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9.Astrophysics - A level Physics > Telescopes > Flashcards

Telescopes Flashcards

1
Q

What are the 2 types of lenses?

A
  • Convex - converging lens
  • Concave - diverging lens
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2
Q

What is the principal axis?

A

An imaginary line that passes through the centre of a lens. A lens is constructed so that it is symmetrical about its principal axis.

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3
Q

What is the focal point?

A

The point at which rays parallel to the principal axis of a lens are brought to a focus.

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4
Q

What is a converging lens?

A

A lens that causes parallel rays (of light) to come to a focus point (on the principal axis).

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5
Q

What is the focal length?

A

The distance between the centre of a lens and the focal point.

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6
Q

What does the focal length depend on?

A

The curvature or thickness of the lens.
(The thicker/ more curved, the shorter the focal length - the shorter the length, the more magnification)

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7
Q

What are 5 products of real images?

A
  • light converges towards a focal point
  • always inverted
  • can be projected onto a screen
  • intersection of 2 solid lines
  • (example = image from a projector onto a screen)
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8
Q

What are 5 products of a virtual image?

A
  • light diverges away from a focal point
  • always upright
  • cannot be projected onto a screen
  • intersection of 2 dashed lines
  • example - image in a mirror
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9
Q

What are 3 main rules for constructing ray diagrams?

A
  1. Rays passing through the principal axis will pass through the optical centre of the lens un-deviated.
  2. Rays that are parallel to the principal axis will be refracted and pass through the focal point f.
  3. Rays passing through the focal point f will emerge parallel to the principal axis.
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10
Q

What are 3 ways images can be described?

A

Nature - real or virtual
Orientation - inverted or upright
Size - magnified, diminished, same size

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11
Q

What is the lens equation?

A

1/f = 1/u + 1/v
1/ focal length = 1/distance of the object from lens + 1/distance of the image from lens
(horizontal distance on ray diagram)

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12
Q

What is the equation for magnification?

A

M = hᵢ / hₒ
image height / object height

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13
Q

What is an example of a convex lens?

A

Long sighted glasses (reading glasses).

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14
Q

What is an example of a concave lens?

A

Short sighted glasses.

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15
Q

If the distance of the image from the lens is positive, is the image real or virtual?

A

Real (as it will form on the right side of the ray diagram).

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16
Q

What is the equation for angular magnitude?

A

angle subtended by image at eye/ angle subtended by object by unaided eye
(unaided eye = without the lens)

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17
Q

What is the structure of a refracting telescope?

A

2 converging lenses:
1 - objective lens - collects the lights from the (celestial) objects and brings it to a focus at its focal length (fₒ)
2 - eyepiece lens - placed at a distance of its focal length (fₑ) away from the image and produces parallel rays of light to be analysed (by the observer).

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18
Q

What does it mean when a telescope is in normal adjustment?

A

It is calibrated and in the optimal position for observing celestial objects.
The image is formed at infinity (very far away so light rays are parallel).

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19
Q

What are the conditions for a refracting telescope to be in normal adjustment?

A
  • Both lenses must be arranged so that their focal points meet in the same place.
  • Focal length of the objective lens must be longer than the focal length of the eyepiece.
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20
Q

How do you construct a refractor ray diagram?

A
  • Both focal points are marked and labelled at the same point on the principal axis with fo>fe
  • 3 off-axis rays drawn through the objective lens
  • 3 rays drawn through the eyepiece lens parallel to a construction line (construction line from focal point to where eyepiece meets axis)
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21
Q

What is the formula for the angular size of an object?

A

θ = h/d (θ in radians)
h is the height of the object being observed
d = the distance from the object to observing point

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22
Q

Where is the angle subtended by the object (with unaided eye) found on a refractor ray diagram?

A

The angle between the light ray that goes through the centre of the objective lens and the principal axis.

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23
Q

Where is the angle subtended by the image found on a refractor ray diagram?

A

angle between the construction line (the image to the centre of the eyepiece lens) to the principal axis.

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24
Q

How can the angle subtended by the object be described as (in a ratio)?

A

α = image height/ focal length of objective lens

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25
Q

How can the angle subtended by the image be described as (in a ratio)?

A

β = image height/ focal length of eyepiece

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26
Q

How does one achieve greater magnification with a refracting telescope?

