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A-Level Astrophysics > Optical Telescopes > Flashcards

Optical Telescopes Flashcards

1
Q

What is the principal axis of a lens?

A

An imaginary line through the centre of the lens, at 90°

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2
Q

What happens to all light rays incident on a converging lens, parallel to the principal axis?

A

The converge onto a single point

“Principal Focus”

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3
Q

What happens to all parallel light rays incident on a converging lens?

A

They converge onto a single point on the focal plane

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4
Q

What is the focal length, f?

A

The distance between the lens axis and the focal plane

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5
Q

What is the lens axis?

A

The plane of the centre of the lens

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6
Q

What are the two types of images?

A

Real and virtual

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7
Q

What defines a real image?

A

The light rays are actually there, and the image can be captured on a screen

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8
Q

How is a real image formed?

A

Light rays from an object are made to pass through another point in space

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9
Q

What defines a virtual image?

A

The light rays aren’t actually where the image appears to be. The image cannot be captured on a screen

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10
Q

How is a virtual image formed?

A

When light rays from an object appear to have come from another point in space

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11
Q

What kind of image can a converging lens form?

A

Both real and virtual

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12
Q

What determines whether a virtual or real image is formed for a converging lens?

A

The distance of the object from the lens

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13
Q

Where will the image of an object sit, if the object is on the principal axis?

A

On the principal axis

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14
Q

If the object is further than the focal length away from the converging lens, what kind of image is formed?

A

Real image

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15
Q

At what distance from a converging lens is the object, for a real image to be formed?

A

Further that the focal length away

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16
Q

If the object is closer than the focal length away from the converging lens, what kind of image is formed?

A

Virtual image

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17
Q

At what distance from a converging lens is the object, for a virtual image to be formed?

A

Closer than the focal length away

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18
Q

When you draw a ray diagram for a converging lens, what two rays do you draw?

A

One parallel to the principal axis (therefore refracted through the principal focus on the other side of the lens)

One passing through the lens’ centre (therefore no refraction)

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19
Q

On a ray diagram for a converging lens, how do you determine if the image will be virtual?

A

If the refracted rays on the other side of the lens will never meet

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20
Q

On a ray diagram for a converging lens, how do you determine where a real image is formed?

A

Where the refracted rays meet

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21
Q

When drawing a ray diagram for a converging lens, what must be true about the distance of the object from the lens?

A

Closer than the focal length for a Virtual image

Further than the focal length for a Real image

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22
Q

Where is the lens equation in the formula book?

A

Medical Physics

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23
Q

What is a refracting telescope made up of?

A

Two converging lenses

Objective lens and Eye lens

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24
Q

What does the objective lens do?

A

Converges the light rays from the object to form a real image

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25
Q

What does the eye lens do?

A

Acts as a ‘magnifying glass’ on the real image to form a magnified virtual image

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26
Q

How can we assume that the real image formed by the objective telescope is formed on the focal plane?

A

If you assume the object is at infinity, the rays from it are parallel.
Therefore, the image must be on the focal plane

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27
Q

In a telescope, where is the principal focus of the objective lens set up to be?

A

In the same position as the principal focus of the eye lens

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28
Q

In a telescope, where is the principal focus of the eye lens set up to be?

A

In the same position as the principal focus of the objective lens

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29
Q

What is the magnification (M) equation relating to angles?

A

θᵢ / θ₀

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30
Q

What is θᵢ?

A

The angle subtended by the image at the eye

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31
Q

What is θ₀?

A

The angled subtended by the object at the eye

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32
Q

Why is θᵢ > θ₀ for the magnification equation?

A

Because it is a magnifying telescope

33
Q

What is f₀?

A

The focal length of the objective lens

34
Q

What is fₑ?

A

The focal length of the eye lens

35
Q

What is the magnification (M) equation relating to the focal length?

A

In the formula book

36
Q

What is a Cassegrain reflecting telescope made up of?

A

Primary mirror,
Secondary mirror,
Eye lens

37
Q

What is the shape of a reflecting telescope’s primary mirror?

A

Parabolic concave

38
Q

What is the shape of a Cassegrain reflecting telescope’s secondary mirror?

A

Convex

39
Q

What is the main characteristic of a Cassegrain arrangement?

A

The presence of a convex secondary mirror

40
Q

What necessitates the presence of a convex secondary mirror for a Cassegrain arrangement?

