5 Optics

Question 1

What is refraction?

Answer 1

the change of direction of light as it passes across an interface between two transparent media of different optical densities

Question 2

what is Snell’s law?

Answer 2

n = sini/sinr which is a constant at a particular wavelength

Question 3

what is another formula for refractive index?

Answer 3

n = c (of air) / c (of transparent medium)

Question 4

what causes dispersion?

Answer 4

because different coloured wavelengths of light travel at different speeds in glass and refract at different angles. this causes the colours of white light to split when shone through a triangular prism

Question 5

what is a ray of light?

Answer 5

a series of wavefronts

Question 6

why do waves refract?

Answer 6

one side of the wavefront enters the slower medium first. This causes the wavefront to rotate as it enters at a boundary

Question 7

what is the critical angle?

Answer 7

the angle i when angle r = 90 degrees

Question 8

what is the normal to a boundary?

Answer 8

an imaginary line perpendicular to the boundary between two materials or a surface

Question 9

when does refraction not take place at a boundary?

Answer 9

if the incident ray is along the normal

Question 10

what is refractive index?

Answer 10

the ratio (Snell’s law) of sin(i)/sin(r)

Question 11

what is the formula for the refractive index of a substance when a ray leaves a block when the refractive index when entering the block is n?

Answer 11

1/n

Question 12

what changes when refraction occurs and what remains the same?

Answer 12

changes - wavelength
constant - frequency

Question 13

what is the other formula for Snell’s law?

Answer 13

n₁sin𝜃₁ = n₂sin𝜃₂

Question 14

What is Total Internal Reflection?

Answer 14

if the angle of incidence is greater than the critical angle, the ray is reflected as if the boundary were replaced by a plane mirror.

Question 15

when can total internal reflection take place?

Answer 15

1. the incident substance (n₁) has a larger refractive index than the other substance
2. the angle of incidence exceeds the critical angle

Question 16

what is the formula for the critical angle?

Answer 16

sin𝜃c = n₂/n₁

Question 17

what are optical fibres?

Answer 17

a thin flexible transparent fibre used to carry light pulses from one end to the other

Question 18

what are the components of a communications optical fibre?

Answer 18

• core
• cladding with a lower refractive index than the core

Question 19

why do optical fibres for communications have cladding?

Answer 19

to reduce light loss and allow for total internal reflection

Question 20

what is modal dispersion?

Answer 20

the lengthening of a light pule as it travels along an optical fibre, due to rays that undergo less total internal reflection

Question 21

why must the core of an optical fibre be very narrow?

Answer 21

to prevent modal dispersion

Question 22

why must monochromatic light be used with communications optical fibres?

Answer 22

to prevent material dispersion which occurs with white light

Question 23

what are young’s fringes?

Answer 23

alternate bright and dark fringes which are evenly spaced and parallel to the double slits

Question 24

why are Young’s fringes formed with a double slit?

Answer 24

the interference of light from the two slits as the light diffracts

Question 25

what happens where the bright fringes are formed?

Answer 25

the light from one slit reinforces the light from the other slit. they arrive in phase with each other

Question 26

what happens to the light where dark fringes are formed?

Answer 26

the light from one slit cancels the light from the other slit. the light from one slit arrives 180° out of phase with the other

Question 27

what is the formula for fringe separation with Young’s double slit?

Answer 27

w = (λD)/s

Question 28

What does coherence mean?

Answer 28

light waves of the same frequency with a constant phase difference.

Question 29

why is the fringe separation of red light larger than that of blue light?

Answer 29

red light has a longer wavelength than blue light
from the formula: w = (λD)/s

Question 30

What is the light from vapour lamps and discharge tubes like?

Answer 30

they produce light with a dominant colour.
other wavelengths are also emitted but they are, in effect, monochromatic sources

Question 31

what is light from a filament lamp or the sun like?

Answer 31

composed of the colours of the spectrum so covers a continuous range of wavelengths

Question 32

what is light from a laser like?

Answer 32

• highly monochromatic
• coherent

Question 33

What is the fringe pattern of white light?

Answer 33

• central fringe is white as every colour contributes at the centre of the pattern
• inner fringes are blue on the inner side and red on the outer side
• outer fringes merge into an indistinct background of white light becoming fainter with increasing distance from the centre. this is because when the fringes merge, different colours reinforce and therefore overlap

Question 34

What are the characteristics of the pattern produced by single slit diffraction?

Answer 34

• central fringe twice as wide as the outer fringes
• peak intensity of each fringe decreases with distance from the centre
• outer fringes are the same width
• the outer fringes are much less intense than the central fringe

Question 35

what is the formula for the width of the central fringe?

Answer 35

W = (λ/a) × 2D
a - width of single slit

Question 36

What are some conditions for interference to happen with double slits?

Answer 36

• slit must be narrow enough to diffract the light sufficiently
• two slits must be close enough so the diffracted waves overlap

Question 37

why are only a limited number of interference fringes seen in double slit experiment?

Answer 37

the interference fringes have to be within the central diffraction fringe to be seen adequately

Question 38

What is a diffraction grating?

Answer 38

a plate with many closely spaced parallel slits ruled on it

Question 39

Why is light transmitted in certain directions only when light is shone through the diffraction grating?

Answer 39

• light passing through each slit is diffracted
• diffracted light waves from adjacent slits reinforce each other in certain directions only and cancel out in all other directions

Question 40

what increases the angle of diffraction of each beam for light shone through a diffraction grating?

Answer 40

• light of longer wavelength
• grating with closer slits is used

Question 41

what is the formula for diffraction grating?

Answer 41

dsin𝜃 = nλ

Question 42

What is the formula for the number of slits per metre of the grating?

Answer 42

N = 1/d

Question 43

What do you do to find the maximum number of orders produced?

Answer 43

• substitute 𝜃 = 90° into dsin𝜃 = nλ

Question 44

what is a line emission spectra?

Answer 44

shows the light at specific wavelengths as narrow lines of different colours

Question 45

what is a line absorption spectra?

Answer 45

a continuous spectrum with narrow dark lines at certain wavelengths
this is because the elements absorb light of the same wavelengths they can emit. the atoms then emit the light but not necessarily in the same direction