P7 Observing the Universe

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P7 Observing the Universe Topics 1,2 and 3.

• Use this loopy starter to highlight areas you
  need to focus on during this revision lesson.
• Stop the presentation after Slide 11 - Q 10.
• Replay at the end of the lesson starting at Slide
  12 this 2nd set gives the answers.

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Question 1
In a telescope how is dish diameter linked to
wavelength ?
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Question 2
In a pinhole camera, why does a small aperture
give a sharp image, and what effect does this
have on the depth of field?
4
Question 3
A lens has a focal length of 25 cm.
What is its power? A 0.04 dioptres B 25
dioptres C 4 dioptres D Not enough
information to work it out
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Question 4
Which lens will produce the largest real
image? A thin concave B thick concave C
thin convex D thick convex
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Question 5
What is the difference between a solar eclipse
and a lunar eclipse, and which is rarer and why?
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Question 6
Parallax angle can be used to calculate distance.
The parallax of a star is 0.5 , how far away is
the star in parsecs?
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Question 7
Why is the parallax method only useful for
finding the distance of close objects? A they
are bright enough to see B the parallax angle
is measurable C they maintain the same
position D they have a known luminosity
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Question 8
Explain the difference between intrinsic
brightness (luminosity) and observed brightness.
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Question 9
Why are Cepheid variable stars important to
astronomers?
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Question 10
The spectrum from a hot star emits A more
intense radiation at higher frequencies B more
intense radiation at all frequencies C very
low levels in the visible spectrum D mostly
large wavelength radiation
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Question 1 answer
In a telescope how is dish diameter linked to
wavelength ? The link is larger diameters are
needed for detecting longer wavelengths. Hence
largest for radio waves and smallest diameters
for gamma.
13
Question 2 answer
In a pinhole camera, why does a small aperture
give a sharp image, and what effect does this
have on the depth of field? One small hole
prevents overlapping images so one sharp but dim
image is formed. Small apertures give a large
depth of field (near and far objects equally
sharp).
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Question 3 answer
A lens has a focal length of 25 cm.
What is its power? C 4 dioptres Why? To
work in dioptres, distances need to be in metres.
Power 1 focal length 1 0.25 m 4
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Question 4 answer
Which lens will produce the largest real
image? C thin convex Why? Convex lenses are
converging lenses that bring parallel rays to a
focal point. Thin lenses need to be further away
from the screen, but produce larger images,
although these are dimmer.
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Question 5 answer
What is the difference between a solar eclipse
and a lunar eclipse, and which is rarer and
why? Lunar Earth between Sun and Moon, and all
aligned (so they only happen at a Full
Moon). Solar Moon between Sun and Earth, and all
aligned (so they only happen at a New Moon).
Solar are rarer as the Moon is small, so only
forms a shadow on a small area of Earth. The
Moon orbits around the Earths equator, but the
Earths tilt affects the alignment.
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Question 6 answer
Parallax angle can be used to calculate distance.
The parallax of a star is 0.5 , how far away is
the star in parsecs? Distance (pc) 1/0.5 2pc
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Question 7 answer
Why is the parallax method only useful for
finding the distance of close objects? B the
parallax angle is measurable Distance objects
have such a small parallax angle (even measuring
in arc seconds) that they are difficult to
measure). Answer A is also possible, but not
the best one.
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Question 8 answer
Explain the difference between intrinsic
brightness and observed brightness. Intrinsic
Brightness depends on temperature and size. It
is how much light a star actually gives out.
Observed brightness depends on intrinsic
brightness. and distance. Its how bright a star
appears to an observer on Earth.
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Question 9 answer
Why are Cepheid variable stars important to
astronomers? These stars vary in brightness over
a regular period. The longer the period, the
brighter the Cepheid. Once the period is know,
the intrinsic brightness can be inferred, and
when compared to its observed brightness, you can
work out very large stellar distances.
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Question 10 answer
The spectrum from a hot star emits A more
intense radiation at higher frequencies Why? Hot
objects emit radiation from all pats of the e-m
spectrum, but more in the higher frequencies /
shorter wavelengths. They emit higher levels of
UV, X ray and gamma and appear blue white as
they emit more in this part of the visible
spectrum. Cooler red stars emit more in the red
orange yellow green visible part than the blue
part.