MCAT Telescope Questions
Telescope Type Objective Focal Length (m) Objective Magnification
Diameter (m)
A reflector 16.8 5 336x
B reflector 14.2 3.6 284x
C reflector 10.7 4 214x
D reflector 3.2 0.9 64x
E reflector 18.9 1 378x
17. Which telescope would require the least amount of time to observe a star
800 kpc away? (note: 1 kpc = 3 x 1016 m)
Telescope A: The larger diameter, the larger the light gathering power, the
more efficient the lens, the less time needed
Lg D 2 Since looking at point object far away
Telescope Type Objective Focal Length (m) Objective Magnification
Diameter (m)
A reflector 16.8 5 336x
B reflector 14.2 3.6 284x
C reflector 10.7 4 214x
D reflector 3.2 0.9 64x
E reflector 18.9 1 378x
18. If an eye has a diameter of 0.5 cm, what is the light gathering power of
Telescope A compared to the eye?
Unclear question if you know about telescopes. Are we looking at a point object or
an extended object? If we blindly follow the reading which is not the whole truth,
we are only given
Lg D 2
Which is only true of point objects. If this is a point object,
Lgeye 0.005 m 2.5 105 m2
2
Lgtelescope A 25 m2
106
Lgtelescope A 5 m 25 m2 2.5 105 m2
2
Lgeye
19. Why are large telescopes most often reflecting telescopes?
I. Lenses always magnify more than mirrors
II. Large lenses cannot hold their shape
III. Large lenses produce greater aberration
-Weight of glass deforms under force of gravity, so changes lens shape. In a
reflecting telescope you can ideally paint on a reflecting surface (make it thin)
-One reason large lenses produce greater aberration is the glass causes
colors to travel at different speeds. Different colors will focus in different
places.
Dispersion
The property of wave where different frequencies (colors, if we talk about
light) travel at different speeds through a dense medium. Index of
refraction depends on color of light.
Why does this happen? nucleus
E
k Fbinding kx
m Electron, -q
Electron responds differently to different frequencies. For example at the
resonance frequency we have the most delay (greatest index of refraction).
In the prism, blue light bends more than red, so
what color travels faster?
20. One of the advantages of using large telescopes as compared to
small telescopes is that:
A. large telescopes magnify distant star images more
B. large telescopes shorten the observing time necessary to
complete an observation
C. large telescopes track a star with greater accuracy.
D. large telescopes have a better dynamic range
A. is only true if the focal length is lengthened at the same time, but it is unclear
that this is the case. The question just considers larger diameter. So B is the only
one that makes sense.
Telescope Type Objective Focal Length (m) Objective Magnification
Diameter (m)
A reflector 16.8 5 336x
B reflector 14.2 3.6 284x
C reflector 10.7 4 214x
D reflector 3.2 0.9 64x
E reflector 18.9 1 378x
21. Which Telescope would enlarge an image of the Andromeda galaxy
the most? :
Don’t over-think this one, the answer is Telescope E. The fact that it is a galaxy just
let’s you know that the telescope will actually be magnifying. Distant point objects
don’t magnify much at all.
22. Which of the following statements describes why the use of mirrors
decreases the degree of chromatic aberration? :
A. Lenses refract light at an angle that depends on its wavelength,
whereas mirror reflect light of all wavelengths at the same
angle.
B. A mirror absorbs much less light in reflecting than a lens
absorbs in refracting
C. Since mirrors are easier to grind accurately than lenses, mirrors
have fewer flaws than lenses.
D. The changes in temperature during the course of an evening
affect mirrors less than lenses
Basically if you use a lens, you will have a prism effect, dispersion. In a
mirror the reflected light does not travel into a dense optical medium like
glass but reflects off of the surface. So with a mirror you will not have
dispersion and therefore no chromatic aberration.
23. The passage states that for observation of faint extended objects,
magnification is not the primary factor. Which of the following best
describes what would happen to an image of a faint galaxy as the
magnification is increased
A. The image would get larger and dimmer, eventually fading
completely because the small amount of light coming from the
galaxy would be spread over a larger image area.
B. The image would remain the same size and become dimmer,
because there is an intrinsic upper limit on how much any
telescope can enlarge an image.
C. The image would get larger indefinitely, remaining at the same
overall brightness, because there is no intrinsic limit on how
much any telescope can enlarge an image.
D. The image would get larger and dimmer, eventually fading
because of dispersion effects.
As you magnify an extended object, it does get magnified and fainter since light is
spread out over a larger area. It doesn’t really fade completely. You could use film
and wait a long time and it would show up.