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AS Physics > Optics (topic 5) > Flashcards

Optics (topic 5) Flashcards

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1
Q

Normal (definition)

A

Imaginary line perpendicular to the boundary between a material or surface

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2
Q

Refraction (definition)

A

Change of direction that occurs when light passes at an angle across a boundary between two transparent substances

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3
Q

How does the density of a material (comparatively to the density of the material it was in) affect how light refracts when crossing the boundary? (2)

A
  • Light ray bends towards the normal in a more dense substance
  • Light ray bends away from the normal in a less dense substance
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4
Q

Method for investigating refraction by glass (5)

A
  • glass block on paper
  • draw around glass block with pencil + draw a normal
  • Shine a ray of light at an angle into the point where normal reaches glass block
  • Mark P (incident ray) and Q (ray leaving block)
  • measure angle of refraction from P/refracted ray to normal
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5
Q

What is always true for a refracted ray of light in a glass block?

A
  • angle of refraction < angle of incidence
  • ratio of sin i/sin r is the same for each ray (Snell’s law)
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6
Q

What is Snell’s law?

A

The ratio of sin i/sin r is the same for every angle of i

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7
Q

Refractive index (definition + equation)

A

The ratio between angle of incidence and angle of refraction in a substance (sin i/sin r)

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8
Q

Apart from refraction, what also occurs when shining a ray of light into a glass block?

A

Partial reflection

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9
Q

Comparison of refractive index from air>glass and glass>air

A

If n for air>glass = n

glass>air = 1/n
(as i(2) = r(1) and r(2) = i(1))

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10
Q

Why does refraction occur, and how does this relate to refractive index?

A

Speed of light waves is different in each substance
(smaller speed = greater refractive index)

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11
Q

What changes about light when crossing a boundary, and what doesn’t change?

A

Speed of light changes
Wavelength changes

Frequency DOES NOT change

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12
Q

Equation for refraction at a boundary between two transparent substances

A

n(1)sin theta(1) = n(2) sin theta(2)

where theta (1) is i and theta (2) is r

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13
Q

Refractive index of air/a vacuum

A

Approximately 1
(Exactly 1 for a vacuum)

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14
Q

Why can a prism be used to split a beam of white light into a spectrum?

A

White light is made up of light with a continuous range of wavelengths
Glass prism refracts light by different amounts depending on wavelength

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15
Q

Rule for wavelength and diffraction, and why this happens (2)

A

Shorter wavelength in vacuum = greater diffraction

Because speed of light in a material depends on wavelength e.g. violet light travels slower than red light in glass

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16
Q

How does the refraction/reflection of light from glass to air change as the angle of incidence is changed?

A

Refracts away from the normal up until the critical angle (where it is reflected along the boundary (90* - the critical angle). An angle of incidence larger than that of the critical angle, total internal reflection takes place

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17
Q

What is total internal reflection?

A

At the boundary between two substances (e.g. glass and air), a ray behaves like it has reached a plane mirror and reflects back into the glass instead of refracting into air.

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17
Q

Equation or critical angle and why it works (2)

A

sin theta(c) = n2 / n1
As sin theta(2) = sin(90) = 1

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17
Q

What are the conditions for total internal reflection?

A
  • Incident substance has a larger refractive index than other substance (n1 > n2)
  • angle of incidence is greater than critical angle
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18
Q

Why do diamonds sparkle when white light is directed at them? (2)

A

Very high refractive index = separates colours more

Low critical angle = light is totally internally reflected many times, which means colours spread out even more

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19
Q

What are optical fibres used for?

A

1) medical endoscopes to see inside body
2) communications to carry signals

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19
Q

Why do slits give an interference pattern?

A

They act as coherent light sources (emit light with constant phase difference and same frequency)

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20
Q

What happens when a single slit is too wide? (2)

A

Each part produces a separate fringe pattern, which is displaced from the others.
The darker fringes become smaller than the lighter fringes, so contrast is lost between dark and bright fringes.

21
Q

Young’s slits pattern

A

Alternating bright and dark fringes, evenly spaced and parallel to the double slits

22
Q

How is a bright fringe formed?

A

Light from one slit reinforces light from the other slit

23
Q

How is a dark fringe formed?

A

Light from one slit cancels light from the other slit

24
Q

What is fringe separation?

A

Distance from the centre of one bright fringe to the centre of the next bright fringe

25
Q

Fringe separation equation

A

w = (lambda)D / s

26
Q

What is path difference (Young’s slits)?

