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AQA A Level Physics Paper 3B - Astrophysics > Telescopes > Flashcards

Telescopes Flashcards

1
Q

What effect do converging (convex) lenses have on light rays?

A

Converging lenses bring light rays together

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2
Q

How do lenses change the direction of light rays?

A

Lenses change the direction of light rays by refraction

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3
Q

Explain how converging (convex) lenses bring light rays together

A

Rays parallel to the principle axis of the lens converge onto a point called the principal focus. Parallel rays that aren’t parallel to the principal axis converge somewhere else on the focal plane

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4
Q

Define the term focal length (f)

A

The focal length (f) is the distance between the lens axis and the focal plane

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5
Q

Draw a labelled diagram to show the effect of parallel rays passing through a converging lens

A

See page 172 in the revision guide

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6
Q

What are the two different types of images?

A

Real and virtual

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7
Q

When is a real image formed?

A

A real image is formed when light rays from an object are made to pass through another point in space. The light rays are actually there and the image can be captured on a screen

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8
Q

When is a virtual image formed?

A

A virtual image is formed when light rays from an object appear to have come from another point in space. The light rays aren’t really there where the image appears to be, so the image can’t be captured on a screen

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9
Q

What type of images can converging lenses form?

A

Converging lenses can form both real and virtual images, depending on where the object is. If the object is further than the focal length away from the lens the image is real. If the object’s closer the image is virtual

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10
Q

What is the focal point (principal focus)

A

The focal point (principal focus) is the point on the principal axis through which all rays parallel to the principal axis pass after being refracted by the lens

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11
Q

How many focal points does a converging lens have?

A

A converging lens has two focal points, one at each side of the lens but they are both the same distance away from the middle of the lens

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12
Q

Explain how to work out where an image will appear by drawing a ray diagram

A
  • To work out where an image will appear draw a ray diagram
  • Draw two rays from the same point on the object (the top is best) one parallel to the principal axis that passes through the principal focus and one passing through the centre of the lens that doesn’t get refracted
  • The image will form where the two rays meet if the image is real or where the two rays appear to have come from if the image is virtual
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13
Q

Draw two ray diagrams showing where an image will appear for a real and virtual image

A

See page 172 in the revision guide

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14
Q

State the lens equation and each of its variables

A
  • 1/f = 1/u + 1/v
  • u is the distance between the object and the lens axis
  • v is the distance between the image and the lens axis ( positive if the image is real and negative if the image is virtual)
  • f is the focal length
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15
Q

How many lenses does a refracting telescope use?

A

A refracting telescope uses two converging lenses

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16
Q

What two converging lenses does a refracting telescope use and what are their purpose?

A
  • The objective lens converges the rays from the object to form a real image
  • The eye lens acts as a magnifying glass on this real image to form a magnified virtual image
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17
Q

What are the two types of optical telescopes?

A

Refracting and Reflecting

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18
Q

For a refracting telescope why is it assumed that the object is at infinity?

A

If you assume the object is at infinity then the rays from it are parallel and the real image is formed on the focal plane

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19
Q

How is a refracting telescope set up?

A

A telescope (in normal adjustment) is set up so that the principal focus of the objective lens is in the same position as the principal focus of the eye lens so the final magnified image appears to be at infinity

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20
Q

What is the focal plane?

A

The focal plane is the point where the rays meet after passing through a lens

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21
Q

How can the magnification, M of the refracting telescope be calculated?

A
  • It can be calculated in terms of angles or the focal length
  • The angular magnification is the angle subtended by the image θi over the angle subtended by the object θo at the eye
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22
Q

What are the two formulas used to calculate the magnification, M of the refracting telescope?

A
  • M = θi/θo
  • M = fo/fe
    fo and fe refer to the objective and eye lenses
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23
Q

What is the relationship between fo and fe for a large magnification

A

A large magnification is needed so fo>fe

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24
Q

What does a reflecting telescope use?

A

A reflecting telescope uses two mirrors and a converging lens

25
Q

Explain how a reflecting telescope works

A
  • A parabolic concave mirror (the primary mirror) converges parallel rays from an object forming a real image
  • An eye lens magnifies the image as before
  • The principle focus of the mirror (where the image is formed) is in front of the mirror so an arrangement needs to be devised where the observer doesn’t block out the light
  • A set-up called the Cassegrain arrangement which uses a convex secondary mirror is a common solution to this problem
26
Q

Draw a diagram showing how a reflecting telescope works

A

See page 173 in the revision guide
See page 8 in the telescopes pack

27
Q

What is the resolving power of a telescope?

