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Physics P7 > 7.2 - Light, telescopes and images > Flashcards

7.2 - Light, telescopes and images Flashcards

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1
Q

What is speed of a wave affected by?

A

The medium it is travelling through.

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2
Q

What does a change of speed cause? Why?

A

A change in wavelength because the FREQUENCY OF WAVES CANNOT CHANGE.

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3
Q

As a result of a change in wavelength, what does this lead to?

A

A change in direction.

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4
Q

What is refraction?

A

When waves hit a boundary between two materials at an angle they change direction as they pass from one medium to another.

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5
Q

As light enters a glass block it slows down and refraction causes it to what?

A

Deviate TOWARDS the normal and SLOWS DOWN.

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6
Q

When light leaves the glass block, what happens?

A

Light bends AWAY from the normal and SPEEDS UP.

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7
Q

What is the ‘normal’?

A

A construction line at right angles to the surface of the glass.

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8
Q

Generally, what is refraction used by?

A

A lens to bring light into focus.

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9
Q

What is a Convex Lens?

A

A lens that is shaped in such a way so that it is thicker in the middle than at the edges.

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10
Q

When light enters a convex lens, what happens?

A

Light will be turned INWARDS as it deviates TOWARDS the normal.

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11
Q

On leaving the lens, what happens?

A

The light deviates AWAY from the normal but the shape of the lens means that this also turns the light INWARDS.

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12
Q

When light rays all CONVERGE on a single point, what is the scientific name for this?

A

Focal point

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13
Q

What do Convex Lens do?

A

Brings rays of light together. Parallel rays entering the lens are brought together at the focal point.

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14
Q

How much the rays are turned inwards depends on….?

A

The material of the lens and how curved it is.

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15
Q

The more curved the surface of the lens is, the….?

A

The further the rays are turned inwards and the shorter the focal length.

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16
Q

What is Power measured in?

A

Dioptres

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17
Q

The more powerful a lens, the….?

A

…The shorter the focal length.

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18
Q

What is the formula for Power?

A

Power (Dioptres) = 1 /Focal Length (metres)

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19
Q

What two features does a more curved lens possess?

A
  1. MORE powerful

2. SHORTER Focal Length

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20
Q

Why do we present light rays as parallel lines?

A

Astronomical objects are so distant that the light rays from them reaching Earth are effectively parallel sets of rays.

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21
Q

When a lens is used to focus the light from a distant star which is on the PRINCIPAL AXIS of the lens then….?

A

The image will be formed at the FOCAL POINT.

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22
Q

What would happen if the object is off the principal axis?

A

The rays will still be brought to a focus but the image will be formed off the principal axis.

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23
Q

When objects look larger than a ‘point’ (due to their distance), what is this called?

A

Extended objects.

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24
Q

How would you construct an image using a Ray Diagram?

A
  1. The Ray that passes through the centre of the lens continues in a straight line. Construct this.
  2. Make all the other rays refract (bend) towards the point of the straight line.
  3. The image is formed.
25
Q

What does a simple telescope contain?

A

Two converging (convex) lens: an EYEPIECE lens and an OBJECTIVE lens.

26
Q

What is the Objective Lens?

A

A LOW power lens with a LONG focal length. It has a large diameter to collect more light and make a brighter image.

27
Q

What is the Eyepiece Lens?

A

A smaller but a HIGH power lens with a SHORT focal length. It acts like a magnifying glass to magnify the image formed by the objective lens.

28
Q

What is the function of a telescope?

A

To make objects appear larger.

29
Q

How does a telescope work?

A

It works by making the angle between the light rays and the axis of the telescope BIGGER.

30
Q

What does the objective lens form when the parallel rays are converged?

A

It forms a REAL image between the two lens.

31
Q

What does the eyepiece lens form when the light enters it?

A

A MAGNIFIED, VIRTUAL image.

32
Q

What is the formula for (angular) magnification?

A

Magnification = Focal Length of Objective Lens / Focal Length of Eyepiece Lens

33
Q

What do most astronomical telescope use?

A

Concave Mirror

34
Q

What is the brief description of a Concave Mirror?

A

They’re shiny on the inside of the curve. Parallel rays of light shining on a concave mirror reflect and converge.

35
Q

What is the centre of the mirror’s surface called?

A

Vertex

36
Q

Where is the focal point in a Concave Mirror? Where do all the points lie on?

A

Halfway between the centre of the curvature and the vertex. These points all lie on the axis.

37
Q

How is the image received by the user?

A

When the light converges from the Concave Mirror, the light reflects on a Secondary Mirror which reflects on the Eyepiece. This performs the final magnification.

38
Q

What are the 4 main advantages that Concave Mirrors have over Convex Lenses?

A
  1. It is easier to manufacture a large mirror than a large lens.
  2. A larger mirror can be much thinner than a large lens so is lighter.
  3. A large mirror can be supported all along its rear side but a lens can only be supported at the edge, otherwise the light passing through the lens is blocked.
  4. It does not cause CHROMATIC ABERRATION.
39
Q

What is Chromatic Aberration?

A

When light passes through a lens it is refracted towards the normal, however different colours of light are refracted.

40
Q

In terms of telescopes, what effect does Chromatic Aberration have on them?

A

The light is separated into different colours and the image will appear coloured where it should not be.

41
Q

How did Isaac Newton solve the issue of Chromatic Aberration when building the Newtonian Reflecting Telescope in 1668?

A

Using a mirror in place of the objective lens.

42
Q

What is diffraction?

A

When waves pass through a gap/obstacle and spread out.

43
Q

How intense will diffraction be if a narrow gap was placed?

A

MORE diffraction.

44
Q

The ______ the wavelength the _____the amount of diffraction.

A

The LONGER the wavelength the GREATER the amount of diffraction.

45
Q

Other than light telescopes, what types of telescopes are there?

A

Radio, X-Ray, etc.

46
Q

What happens when radiation enters a telescope?

A

It is diffracted by the APERTURE of the telescope.

47
Q

What is the Aperture of a telescope?

A

The size of the objective lens/objective mirror.

48
Q

Because the aperture is circular, what would happen to a diffracted image of a star?

A

It will appear blurry with a bright central disc surrounded by circular rings instead of a sharp point.

49
Q

How can we produce sharp images?

A

The aperture must be very much larger than the wavelength. The longer the wavelength of the radiation being detected, the larger the aperture needs to be.

50
Q

In terms of radio telescopes, what kind of aperture is needed?

A

Radio waves have a LONG wavelength so radio telescopes are very large and are often joined together in an array to create a telescope with an aperture HUNDREDS OF METRES across.

51
Q

When studying the spectrum of radiation emitted by a star, what information can that give us?

A

The star’s COMPOSITION and TEMPERATURE.

52
Q

How can we separate light into the spectrum of colours?

A

By using a prism.

53
Q

What is white light?

A

A mixture of ALL the colours in the spectrum.

54
Q

The ______ the wavelength of light, the _____ it is refracted.

A

The SHORTER the wavelength the MORE it is refracted.

55
Q

How else can we produce a spectrum?

A

A diffraction grating.

56
Q

What is a diffraction grating?

A

Either a series of thousands of finely spaced gaps that light shines through OR a series of finely spaced lines drawn on a reflective surface that light reflects off.

57
Q

When astronomers discovered quasars, what did they notice?

A

That their spectrum was highly shifted to the RED end of the spectrum.

58
Q

What does this very large redshift mean?

A

That the Quasar must be moving away from us at a very high speed (and must be very far away).

Physics P7 (5 decks)

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