Telescopes and Diffraction

Question 1

How are simple refacting optical telescopes made up?

Answer 1

From two convex lenses with different powers: An objective lens and a more powerful eye lens / eyepiece.

Question 2

How does the objective lens in a telescope work?

Answer 2

It collects the light from the object being observed and forms an image of it

Question 3

How does the eyepiece in a telescope work?

Answer 3

It magnifies the image so we can view it.

Question 4

How are the two lenses positioned?

Answer 4

So that they have the same principal axis and their focal points are in the same place

Question 5

Which of the two lenses is more powerful?

Answer 5

The eyepiece

Question 6

What images do the 2 lenses make?

Answer 6

The objective lens makes a real image

The eyepiece lens makes a virtual image

Question 7

What is the formula for angular magnification if you’re given the focal lengths?

Answer 7

Focal length of objective lens
(Divided by)
Focal length of eye lens

Question 8

What is the formula for angular magnification if you’re given the powers?

Answer 8

Power of the eyepiece
(Divided by)
Power of the objective lens

Question 9

What do astronomical telescopes use?

Answer 9

A concave mirror

Question 10

What is a vertex?

Answer 10

The centre of the mirror’s surface

Question 11

Where is the focal point on a concave mirror?

Answer 11

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

Question 12

When do waves spread out / ‘diffract’?

Answer 12

When they pass through a gap or past an object

Question 13

What does the amount of diffraction depend on?

Answer 13

The size of the gap relative to the wavelength of the wave: the narrower the gap (or the longer the wavelegth) then the more the wave spreads out

Question 14

What is aperture?

Answer 14

The diameter of the objective lens

Question 15

How does the aperture effect the image formed from a telescope?

Answer 15

The bigger the aperture, the more radiation can get into the telescope and the better the image formed

Question 16

What is a reason for why many telescopes use concave mirrors instead of lenses?

Answer 16

Because making large lenses is difficult and expensive, whereas big mirrors are much easier to make accurately

Question 17

Why must the aperture size be larger than the wavelength?

Answer 17

Because when the light passes through the gap it diffracts and spreads out, causing the image to blur. Having a wider aperture means that the radiation passing through won’t diffract much and it’ll produce a sharp image

Question 18

What happens when white light passes through the gaps in a diffraction grating?

Answer 18

The different wavelengths of coloured light are all diffracted but by different amounts, creating a spectrum of coloured light

Question 19

How do astronomers use the spectrums produced by a diffraction gratings?

Answer 19

To analyse the light coming from stars