A
  • The resolving power of a telescope is just a measure of how much detail you can see
  • It’s dependent on the minimum angular resolution - the smallest angular separation at which the instrument can distinguish two points
28
Q

The smaller the minimum angular resolution of a telescope …

A

the better the resolving power

29
Q

What is the relationship between resolution and diffraction?

A

Resolution is limited by diffraction. If a beam of light passes through a circular aperture then a diffraction pattern is formed. The central circle is called the Airy disc

30
Q

When can two light sources be distinguished?

A

Two light sources can just be distinguished if the centre of the Airy disc from one source is at least as far away as the first minimum of the other source

31
Q

State the Rayleigh criterion

A

Two objects can just be resolved when the central maximum of the diffraction pattern of one coincides with the first minimum of the other

32
Q

State the equation given by the Rayleigh critierion

A
  • θ = λ/D
  • θ is the minimum angular resolution in radians
  • λ is the wavelength of the light in metres
  • D is the diameter of the aperture in metres or for telescopes it is the diameter of the objective lens or the objective mirror
33
Q

What are the disadvantages of refracting telescopes?

A

1- Glass refracts different colours of light by different amounts and so the image for each colour is in a slightly different position. This blurs the image and is called chromatic aberration
2- Any bubbles and impurities in the glass absorb some of the light which means that very faint objects aren’t seen. Building large lenses that are of a sufficiently good quality is difficult and expensive
3- Large lenses are very heavy and can only be supported from their edges so their shape can become distorted
4- For a large magnification the objective lens needs to have a very long focal length. This means that refracting telescopes have to be very long, leading to very large and expensive buildings needed to house them

34
Q

What are the disadvantages of reflecting telescopes despite being better than refracting telescopes?

A

1- Large mirrors of good quality are much cheaper to build than large lenses. They can also be supported from underneath so they don’t distort as much as lenses
2- Mirrors don’t suffer from chromatic aberration but can have spherical aberration. If the shape of the mirror isn’t quite parabolic, parallel rays reflecting off different parts of the mirror do not all converge onto the same point

35
Q

What are Charged-Coupled Devices (CCDs)?

A

CCDs are very sensitive image detectors

36
Q

Explain how CCDs work

A
  • CCDs are silicon chips about the size of a postage stamp divided up into a grid of millions of identical pixels
  • When photons hit the silicon in a pixel they cause electrons to be released. These electrons alter the charge on each pixel, this charge can be measured and used to create a digital signal
  • This signal describes not only where the light hits but its brightness/intensity too as the charge on each pixel will vary depending on how many photons hit it. This allows a digital image of an object to be created
37
Q

Where are CCDs used?

A
  • Digital cameras
  • Barcode scanners
  • Giant astronomical telescopes
38
Q

Define Quantum efficiency

A

Quantum efficiency is the proportion of the incident photons that are detected

39
Q

Compare CCDs and the Human eye as image detectors in terms of quantum efficiency

A

For a CCD quantum efficiency is typically 80% or more. The quantum efficiency of the eye is of the order of 1% so CCDs detect far more of the light that falls on them than the eye does

40
Q

Compare CCDs and the Human eye as image detectors in terms of detectable light spectrum

A

The eye can only detect visible light whereas CCDs can detect infrared, visible and UV light

41
Q

Compare CCDs and the Human eye as image detectors in terms of resolution

A
  • If you were to project the whole visual field of an eye onto a screen you’d need over 500 megapixels for the eye not to see any pixelation. CCDs on the other hand have the order of 50 megapixels so it seems like the eye captures more detail than a CCD
  • It is also important to see how far apart different parts of the object being viewed need to be in order for them to be distinguishable - this is called spatial resolution
  • The minimum resolvable distance of the human eye is around 100 µm whereas CCDs can have a spatial resolution of around 10 µm. So CCDs are better for capturing fine detail
42
Q

Compare CCDs and the Human eye as image detectors in terms of convenience

A

The human eye doesn’t need any extra equipment and looking down a telescope is simpler than setting up a CCD but CCDs produce digital images which can be stored, copied and shared globally

43
Q

Explain how radio telescopes work and list their features

A
  • The most obvious feature of a radio telescope is its parabolic dish. This works in exactly the same way as the objective mirror of an optical reflecting telescope
  • An antenna is used as a detector at the focal point instead of an eye or camera in an optical telescope but there is no equivalent to the eye lens
  • Most radio telescopes are manoeuvrable allowing the source of the waves to be tracked (in the same way as optical telescopes). The telescope moves with the source stopping it slipping out of view as the Earth rotates
44
Q

What would be needed for a radio telescope to have the same resolving power as an optical telescope?

