Lenses and Telescopes (Unit 5)

Question 1

Definition of Principal focus (Focal Point)

Answer 1

The point through which all light parallel to the axis of the lens passes through.

Question 2

Definition of Focal length

Answer 2

The distance between the centre of the lens and the focal point.

Question 3

Constructing ray diagrams through converging lenses

Answer 3

See sheet

Question 4

Letters in Lens makers formula

1/u + 1/v = 1/f

Answer 4

u is the Object distance
v is the Image distance
f is the focal length of lens
u comes before v in the alphabet, object comes before image in lens
RIP – Real Is Positive
(Real image has positive distance, Virtual image has negative distance)

Question 5

Description of images formed with different object distances

Answer 5

See sheet

Question 6

Formation of images by converging lenses

Answer 6

See sheet

Question 7

Ray diagram of a refracting telescope in normal adjustment

Answer 7

See sheet

Question 8

Angular magnification in normal adjustment

Answer 8

See sheet

Question 9

Ray diagram of a Cassegrain telescope

Answer 9

See sheet

Question 10

Definition spherical aberration – lens

Answer 10

Where parallel rays at different distances from principal axis are brought to focus at different points (on principal axis).

Question 11

Definition spherical aberration - mirror

Answer 11

Where parallel rays furthest from the principal axis are brought to focus closer to reflector than rays closest to principal axis.

Question 12

How can spherical aberration be prevented in a mirror

Answer 12

Use a parabolic mirror.

Question 13

Diagram of spherical aberration of a mirror

Answer 13

See sheet

Question 14

Effect on image of spherical aberration

Answer 14

Image is blurred

Question 15

Definition chromatic aberration

Answer 15

Where different wavelengths are refracted by different amounts resulting in different focal lengths for different wavelengths (colours).

Question 16

Effect on image of chromatic aberration

Answer 16

Images have multicoloured, blurred edges

Question 17

Diagram of chromatic aberration from a lens

Answer 17

See sheet

Question 18

Resolving power of telescopes

Answer 18

the smallest angular separation that two point objects can still be discerned as separate

Question 19

Definition Rayleigh criteria

Answer 19

two point objects can be just be resolved when the central maximum of the diffraction pattern of one object lies over the first minimum of the diffraction pattern of the other.

Question 20

Definition of Airy Disc

Answer 20

The bright central maximum (spot) of the diffraction pattern produced when light from a point source passes through a circular aperture.

Question 21

Sketch of diffraction pattern from a circular aperture

Answer 21

See sheet

Question 22

Sketch illustrating Rayleigh criteria

Answer 22

See sheet

Question 23

Structure and operation of a CCD

Answer 23

• A CCD is silicon chip divided into picture elements (pixels).
• Incident photons cause electrons to be released.
• The number of electrons liberated is proportional to the intensity of the light.
• These electrons are trapped in ‘potential wells’ in the CCD.
• An electron pattern is built up which is identical to the image formed on the CCD.
• When exposure is complete, the charge is processed to give an image.

Question 24

Quantum efficiency of a CCD

Answer 24

Quantum efficiency of pixel >70%

a measure of the proportion of the incident photons that release electrons

Question 25

Definition of quantum efficiency

Answer 25

The percentage of the photons incident on the CCD that release electrons and so are detected

Question 26

Why are telescopes put into space?

Answer 26

• Electromagnetic waves are absorbed by the atmosphere
• Light pollution and other interference at ground level
• The effect the atmosphere has on the path of the light as it passes through

Question 27

Non-optical telescopes positioned in space

Answer 27

See sheet

Question 28

Non-optical telescopes positioned on the ground

Answer 28

See sheet

Question 29

What does collection power of a telescope depend upon?

Answer 29

Collection power of a telescope (rate at which energy is collected) is proportional to the cross-sectional area of the objective (lens or mirror).
As cross-sectional area = pi x r2 = pi(d/2)^2 = (pi x d^2)/4
Where d is the diameter of the objective, therefore
Collecting power of a telescope is proportional to d^2

Question 30

What absorbs X-rays, UV and infra-red in Earth’s atmosphere?

Answer 30

• X-rays are absorbed by Ozone (and Oxygen)
• UV are absorbed by Ozone (and Oxygen)
• Infra-red are absorbed by water vapour (and carbon-dioxide)

Question 31

Similarities and differences between radio and optical telescopes

Answer 31

Similarities
• Radio telescopes have a similar structure in that a concave reflecting surface reflects electromagnetic radiation to a detector at the focal point (of the reflecting surface).
Differences
• Radio telescopes are much larger – radio wavelengths are much longer than optical wavelengths
• Radio telescopes have a lower resolving power – the wavelengths of radio waves are very much larger than optical wavelengths (even though the diameters of radio telescopes are larger).
• Radio telescopes have a greater collecting power – collecting power depends on the area of the objective which is much larger for radio telescopes (depends on the square of the diameter).
• Radio telescopes are not as affected by atmosphere so their positioning is less critical
• Radio telescopes have only 1 reflecting surface not 2

Question 32

Advantages of mirrors in telescopes rather than convex lens

Answer 32

• No chromatic aberration – mirrors do not refract light
• no spherical aberration – use of parabolic mirror
• no distortion – mirrors can be supported across its whole surface (lenses can only be supported around their edge)
• better resolving power and/or greater brightness – mirrors can be made larger
• more light gets through (image is brighter) – lens absorbs more light

Question 33

Disadvantages of mirrors in telescopes rather than convex lens

Answer 33

• they have a frame holding the central mirror (secondary), that:
o reduces the light intensity (as it blocks some light from hitting the primary mirror)
o creates diffraction effects around the edges causing blurring of the image

Question 34

Advantage of larger objective diameter (mirror or lens)

Answer 34

• It collects more light – so can observe fainter objects

* It has a higher resolution – possible to distinguish (resolve) two objects close together.