refraction, diffraction, interference

Question 1

define refraction

Answer 1

Refraction is the change of direction that occurs when light passes at an angle across a boundary between two transparent substances.

Question 2

what speed do visible light and other waves in EM spectrum travel

Answer 2

Visible light and other waves in the electromagnetic spectrum travel at a speed of 3.00X108 ms*’ in a vacuum. Light travels slower in all other substances.

Question 3

what is the refractive index

Answer 3

The refractive index n of a substance is the ratio of the speed of light in a vacuum c and the speed of light in that substance cs

n= speed of light in a vacuum / speed of light in substance
n= c/cs

Question 4

what is the refractive index of air?

Answer 4

1

Question 5

what is the refractive index of water

Answer 5

1.33

Question 6

what is the refractive index of glass?

Answer 6

1.50

Question 7

the greater the refractive index….

Answer 7

The greater the refractive index of a substance, the more slowly light travels through it, and the higher its optical density.

Question 8

what is snell’s law

Answer 8

When light travels from one substance into another, the angle of incidence, angle of refraction, and refractive index of each substance are related by Snell’s law:

n1 sin 01 = n2 sin 02

Question 9

what do rays of light travelling from a less optically dense substance into a more optically dense substance do?

Answer 9

refract towards the normal

Question 10

what do rays of light travelling from a more optically dense substance into a less optically dense substance do?

Answer 10

refract away from the normal.

Question 11

when does total internal reflection occur?

Answer 11

Total internal reflection occurs when light is travelling from an optically-dense material
to a less optically-dense material, and the angle of incidence is greater than the critical angle.

n1>n2
i > c

Question 12

what is the critical angle?

Answer 12

The critical angle is the angle at which the angle of refraction is 90° for light going from an optically-dense material to a less optically-dense material:

sin0c = n2/n1

where n2 <n1

Question 13

what do optical fibres do?

Answer 13

optical fibres make use of total internal reflection to transmit information encoded in light over long distances and a round corners.

Question 14

describe the structure of an optical fibre

Answer 14

The core of an optical fibre is made of glass with a higher refractive index than the cladding. The
cladding protects the core from being damaged and prevents light from leaking out.

Question 15

what is signal degradation?

Answer 15

Signal degradation in an optical fibre can cause intormation to be lost. It can occur due to absorption or pulse broadening.

Question 16

what is absorption in optical fibres

Answer 16

Some of the light is absorbed by the optical fibre every time it reflects, resulting in a decrease in the amplitude of the signal.

Question 17

what is pulse broadening in optical fibres?

Answer 17

The signal pulse gets broader as it travels along the fibre, reducing the quality of the signal and leading to loss of information if the pulses overlap.

it can be caused by modal or material dispersion.

Question 18

what is MODAL dispersion?

Answer 18

If the core is wide enough, rays of light entering it at different angles will take different paths through it and travel different distances.

Question 19

how can modal dispersion be reduced?

Answer 19

by using an optical fibre with a narrower core , known as a single mode fibre.

Question 20

what is MATERIAL dispersion?

Answer 20

if white light is used for the signal, the different wavelengths in it travel at slightly different speeds through the fibres, causing them to separate as they travel along the fibre.

Question 21

how can material dispersion be eliminated?

Answer 21

by using mono chromatic light

Question 22

when are waves COHERENT

Answer 22

when they have the same frequency and a constant phase difference

Question 23

constructive interference…

Answer 23

constructive interference occurs when two waves in phase superpose to give a wave with a larger amplitude.

Question 24

destructive interference….

Answer 24

destructive interference occurs when two waves out of phase superpose to cancel each other out.

Question 25

when does an interference pattern occur?

Answer 25

an interference pattern occurs when waves with a constant phase difference superpose forming cancellation and reinforcement at fixed points

Question 26

what is the path difference?

Answer 26

the path difference between two waves is the difference in the distance travelled by the waves to the point at which they superpose

Question 27

what happens if the path difference is a whole number of wavelengths n λ?

Answer 27

if the path difference is a whole number of wavelengths n λ ,the waves always arrive in phase at that point and constructive interference occurs producing a maximum.

Question 28

what happens if the path difference is (n+ 1/2 λ)

Answer 28

if the path difference is (n+1/2λ) then destructive interference occurs, producing a minimum

Question 29

what is two source interference?

Answer 29

two source interference occurs when coherent waves from two sources superpose to produce an interference pattern

Question 30

what does young’s double slit provide evidence for?

Answer 30

the wave nature of light, because the resulting fringe pattern is due to diffraction and interference which cannot happen with particles.

Question 31

what happens in young’s double slit experiment?

Answer 31

a single beam of light is split into two coherent beams as it passes through the closely spaced double fringe. this produces a pattern of dark and bright fringes on a screen

Question 32

where do bright fringes form?

Answer 32

bright fringes form where the path difference between light from each slit is n λ, so waves arrive in phase and superpose constructively

Question 33

where do dark fringes form?

Answer 33

dark fringes form where the path difference is ( n + 1/2 λ ) so the waves arrive out of phase and superpose destructively

Question 34

define fringe separation w

Answer 34

fringe separation w is the distance between the centres of two adjacent maxima or minima (bright or dark fringes)

Question 35

what is the equation for fringe separation?

Answer 35

w = λD/ s w is fringe separation λ is wavelength D is distance from screen s is slit separations

Question 36

define diffraction

Answer 36

Diffraction is the spreading of waves when they pass through a gap or round an obstacle

Question 37

what happens to light passing through a narrow slit?

Answer 37

Light passing through a narrow single slit is diffracted, producing an intenterence pattern on a screen. For monochromatic light: • the central fringe is much brighter and is twice as wide as the outer fringes • the outer fringes are the same width and their brightness decreases from the centre for white light, the different wavelengths are diffracted by different amounts, resulting In • a bright white cental maximum. • less bright fringes that are spectra, with blue light on the inner edge and red light on the outer edge.

Question 38

what’s the difference between young’s double slit and the single slit?

Answer 38

for the single slit, the central maximum is very bright compared to fringes and it’s double the width of the fringes

Question 39

diffraction grating vs single or double slit

Answer 39

Monochromatic light passing through a diffraction grating will produce an interference pattern where the maxima are much sharper than for a single or double slit. the central maximum is the brightest and is called the zero order maximum.

Question 40

as light from one source is diffracted through each slit of a grating,

Answer 40

As light from one source is diffracted through each slit of a grating, the diffracted waves constructively superpose in certain directions only, and destructively superpose in all other directions.

Question 41

what is the equation used for diffraction grating?

Answer 41

n lambda = d sin theta where n is the order of maxima d is the slit width

Question 42

what is D for diffraction gratings?

Answer 42

lines per mm eg 100 lines per mm d = 1/D d = 1/100 = 0.01mm = 1x10^-5 m

Question 43

how to find the maximum number of order?

Answer 43

find n at 90° n = dsin theta/ lambda n= d/ lambda because sin 90 is 1

Question 44

to find the maximum order

Answer 44

n = d/ lambda