a core of optically dense material that is covered by a layer of cladding which is much lower optically dense giving a very small critical angle

Question 35

what causes material dispersion

Answer 35

it’s due to different wavelengths of light arriving at the end of the fibre at different times. this is because the refractive index changes with frequency. this causes pulse broadening. To avoid this monochromatic light can be used

Question 36

what does pulse broadening in fibre optic cables cause

Answer 36

it degrades the signal so they may now overlap with each other.

Question 37

what is modal dispersion

Answer 37

this is due to the different paths a signal will take in a fibre. this means different parts of a signal arrive at different times. this results in pulse broadening. to avoid this a monomode fibre is used.

Question 38

what is pulse absorption

Answer 38

a signal may be absorbed by the material. this reduces the signal amplitude.

Question 39

how can signal absorption and broadening be stopped

Answer 39

signals can be boosted by repeaters to counteract signal absorption and broadening

Question 40

define diffraction

Answer 40

the spreading of a wave as it passes through a gap. diffraction is maximised if the gap is roughly the same width as the wavelength of the wave

Question 41

what are the 3 ways to make a standing wave

Answer 41

closed pipe. antinode and the start, node at the end
open pipe. anti node and start and end
string- node at start and end

Question 42

define reflection

Answer 42

when light changes direction at a boundary and remains in the original medium

Question 43

law of reflection

Answer 43

the angle of incidence = angle of reflection

Question 44

what properties of waves change when they are reflected or refracted

Answer 44

reflected- none

refracted- wavelength

Question 45

principle of superposition

Answer 45

when two waves of the same type meet at a point the resultant displacement is equal to the sum of the displacements of the individual waves

Question 46

define constructive interference

Answer 46

superposition of two waves causing a larger displacement than the original waves

Question 47

define destructive interference

Answer 47

superposition of two waves causing a smaller displacement than the original waves.

Question 48

define phase difference

Answer 48

the difference between the displacement of particles along a wave