A

Longer objective focal lengths and shorter eyepiece focal lengths.
(refractor must be very long (since length = fₒ +fₑ)

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27
Q

What is the most common reflecting telescope?

A

The Cassegrain telescope.

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28
Q

What is the law of reflection?

A

The angle of incidence = the angle of reflection

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29
Q

How are reflecting telescopes structured?

A

They use 2 mirrors and an eyepiece:
- The primary mirror - large and concave - incident light reflects towards the focal point which is behind the secondary mirror
- The secondary mirror - smaller and convex - light reflects again to form a real, magnified image at the eyepiece
- Eyepiece - rays directed through an aperture towards this lens which is located behind the primary mirror

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30
Q

What are 4 important features of a ray diagram for a Cassesgrain telescope?

A
  • Rays enter telescope parallel to principal axis
  • The curvature of the mirrors do NOT have to be the same
  • Rays do NOT cross the secondary mirror, they only cross in the aperture of the primary mirror
  • Shading (or the lines on the mirror) indicates the non-reflective side of the mirrors
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31
Q

Which type of telescope is distorted by chromatic aberration?

A

Refracting telescopes.

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32
Q

What is chromatic abberation?

A

Different wavelengths of light are refracted by different amounts causing the edges of an image to appear coloured.

33
Q

Why does chromatic aberration occur?

A
  • Blue light has a shorter wavelength than red light, meaning blue light refracts more by a lens than red light.
  • Different colours are brought to focus at different points.
34
Q

How can chromatic aberration be reduced?

A

A second diverging lens in the refracting telescope which refracts the light in the opposite direction (this allows the red light to be brought to the same focal point as the blue.

35
Q

What type of telescope is distorted by spherical aberration?

A

BOTH refracting and reflecting telescopes.

36
Q

What is spherical aberration?

A

Rays of light come to focus at different points due to the spherical curvature of a lens, or mirror, causing the image to become very blurred.

37
Q

Why does spherical aberration occur?

A
  • The positions of the focal points depend son where the rays of light meet the lens/mirror
  • The further away the ray is from the principal axis, the shorter its focal length
38
Q

How do you reduce spherical aberration in refracting telescopes?

A
  • Using a parabolic lens - there is a limit to how much this improves the image quality, as this further increases the size and weight of the lens.
39
Q

How do you overcome spherical aberration in reflecting telescopes?

A

A parabolic mirror - gets rid of it entirely.

40
Q

What are 3 advantages to reflecting telescopes?

A
  • parabolic mirrors = no aberration
  • up to 8m + huge diameters = greater magnification
  • Can observe non-visible light wavelengths (and can be sent to space to observe these)
41
Q

What are 2 advantages to refracting telescopes?

A
  • Require less maintenance than reflectors
  • Not as sensitive to temperature changes as reflectors
42
Q

What are 3 disadvantages to refracting telescopes?

A
  • Difficult to have large diameter glass lenses which are completely free form defects (smaller mag)
  • heavy - difficult to manoeuvre
  • Image quality - aberration
43
Q

What are 3 disadvantages to reflecting telescopes?

A
  • Secondary mirror blocks light (entering primary mirror)
  • Diffraction at secondary mirror
  • Regular maintenance (since they’re exposed to air)
44
Q

What is the Rayleigh criterion?

A

Two sources will be resolved if the central maximum of one diffraction pattern coincides with the first minimum of the other.

45
Q

What is the airy disc?

A

The central bright maximum of the pattern produced by light diffracted when passing through a circular aperture.

46
Q

When is a circular interference pattern found?

A

When light is diffracted through a circular aperture instead of a rectangular slit.

47
Q

How would you increase the resolving power of a telescope?

A
  • Increase the diameter of the aperture
  • Operating at shorter wavelengths of light
48
Q

What is the equation for angular separation?

A

θ = s/d
θ = angular separation (rad)
s = distance between the two sources (m)
d = distance between the sources and the observer (m)

49
Q

What is the equation for the Rayleigh Criterion?

A

θ ≈ λ/D
θ = minimum angular resolution of the telescope (rad)
λ = operating wavelength of the telescope (m)
D = diameter of the telescope’s aperture (m)

50
Q

Using the Rayleigh Criterion equation, how can you know when sources are resolvable?

A

θ > λ/D

51
Q

Using the Rayleigh Criterion equation, how can you know when sources are just about resolvable?

