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A level AQA Physics > 5 Optics > Flashcards

5 Optics Flashcards

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1
Q

What is refraction?

A

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

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2
Q

what is Snell’s law?

A

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

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3
Q

what is another formula for refractive index?

A

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

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4
Q

what causes dispersion?

A

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

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5
Q

what is a ray of light?

A

a series of wavefronts

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6
Q

why do waves refract?

A

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

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7
Q

what is the critical angle?

A

the angle i when angle r = 90 degrees

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8
Q

what is the normal to a boundary?

A

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

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9
Q

when does refraction not take place at a boundary?

A

if the incident ray is along the normal

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10
Q

what is refractive index?

A

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

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11
Q

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?

A

1/n

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12
Q

what changes when refraction occurs and what remains the same?

A

changes - wavelength
constant - frequency

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13
Q

what is the other formula for Snell’s law?

A

n₁sin𝜃₁ = n₂sin𝜃₂

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14
Q

What is Total Internal Reflection?

A

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.

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15
Q

when can total internal reflection take place?

A
  1. the incident substance (n₁) has a larger refractive index than the other substance
  2. the angle of incidence exceeds the critical angle
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16
Q

what is the formula for the critical angle?

A

sin𝜃c = n₂/n₁

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17
Q

what are optical fibres?

A

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

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18
Q

what are the components of a communications optical fibre?

A
  • core
  • cladding with a lower refractive index than the core
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19
Q

why do optical fibres for communications have cladding?

A

to reduce light loss and allow for total internal reflection

20
Q

what is modal dispersion?

A

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

21
Q

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

A

to prevent modal dispersion

22
Q

why must monochromatic light be used with communications optical fibres?

A

to prevent material dispersion which occurs with white light

23
Q

what are young’s fringes?

A

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

24
Q

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

A

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

25
Q

what happens where the bright fringes are formed?

A

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

26
Q

what happens to the light where dark fringes are formed?

A

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

27
Q

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

A

w = (λD)/s

28
Q

What does coherence mean?

A

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

29
Q

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

A

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

30
Q

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

A

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

31
Q

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

A

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

32
Q

what is light from a laser like?

A
  • highly monochromatic
  • coherent
33
Q

What is the fringe pattern of white light?

A
  • 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
34
Q

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

A
  • 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
35
Q

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

A

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

36
Q

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

A
  • slit must be narrow enough to diffract the light sufficiently
  • two slits must be close enough so the diffracted waves overlap
37
Q

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

A

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

38
Q

What is a diffraction grating?

A

a plate with many closely spaced parallel slits ruled on it

39
Q

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

A
  • 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
40
Q

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

A
  • light of longer wavelength
  • grating with closer slits is used
41
Q

what is the formula for diffraction grating?

A

dsin𝜃 = nλ

42
Q

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

A

N = 1/d

43
Q

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

A
  • substitute 𝜃 = 90° into dsin𝜃 = nλ
44
Q

what is a line emission spectra?

A

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

45
Q

what is a line absorption spectra?

A

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

A level AQA Physics (27 decks)

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