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Physics (Elliott) > telescopes > Flashcards

Flashcards in telescopes Deck (26):
1

Why were telescopes invented?

The invention of refracting and reflecting telescopes has enabled scientists to study the solar system, galaxy and universe in more detail than they were able to do with the naked eye.

2

What is refraction?

The speed of a wave (eg light) is affected by the medium it passes through. When light passes from one substance into one with a different optical density, it will speed up or slow down.

Because the frequency (number of cycles in a given time period) of the light remains constant, the wavelength changes, and this causes a change in direction. This effect is called refraction.

3

what happens with refraction of a light ray passing into a Perspex block from air:

Light rays slow down when passing from the air into the Perspex block, so they bend towards the normal (a line drawn at 90o from the edge of the block of Perspex at the point where the light hits it). As they leave the block and pass into the air, they speed up and bend away from the normal.

4

How do converging lenses work?

A converging lens uses refraction to bend the light rays coming out of it inwards. This has the effect of focusing the light to a point called the focal point.

5

does a convex Lens have more power?




Lens power

A convex lens with a greater degree of curvature, but made from the same material, will have a shorter focal length and therefore be a more powerful lens

6

How is the power of a lens calculated ?

The power of a lens can be calculated using the following formula:

power of lens (dioptres) = 1/focal length of lens (metres)

For example, a lens which has a focal length of 0.1 metres has a power of 10 dioptres

7

why do rays of light enter Telescopes as parallel?

Telescopes

Astronomical objects such as stars are so far away that the light rays that reach us from them are effectively parallel. We can therefore draw the rays of light entering a telescope as parallel rays.

Because the light from distant stars will be very faint, a large telescope will be needed to collect enough light to make a visible image.

8

Why are large telescopes needed?

Because the light from distant stars will be very faint, a large telescope will be needed to collect enough light to make a visible image.

9

Describe a simple optical telescopes

A simple optical telescope will have two converging lenses of different powers. The more powerful lens will be the eyepiece lens.

10

Describe the objective lens and the eyepiece lens.

The objective lens focuses the light to the focal point of the lens. This point is also the focal point of the more powerful eyepiece lens. The eyepiece lens produces a magnified image of the image from the objective lens which the viewer can see.

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11

How do you calculate the distance between the lenses?

The distance between the lenses will be the sum of the two focal lengths of the lenses

12

How does an Astronomical (reflecting) telescope work?

In a reflecting telescope, instead of a convex objective lens, a concave mirror is used to collect parallel rays from the object and form an image at the focal point. Then the convex eyepiece lens is used to magnify this image for the viewer

13

How do concave mirrors work?

Concave mirrors focus light at a focal point using reflection, not refraction.

14

Why are mirrors are used instead of lenses?

Mirrors are used instead of lenses in large, professional telescopes. This is because an objective lens can have a maximum diameter of approximately 1 metre before it begins to sag, whereas a mirror of several metres in diameter can be used.

15

what advantages do mirrors have (rather than lenses)?

Mirrors (rather than lenses) have other advantages:
They can be made very smooth - resulting in undistorted images.
Suitable reflectors can be used to focus all types of electromagnetic radiation.
Mirrors reflect rays of all colours in the same way.

16

what is Diffraction?

When waves move through a narrow gap or past an obstacle, they spread out from the edges. This is called diffraction.

Diffraction is most significant when the gap or obstacle is very small compared to the wavelength of the light.

When waves move through a narrow gap or past an obstacle, they spread out from the edges. This is called diffraction.

Diffraction is most significant when the gap or obstacle is very small compared to the wavelength of the light.

17

How is a spectrum produced?

When light passes from air through a glass block that has parallel sides, it emerges parallel to the path of the light ray that entered the block.

18

what is dispersion?

Red light is refracted the least when it enters the block, and violet light is refracted the most. This effect is called dispersion. However, this difference is corrected when the light leaves the block, so the effect is not noticeable.

19

what is refraction ?

Refraction of white light through a glass prism, which has sides that are not parallel, causes this difference in refraction of different colours of light to be exaggerated - so we see a spectrum of light emerging from the prism

20

how can a spectrum also be produced ?

spectrum can also be produced when white light passes through a diffraction grating.

21


How is a distant extended light source seen ?

Light from a distant extended source

A distant extended source might be a planet, galaxy or moon. The image seen will be inverted (upside down).

22

what is the formula for calculating the angular magnification of a telescope?

angular magnification = focal length of objective lens/focal length of eyepiece lens

23

Professional telescopes are quite large. why?

Professional telescopes are quite large. One reason for this is that they need to collect visible light and other electromagnetic radiation to detect faint objects.

24

radiation is diffracted by what ?

radiation is diffracted by the aperture of a telescopex

25

What is the aperture?

The aperture is the hole through which the light must pass.

26

How are sharp images produced?

In order to produce sharp images, the aperture must be very much larger than the wavelength of the radiation which is detected by the telescope.