Astrophysics: Lenses and Telescopes

Question 1

Do mirrors have a principle axis

Answer 1

Yes

Perpendicular to its surface and passes through its centre

Question 2

What do lenses do

Answer 2

Change the direction of light rays

By refraction since light travels slower in glass than in air

Question 3

Converging lenses

Answer 3

Convex
Thicker across the middle than the edges
So refract light rays towards each other

Question 4

Principle axis

Answer 4

Horizontal axis through the centre of a lens

Question 5

Lens axis

Answer 5

Vertical axis though the middle of a lens

Question 6

What are axial rays

Answer 6

Rays parallel to the principle axis of a lens

Converge at the principle focus

Question 7

What are non-axial rays

Answer 7

Rays that aren’t parallel to the principle axis

Converge somewhere along the focal plane but not the principle focus

Question 8

Focal plane

Answer 8

The plane perpendicular to the principle axis that contains the principle focus

Question 9

Focal length

Answer 9

Perpendicular distance between the lens axis and the focal plane
f

Question 10

What do ray diagrams show

Answer 10

How light rays passing through a lens travels and where and what type of image is formed when they converge

Question 11

How do you draw a ray diagram

Answer 11

One ray parallel to the principle axis straight through the top of the lens
That gets refracted through the principle focus

Another ray passing straight though the centre of the lens
That doesn’t get refracted at all

Question 12

What 7 things can a ray diagram tell you

Answer 12

Real or virtual
Magnified or diminished or same size
Inverted or not

Question 13

If an object hasn’t been magnified or diminished where will it be

Answer 13

The same distance from the lens axis as the object is from the lens axis

Question 14

If an object has been magnified then where will it be

Answer 14

Image is further from the lens axis than the object is from the lens axis

Question 15

If an object has been diminished then where will it be

Answer 15

Image is closer to the lens axis than the object is to the lens axis

Question 16

What is a real image

Answer 16

Formed when light rays from one object are made to pass through another point in space
The light rays are actually there
Can be captured on a screen

Question 17

What is a virtual image

Answer 17

Formed when light rays from a point on an object seem to have come from another point in space
Light rays aren’t where the image appears to be
So can’t be captured on a screen
Image is located at infinity

Question 18

What does how big an object appears depend on

Answer 18

The angle which it subtends at the eye

Question 19

Equation for how powerful a telescope is

Answer 19

Angle subtended by image at eye / angle subtended by object at unaided eye

Question 20

Equation for angular magnification

Answer 20

Angle subtended by image at eye / angle subtended by object at unaided eye

Question 21

How do reflecting telescopes work

Answer 21

Use a mirror to collect light and form an image

Question 22

How do refracting telescopes work

Answer 22

Use two converging lenses

Question 23

Function of objective lens in astronomy/normal adjustment

Answer 23

The object/planet is effectively at infinity

A large lens known as the objective lens forms a real, inverted and diminished image at its focal point

Question 24

Two lenses in astronomy

Answer 24

Objective lens

Eye piece lens

Question 25

Function of eye piece lens in astronomy/normal adjustment

Answer 25

Produce a final image of the first image at infinity
Has principle focus coinciding with the focal point of the objective lens
Final image is inverted

Question 26

Assumption for ray diagrams

Answer 26

The bottom of the object lies on the principle axis

Question 27

Angular magnification equation in normal adjustment

Answer 27

angle at objective/angle at eye piece

Question 28

Expression for the length of a telescope in normal adjustment diagrams

Answer 28

fo+fe

Essentially the length from the centre of objective to centre of eye piece

Question 29

Equation linking angular magnification to fo and fe

Answer 29

a/b=fo/fe

Obtained using trigonometry and the small angle approximation

Question 30

Disadvantages of refracting telescopes

DDDMM

Answer 30

Using glass of sufficient clarity and purity free from defects to make a large diameter telescope is extremely difficult

Large diameter lenses are heavy and tend to distort under their own weight

Suffer from distorting factors (chromatic and spherical aberration)

Heavy and difficult to manoeuvre quickly

Large magnifications require large objective lenses and long focal lengths

Question 31

Advantages of reflecting telescopes

Answer 31

Large single mirrors can be made which are light and easily supportable from behind

Mirror surfaces can be made a few nanometres thick so give excellent image properties

No chromatic or spherical aberration when using parabolic mirrors

Relatively light mirrors allow rapid responses to astronomical events

Question 32

Two types of aberration

Answer 32

Chromatic

Spherical

Question 33

Explain chromatic aberration

Answer 33

Different wavelengths of light are refracted by different amounts
Issue with lens telescopes since they use refraction
Effect is more pronounced at the edges

Question 34

Which wavelengths of light are refracted most in chromatic aberration

Answer 34

Blue since shorter wavelength

Question 35

Explain spherical aberration

Answer 35

Blurry images

Refraction increases at lens edges