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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
What does the eye lens do?
Acts as a 'magnifying glass' on the real image to form a magnified virtual image
26
How can we assume that the real image formed by the objective telescope is formed on the focal plane?
If you assume the object is at infinity, the rays from it are parallel. Therefore, the image must be on the focal plane
27
In a telescope, where is the principal focus of the objective lens set up to be?
In the same position as the principal focus of the eye lens
28
In a telescope, where is the principal focus of the eye lens set up to be?
In the same position as the principal focus of the objective lens
29
What is the magnification (M) equation relating to angles?
θᵢ / θ₀
30
What is θᵢ?
The angle subtended by the image at the eye
31
What is θ₀?
The angled subtended by the object at the eye
32
Why is θᵢ > θ₀ for the magnification equation?
Because it is a magnifying telescope
33
What is f₀?
The focal length of the objective lens
34
What is fₑ?
The focal length of the eye lens
35
What is the magnification (M) equation relating to the focal length?
In the formula book
36
What is a Cassegrain reflecting telescope made up of?
Primary mirror, Secondary mirror, Eye lens
37
What is the shape of a reflecting telescope's primary mirror?
Parabolic concave
38
What is the shape of a Cassegrain reflecting telescope's secondary mirror?
Convex
39
What is the main characteristic of a Cassegrain arrangement?
The presence of a convex secondary mirror
40
What necessitates the presence of a convex secondary mirror for a Cassegrain arrangement?
The principal focus of the primary mirror is in front of the mirror, so an observer would block out the light
41
What is the resolving power of a telescope?
A measure of how much detail you can see
42
What is the resolving power fo a telescope dependent on?
The minimum angular resolution
43
What is the minimum angular resolution?
The smallest angular separation at which the telescope can distinguish two points
44
What phenomena limits resolution?
Diffraction
45
What is formed when a beam of light passes through a circular aperture?
A diffraction pattern is formed
46
What is the central circle of the diffraction pattern formed when a beam of light passes through a circular aperture?
Airy disc
47
At what distance can two light sources just be distinguished from each-other?
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
What formula is used to calculate the minimum angular resolution?
Rayleigh Criterion
49
What is D in the Rayleigh Criterion?
The diameter of the objective lens or the objective mirror
50
What is θ in the Rayleigh Criterion?
Minimum angular resolution
51
What size lenses are needed to see fine detail?
Very large
52
What are the problems with refracting telescopes?
Chromatic aberration Bubbles and impurities Distorted lenses Need to be very large
53
What is chromatic aberration?
Glass refracts different colours of light to slightly different positions, causing the final image to be blurred
54
Why do glass lenses have bubbles and impurities?
Good-quality glass is expensive and difficult to make
55
What effect do bubbles and impurities in the glass have?
They absorb some of the light, so faint objects aren't seen
56
How can lenses become distorted?
Large lenses are heavy and must be supported from the edges, distorting their shape
57
Why do refracting telescopes need to be very large?
The objective lens must have a very long focal length for a large magnification
58
Why are are large mirrors of good quality better than large lenses?
They are cheaper to build, and can be supported from underneath
59
Do mirrors suffer chromatic aberration?
No
60
Do lenses suffer chromatic aberration?
Yes
61
What is spherical aberration?
In mirrors that aren't quite parabolic, parallel rays that are reflected do not converge exactly onto the same point
62
What notable telescope suffered from spherical aberration?
Hubble Space Telescope
63
What problem did the Hubble Space Telescope suffer from?
Spherical aberration
64
What is a Charge-Coupled Device (CCD)?
A small silicon chip divided into a grid of millions of identical pixels
65
How do Charge-Coupled Devices (CCDs) work?
Arriving photons excite electrons in each silicon pixel, creating a charge that can be measured to create a digital signal
66
What two properties does a digital signal in a pixel in a Charge-Coupled Device (CCD) describe?
Where it is, how bright it is
67
What are the applications of Charge-Coupled Devices (CCDs)?
Digital cameras, barcode scanners, giant astronomical telescopes
68
What is quantum efficiency?
The proportion of incident photons that are detected
69
What is the quantum efficiency of the human eye?
Around 1%
70
What is the quantum efficiency of a Charge-Coupled Device (CCD)?
80% +
71
What is the detectable light spectrum of the human eye?
Only visible light
72
What is the detectable light spectrum of a Charge-Coupled Device (CCD)?
Infrared, visible, UV
73
What is the resolution of the human eye?
500 megapixels
74
What is the resolution of a Charge-Coupled Device (CCD)?
50 megapixels
75
What is the minimum resolvable distance of the human eye?
100 μm
76
What is the minimum resolvable distance of a Charge-Coupled Device (CCD)?
10 μm
77
Which is better at capturing fine detail - the human eye or Charge-Coupled Devices (CCDs)?
Charge-Coupled Devices (CCDs)?
78
Which is more convenient to set up; a telescope or a Charge-Coupled Device (CCD)?
Telescope
79
Why might a Charge-Coupled Device (CCD) be more convenient than a telescope?
Images are digital, so they can be stored and copied

A-Level Astrophysics (11 decks)

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