A

The principal focus of the primary mirror is in front of the mirror, so an observer would block out the light

41
Q

What is the resolving power of a telescope?

A

A measure of how much detail you can see

42
Q

What is the resolving power fo a telescope dependent on?

A

The minimum angular resolution

43
Q

What is the minimum angular resolution?

A

The smallest angular separation at which the telescope can distinguish two points

44
Q

What phenomena limits resolution?

A

Diffraction

45
Q

What is formed when a beam of light passes through a circular aperture?

A

A diffraction pattern is formed

46
Q

What is the central circle of the diffraction pattern formed when a beam of light passes through a circular aperture?

A

Airy disc

47
Q

At what distance can two light sources just be distinguished from each-other?

A

If the centre of the airy disc from one source is at least as far away as the first minimum of the other source

48
Q

What formula is used to calculate the minimum angular resolution?

A

Rayleigh Criterion

49
Q

What is D in the Rayleigh Criterion?

A

The diameter of the objective lens or the objective mirror

50
Q

What is θ in the Rayleigh Criterion?

A

Minimum angular resolution

51
Q

What size lenses are needed to see fine detail?

A

Very large

52
Q

What are the problems with refracting telescopes?

A

Chromatic aberration
Bubbles and impurities
Distorted lenses
Need to be very large

53
Q

What is chromatic aberration?

A

Glass refracts different colours of light to slightly different positions, causing the final image to be blurred

54
Q

Why do glass lenses have bubbles and impurities?

A

Good-quality glass is expensive and difficult to make

55
Q

What effect do bubbles and impurities in the glass have?

A

They absorb some of the light, so faint objects aren’t seen

56
Q

How can lenses become distorted?

A

Large lenses are heavy and must be supported from the edges, distorting their shape

57
Q

Why do refracting telescopes need to be very large?

A

The objective lens must have a very long focal length for a large magnification

58
Q

Why are are large mirrors of good quality better than large lenses?

A

They are cheaper to build, and can be supported from underneath

59
Q

Do mirrors suffer chromatic aberration?

A

No

60
Q

Do lenses suffer chromatic aberration?

A

Yes

61
Q

What is spherical aberration?

A

In mirrors that aren’t quite parabolic, parallel rays that are reflected do not converge exactly onto the same point

62
Q

What notable telescope suffered from spherical aberration?

A

Hubble Space Telescope

63
Q

What problem did the Hubble Space Telescope suffer from?

A

Spherical aberration

64
Q

What is a Charge-Coupled Device (CCD)?

A

A small silicon chip divided into a grid of millions of identical pixels

65
Q

How do Charge-Coupled Devices (CCDs) work?

A

Arriving photons excite electrons in each silicon pixel, creating a charge that can be measured to create a digital signal

66
Q

What two properties does a digital signal in a pixel in a Charge-Coupled Device (CCD) describe?

A

Where it is, how bright it is

67
Q

What are the applications of Charge-Coupled Devices (CCDs)?

A

Digital cameras, barcode scanners, giant astronomical telescopes

68
Q

What is quantum efficiency?

A

The proportion of incident photons that are detected

69
Q

What is the quantum efficiency of the human eye?

A

Around 1%

70
Q

What is the quantum efficiency of a Charge-Coupled Device (CCD)?

A

80% +

71
Q

What is the detectable light spectrum of the human eye?

A

Only visible light

72
Q

What is the detectable light spectrum of a Charge-Coupled Device (CCD)?

A

Infrared, visible, UV

73
Q

What is the resolution of the human eye?

A

500 megapixels

74
Q

What is the resolution of a Charge-Coupled Device (CCD)?

A

50 megapixels

75
Q

What is the minimum resolvable distance of the human eye?

A

100 μm

76
Q

What is the minimum resolvable distance of a Charge-Coupled Device (CCD)?

A

10 μm

77
Q

Which is better at capturing fine detail - the human eye or Charge-Coupled Devices (CCDs)?

A

Charge-Coupled Devices (CCDs)?

78
Q

Which is more convenient to set up; a telescope or a Charge-Coupled Device (CCD)?

A

Telescope

79
Q

Why might a Charge-Coupled Device (CCD) be more convenient than a telescope?

A

Images are digital, so they can be stored and copied

A-Level Astrophysics (11 decks)

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