Unit 5 - Astrophysics - Lenses

Question 1

Draw a lens diagram when the object is placed between the optical centre and the principal focus.

Answer 1

Question 2

Draw a lens diagram when the object is between the focal length and twice the focal length.

Answer 2

Question 3

Draw a lens diagram when the object is beyond twice the focal length.

Answer 3

Question 4

Properties of the image of an object placed between the optical centre and principal focus.

Answer 4

Upright

Magnified

Virtual

Question 5

Properties of the image of an object placed between 1 and 2 focal lengths.

Answer 5

Inverted

Magnified

Real

Question 6

Properties of the image of an object beyond 2f.

Answer 6

Inverted

Diminished

Real

Question 7

How is linear magnification defined?

Answer 7

Question 8

Draw an astronomical refracting telescope in normal adjustment.

Answer 8

Question 9

How do we define angular magnification?

Answer 9

Question 10

Draw the Cassegrain arrangement for a reflecting telescope.

Answer 10

Question 11

What are the advantages of reflecting telescopes?

Answer 11

Lenses of large aperture or high magnificatoins produce distortions such as chromatic aberration.

It is easier to grind a large mirror than a large lens

A mirror can be supported by a massive structure from one whole face.

Question 12

Describe Chromatic Aberration.

Draw a digram of Chromatic Aberration.

Answer 12

An image is produced with colour fringes as glass refracts violet light more than red light.

Question 13

Describe spherical aberration for a lens.

Draw a diagram of spherical aberration for a lens.

Answer 13

The outer parts of a lens refract light more than the parts of the lens near to the optical centre. This produces an image that appears out of focus at the edges.

Question 14

How can spherical aberration for a mirror be reduced?

Answer 14

Use a parabolic mirror.

Question 15

What is Airy’s Disc

Answer 15

The bright central disc for a diffraction pattern of a circular aperture.

Question 16

How do you find the resolving power of a telescope?

Answer 16

Question 17

Define the rayleigh criterion.

Answer 17

2 sources are just resolved is the central bright maximum of the diffraction image of one source lies above the first minimum of the diffraction image of the second source.

OR

The centre of the first Airy’s Disc must lie outside the second Airy’s Disc

Question 18

Simlarities between optical and radio telescopes.

Answer 18

• Both have a parabolic reflector as an objective
• They both track the source as it moves across the sky to build up an image
• The power of each is proportional to the square of the objective’s diameter

Question 19

Differences between optical and radio telescopes.

Answer 19

• The resolution of a radio telescope is much less than an optical telescope
• Therefore are normally much larger in aperture
• Radio signals carry less energy so must scan source
• Radio telescopes have one reflecting surface whereas optical telescopes have 2

Question 20

How big can the imperfections be in a telescope dish?

Answer 20

One twentieth of the wavelength

Question 21

Define quantum efficiency.

Answer 21

The % of photons arriving that produce a response in the detector.

Question 22

How does a C.C.D. work?

Answer 22

• The silicon chip is divided into pixels
• Incident photons release electrons
• Number of electrons released proportional to intensity of the radiation
• When the exposure is complete the charge collected is processed
• Image produced is identical to the electron pattern
• Quantum efficiency is around 70%