A

The difference in distance from each slit to one point on the screen

27
Q

Why do bright fringes appear?

A

Reinforcement as the path difference for the light from the slits is a whole number of wavelengths

28
Q

Why do dark fringes appear?

A

Cancellation as the path difference for the light from slits is a whole number of wavelengths + 1/2 a wavelength

29
Q

Why measure w from dark fringes?

A

centre of dark fringes is easier to locate

30
Q

Why does a single slit create a coherent source?

A

Diffraction makes the waves behave as if they are coming from a single point

31
Q

Why can a normal light not create an interference pattern with slits?

A

Not coherent - emits waves at random

32
Q

Which colour of light has the largest wavelength?

A

Red

33
Q

Which colour of light has the largest frequency?

A

Violet

34
Q

Would two sources that are not in phase produce an interference pattern?

A

Not necessarily - they can still produce a pattern if they have a constant phase difference and the same frequency

35
Q

Examples of monochromatic light sources (3)

A
  • vapour lamps/discharge tubes - e.g. sodium vapour lamp (mostly - wavelengths are dominated by orange/yellow)
  • laser light
  • white light through a colour filter
36
Q

Examples of non-monochromatic light sources (2)

A
  • the sun
  • filament lamp
37
Q

Why are lasers the best choice to demonstrate double slit interference? (2)

A
  • highly monochromatic (specify wavelength to a nanometre)
  • highly coherent (so single slit is not needed)
38
Q

Describe fringes formed by white light (3)

A
  • Central fringe is white as every component colour overlaps

-outer fringes form spectra of overlapping fringes with red on outside and blue on inside, as red has widest fringes

  • outer fringes are wider
39
Q

What is diffraction?

A

Spreading of waves through a gap or by an edge

40
Q

How does diffraction improve telescopes?

A

Telescope lenses are wider than the human eye so less diffraction occurs, allowing us to see more detail

41
Q

How can the amount of diffraction be changed? (2)

A
  • narrower gap
  • larger wavelength
42
Q

Describe the single slit diffraction pattern

A
  • Central fringe is brightest and widest (2x width of others)
  • outer fringes decrease in intensity with distance from centre
  • all outer fringes same width
43
Q

How can the width of a fringe in a slit pattern be increased?

A
  • Narrower slit
  • Larger wavelength
44
Q

Width of (outer) fringe for single slit pattern

A

w = (lambda)D / a

where “a” is width of single slit

45
Q

Why might an interference pattern not be observed with Young’s slits? Why does this prevent fringes forming? (2 + reason)

A
  • Slits too wide
  • Slits too far apart

Light from slits does not overlap so no pattern forms

46
Q

What is a diffraction grating?

A

Plate with many closely spaced parallel slits ruled onto it

47
Q

Why does a parallel beam of monochromatic light create beams when passed through a diffraction grating? (2)

A

Light passing through each slit is diffracted.
Diffracted light waves from adjacent slits reinforce each other only in some directions, and cancel everywhere else.

48
Q

How to increase angle of diffraction between each beam transmitted through a diffraction grating? (2)

A
  • longer wavelength
  • closer slits
49
Q

equation for nth order beam from a diffraction grating

A

d sin (theta) = n(lambda)

where d = 1/number of slits

50
Q

equation for maximum order beam from diffraction grating

A

n = d/(lambda)

  • round DOWN to nearest whole number
51
Q

How does a communications optical fibre work? (3)

A
  • light enters through transmitter and reaches receiver at the other end
  • TIR happens around bends as cladding has lower refractive index
52
Q

What properties should communications optical fibre have? why? (3)

A
  • Highly transparent - minimises absorption of light which would reduce amplitude
  • Cladding of lower refractive index - prevents light loss through refraction which would reduce amplitude; also prevents signals mixing between fibres in contact
  • Narrow - prevents multi-path dispersion where light becomes longer in wide core and merges with next pulse
53
Q

Why does pulse dispersion happen? (2)

A
  • Core is too wide which causes some light to cover greater distance in the same time, causing pulses to merge
  • White light spreads out based on component colours’ speed in glass, creating a longer pulse
54
Q

How does a medical endoscope work? (3)

A
  • made up of two bundles of fibres which are inserted into a cavity and used to see into the body (light sent through one bundle, lens on other)
  • light from image travels up other fibre where it can be observed
  • must be coherent bundle (i.e. same relative positions - cannot cross)

AS Physics (13 decks)

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