A

θ ≈ λ/D
(minimum angular separation)

52
Q

Using the Rayleigh Criterion equation, how can you know when sources are not resolvable?

A

θ < λ/D

53
Q

For a circular aperture, what value is it multiplied by?

A

1.22

54
Q

What is collecting power defined as?

A

A measure of the amount of light energy it collects per second.

55
Q

What is the relationship between the collecting power of a telescope and its diameter? (and why?)

A

collecting power ∝ diameter²
- Intensity ∝ surface area
- surface area of circular object = πD²/4

56
Q

What are 2 advantages of larger aperture diameter telescopes?

A
  • They have a greater collecting power so images are brighter
  • They have a greater resolving power so images are clearer
57
Q

If given 2 telescopes, how would you find the collecting power of one? (RATIOS)

A

collecting power of tele1/collecting power of tele2 = (D1/D2)²
(can find one unknown with this)
(also ratios can be used with resolving power as well)

58
Q

What is an optical telescope?

A

A telescope that detects wavelengths of visible light.

59
Q

What wavelengths of light are completely absorbed by Earth’s atmosphere?

A

Gamma rays, x-rays, UV - blocked by upper atmosphere
Longer radio waves - blocked by atmosphere

60
Q

What wavelengths of light are partially absorbed by Earth’s atmosphere?

A

Infra-red - Mostly absorbed by atmospheric gases (best observed from space).

61
Q

What wavelengths of light are detectable from earth?

A

Shorter radio waves
Visible light (a little atmospheric distortion)

62
Q

What are 2 main advantages to putting telescopes into space?

A
  • No absorption of electromagnetic waves by atmosphere
  • No light pollution
63
Q

What is the structure of a radio telescope?

A
  • 1 primary reflecting parabolic dish reflects radio waves
  • A detector in the middle
64
Q

What is the similarity in structure between radio and optical telescopes?

A
  • Both use parabolic surfaces to reflect waves.
65
Q

What are 2 differences in structure between radio and optical telescopes?

A

Radio has a single primary reflector and optical has 2 mirrors
- Radio dish does not need to be as smooth as optical mirrors

66
Q

What is the similarity in the positioning of optical and radio telescopes?

A
  • Both can be ground-based as the atmosphere is transparent to these wavelengths
67
Q

What are the differences in the positioning of optical and radio telescopes?

A
  • Optical must be placed higher up (to avoid atmospheric distortions) and away from cities (no light pollution)
  • Radio must be located remotely (away from radio sources)
68
Q

What can optical and radio wave telescopes both be used for?

A

To detect hydrogen emission lines

69
Q

What can radio telescopes be used for that optical telescopes aren’t?

A

Radio waves aren’t absorbed by dust (like optical waves are) so they can be used to map the Milky Way

70
Q

How does the resolving power of optical and radio telescopes compare?

A
  • Radio waves are longer than optical waves, so they have a much lower resolving power
    (Optical = more detailed imaging)
71
Q

How does the collecting power of radio and optical telescopes compare?

A
  • Radio telescopes are larger in diameter so have a greater collecting power than optical telescopes
    (Radio = Brighter images)
72
Q

What is a Charge-Coupled Device? (CCD)

A

A detector (in the form of a chip) is a highly photon sensitive device, made of silicon. It is an array of light sensitive pixels.

73
Q

How do CCDs work?

A
  • Incident photons cause electrons to be released
  • The number of electrons released is proportional to the intensity of the incident light
  • An image is formed on the CCD, which can be processed electronically to give a digital image.
74
Q

What is the definition of quantum efficiency?

A

The percentage of incident photons which cause an electron to be released.

% = number of electrons produced per second/ number of photons absorbed per second x100

75
Q

What is the quantum efficiency of a human eye?

A

1-4%

76
Q

What is the quantum efficiency of a photographic film?

A

4-10%

77
Q

What is the quantum efficiency of a CCD?

A

70-90%

78
Q

What is the resolution of a CCD in comparison to the human eye?

A

CCD - average is 10μm and the human eye averages 100μm
(The smaller the size of the pixel, the better the resolution, therefore the clearer the image)

79
Q

What is the difference in convenience between CCDs and the human eye?

A

CCD - needs to be set up but images are digital
Human eye - simpler to use as there is no extra equipment needed.

9.Astrophysics - A level Physics (3 decks)

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