Astrophysics

Question 1

What are the three lines you draw on a ray diagram?

Answer 1

Central ray, parallel ray and focal ray

Question 2

What is the central ray?

Answer 2

The line through the centre of the lens

Question 3

What is the parallel ray?

Answer 3

The ray parallel to x-axis that goes through the second focal point

Question 4

What is the focal ray?

Answer 4

The ray through the first focal point that exits parallel to x-axis

Question 5

What are the two types of telescope?

Answer 5

Refracting and reflecting

Question 6

How do refracting telescopes work?

Answer 6

They use 2 convex lenses to capture light

Question 7

How do reflecting telescopes work?

Answer 7

They use a big mirror to capture light

Question 8

What are the two lenses refracting telescopes are made of?

Answer 8

The objective lens and the eye lens

Question 9

What does the objective lens do?

Answer 9

Converges rays from object to form real image inside the telescope

Question 10

What does they eye lens do?

Answer 10

Acts as a magnifying glass on real image to give a magnified virtual image that observer sees

Question 11

Define the angular magnification of a refracting telescope.

Answer 11

Acts as a magnifying glass on real image to give a magnified virtual image that observer sees

Question 12

What are the problems with refracting telescopes?

Answer 12

Chromatic abberation, absorb light, difficult to build, need to be long`

Question 13

Define chromatic abberation.

Answer 13

Glass refracts different colours of light by different amounts and so the image for each colour is in a slightly different position which blurs the image

Question 14

Why does the absorption of light become a problem for refracting telescopes?

Answer 14

Impurities and bumps in glass absorb light so faint objects can be difficult to see

Question 15

Why are refracting telescopes difficult to build?

Answer 15

Building large lenses is difficult and expensive. Lenses are heavy and can only
be supported from edges – this means they can deform under their own weight

Question 16

Why do refracting telescopes need to be long?

Answer 16

For large magnification object lens needs very long focal length so telescope
has to be very long (expensive and needs a large building to put it in)

Question 17

Define focal point.

Answer 17

Point where all rays meet

Question 18

What are Cassegrain telescopes made of?

Answer 18

They use two curved mirrors big concave and small convex) and one convex lens

Question 19

What abberation do mirrors suffer from, why?

Answer 19

Spherical, if the shape of the mirror isn’t
quite parabolic light reflects at wrong angles and doesn’t focus on
one point only, so don’t have a sharp image

Question 20

What is a CCD?

Answer 20

Charged Coupled Devices, silicon chips that record light

Question 21

How do CCDs work?

Answer 21

They use the photoelectric effect – photons hit the surface of chip – release electrons that build up – places with more electrons form brighter digital image

Question 22

What sort of EM radiation can CCDs detect?

Answer 22

They detect infra-red, visible and UV

Question 23

Why are CCDs better for detecting details in images?

Answer 23

Human eye needs about 500 Megapixels to see a smooth image, CCDs only need a resolution of 50 Megapixels to see a smooth image

Question 24

Define minimum angular resolution.

Answer 24

The smallest angle for which we can still see two distinct objects using a telescope

Question 25

Define resolving power.

Answer 25

A measure of how much detail you can see

Question 26

What is an Airy disc?

Answer 26

When you look through a telescope you get diffraction called an Airy disc

Question 27

Two objects can be distinguished if?

Answer 27

The distance between the centre of the Airy disc of object 1 and the centre of the Airy disc of object 2 = distance of centre of disc 2 to first minimum of disc 2

Question 28

How do you calculate minimum angular resolution in radians?

Answer 28

θ = λ / D D = diameter of objective lens, m

Question 29

What is the equation for resolving power for a radio telescope?

Answer 29

θ = λ / D D = dish diameter

Question 30

How do X-Ray telescopes work?

Answer 30

X-ray telescopes use metal mirrors to collect x-rays that graze them

Question 31

How do you find the collecting power of a telescope?

Answer 31

The collecting power (C) of a telescope is proportional to the square of the dish diameter (the area)

Question 32

What positioning do infra red telescopes need to be in?

Answer 32

Infra-Red is absorbed by water vapour so IR telescopes on Earth they need to be in dry places high up

Question 33

What positioning do infra red/ UV/ X-ray telescopes need to be in?

Answer 33

Absorbed by the higher layers of the atmosphere, need to be above the atmosphere (on weather balloons or aeroplanes or in space)

Question 33

What positioning do infra red/ UV/ X-ray telescopes need to be in?

Answer 33

Absorbed by the higher layers of the atmosphere, need to be above the atmosphere (on weather balloons or aeroplanes or in space)

Question 34

What positioning do radio telescopes need to be in?

Answer 34

Can use radio telescopes on Earth because radio waves reach the ground

Question 35

What positioning do optical telescopes need to be in?

Answer 35

On Earth, still and advantage to being higher up, less atmospheric ‘seeing’ - wobble from moving air