Read the following and answer any four
questions:
Atmospheric refraction is the deviation of
light from a straight line as it passes
through the atmosphere due to the
variation in air density as a function of
altitude. Atmospheric refraction causes
astronomical objects to appear higher in
the sky than they are in reality. As the
atmospheric refraction is not
homogeneous, there is turbulence in the
air.
(a) Name the phenomenon which causes
twinkling
(b) Is the position of a star seen by us its
true position? Justify.
(c) Give another example of the above said
phenomenon .
(d) What is the time difference between the
actual sunset and the apparent sunset?
(e) Explain with reason- why planets don’t
twinkle?
Answers
Answer:
a.The atmospheric refraction is responsible for twinkling of stars.
b.No, it is not true position. It is apparent position we see because we know there are many layers in atmosphere, therefore light coming from stars undergo refraction many times which occurs in medium of gradually changing refractive index and therefore we see the apparent position of the star.
d.Due to atmospheric refraction, the sun is visible two minutes after actual sunset. Thus, the time difference between actual sunset and apparent sunset is 2 minutes.
e.Planets appear larger to human eyes as compared to stars because they are closer. As the angle subtended by the planets at the human eye is more, the shift in their position(as compared to stars) is smaller. Since, the shift is smaller, they do not twinkle.
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Answer:
a. atmospheric refraction
b. no, it is apparent position. This is because star light bends due to atmospheric refraction before it reaches our eyes.
c. the apparent flattening of the Sun's disc at sunrise and sunset
d. 2 minutes
e. planets are closer to earth and are seen as extended extended sources. The total variation in the amount of light entering our eye from all the individual point sized sources will average out to zero
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