A

1- The wavelengths of radio waves are about a million times longer than the wavelengths of light
2- The resolving power of a telescope is dependent on the Rayleigh criterion which is θ = λ/D
3- So for a radio telescope to have the same resolving power as an optical telescope, its dish would need to be a million times bigger. The resolving power of a radio telescope is worse than the unaided eye

45
Q

How do radio astronomers get around the poor resolving power of radio telescopes?

A
  • Radio astronomers get around this problem by linking lots of telescopes together
  • Using some computer programming, their data can be combined to form a single image. This is equivalent to one huge dish the size of the separation of the telescopes
  • Resolutions thousands of times better than optical telescopes can be achieved this way
46
Q

Why is making radio telescopes easier than making optical reflectors?

A

1- Instead of a polished mirror, a wire mesh can be used since the long wavelength radio waves don’t notice the gaps. This makes their construction much easier and cheaper than optical reflectors
2- The shape of the dish has to have a precision of about λ/20 to avoid spherical aberration. So the dish does not have to be anywhere near as perfect as a mirror
3- However, unlike an optical telescope a radio telescope has to scan across the radio source to build up the image

47
Q

What type of wavelengths of EM radiation does the atmosphere let through?

A

The atmosphere only lets certain wavelengths of electromagnetic radiation through and is opaque to all the others

48
Q

How does infrared radiation reach the Earth’s surface?

A
  • A few wavelengths of infrared radiation can reach the Earth’s surface but most are absorbed by water vapour in the atmosphere
  • On Earth, the best way to observe IR radiation is to set up in high and dry places
49
Q

Why does being in a high place such as a mountain not help when observing ultraviolet and X-ray radiation?

A
  • Most ultraviolet and X-ray radiation is absorbed higher up in the atmosphere so being in a high place doesn’t help
  • One way to get around this problem is to strap UV and X-ray telescopes to high altitude weather balloons or aeroplanes. They can take the telescope high enough into the atmosphere to detect the radiation
  • The ideal situation is to get your telescope above the atmosphere altogether by launching it into space and setting it in orbit around the Earth
50
Q

How do Infrared and Ultraviolet telescopes have a similar structure to optical telescopes?

A

1- Infrared and ultraviolet telescopes are very similar to optical reflecting telescopes. They both use the same parabolic mirror set-up to focus the radiation onto a detector
2- In both cases, CCDs or spherical photographic paper are used as the radiation detectors just as in optical telescopes
3- The longer the wavelength of the radiation, the less it’s affected by imperfections in the mirror. So the mirrors in infrared telescopes don’t need to be as perfectly shaped as in optical telescopes, but the mirrors in UV telescopes have to be even more precisely made

51
Q

What is a problem that Infrared telescopes have?

A

IR telescopes have the added problem that they produce their own infrared radiation due to their temperature. They need to be cooled to very low temperatures using liquid helium or refrigeration units

52
Q

How do X-rays reflect off surfaces differently to other EM radiation?

A

X-rays don’t reflect off surfaces in the same way as most other EM radiation. Usually X-ray radiation is either absorbed by a material or it passes straight through it

53
Q

In what situation do X-rays reflect?

A

X-rays do reflect if they just graze a mirror’s surface. By having a series of nested mirrors you can gradually alter the direction of X-rays enough to bring them to a focus on a detector. This type of telescope is called a grazing telescope

54
Q

How can X-rays be detected in a X-ray telescope?

A

The X-rays can be detected using a modified Geiger counter or a fine wire mesh. Newer X-ray telescopes such as the XMM-Newton telescope use highly sensitive X-ray CCD cameras

55
Q

What two main factors is the resolving power of a telescope limited by?

A

1- The Rayleigh criterion: This depends on the wavelength of the radiation and the diameter of the objective mirror or dish. So, for the same size of dish, a UV telescope has a much better resolving power than a radio telescope
2- The quality of the detector: Just like in digital cameras, the resolving power of a telescope is limited by the resolution of the detector. That can be how many pixels there are on a CCD, or for a wire mesh X-ray detector, how fine the wire mesh is

56
Q

What is the relationship between the collecting power and the collecting area of a telescope and state the corresponding proportionality formula

A
  • The collecting power of a telescope is proportional to its collecting area
  • Power ∝ Diameter^2
57
Q

As the collecting power of a telescope is proportional to its collecting area, what is this area for a radio, optical, UV or IR telescope?

A

For a radio, optical, UV, or IR telescope this is the area of the objective mirror or dish

58
Q

As the collecting power of a telescope is proportional to its collecting area, what is this area for X-ray telescopes?

A

For X-ray telescopes this is the area of the opening through which X-rays can enter the telescope

59
Q

In general how does the collecting power of X-ray telescopes compare to other types of telescopes?

A

In general X-ray telescopes have a much smaller collecting power than other types of telescope

AQA A Level Physics Paper 3B - Astrophysics (2 decks)

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