Assertion & Reason

Question 11 Mark

Assertion : The stars twinkle while the planets do not.
Reason : The stars are much bigger in size than the planets.

Answer

(b) Stars twinkles due to variation in density of atmospheric layer. Also stars are much bigger in size than planets but it has nothing to deal with twinkling phenomenon.

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Question 21 Mark

Assertion : A pulsor is a source of radio waves which change in terms of intensity at regular interval of time
Reason : A pulsor is a rotating neutron star

Answer

(b) Pulsar is a source of radio waves which emits pulses of radio waves at short and regular time of intervals.Pulsar is formed, due to super nova explosion, when super nova explosion occurs, the core of the star is compressed and electrons and protons combine to form a neutron. Due to this region pulsar is called neutron star.

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Question 31 Mark

Assertion : The comet do not obey Kepler's laws of planetary motion
Reason : The comet do not have elliptical orbit

Answer

(b) Comets do not revolve around the sun in fixed elliptical orbit like other planets and don't obey Kepler's law for planetary motion.

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Question 41 Mark

Assertion : The value of Hubble's constant is $16 km / s$
Reason : Hubble's constant means that a galaxy at 1 million light years away is receding at the rate of $16 km / s$.

Answer

(e) The assertion is not true. Infect, the value of Hubble's constant is $16 \mathrm{~km}$ per sec per million light years.

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Question 51 Mark

Assertion : Moon is seen as it partly reflects the sun light falling on it
Reason : Moon is a satellite of earth. It does not emit light of its own

Answer

(a) Both the assertion and reason are true and reason is a correct explanation of the assertion.

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Question 61 Mark

Assertion : Sun is at the galactic centre $C$ of the milky way
Reason : All planets of solar system revolve around the sun.

Answer

(e) The reason is true, but the assertion is false. Infect, distance of sun of our solar system from galactic centre is $3 \times 10^4$ light years.

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Question 71 Mark

Assertion : Red shift confirms that the universe is expanding
Reason : Wavelength of red light is maximum in the visible region

Answer

(b) Red shift means that wavelength of light received from stars is increasing i.e., apparent frequency is decreasing. Therefore, the stars/galaxies must be receding away. Hence the universe is expanding. Reason is also true, but it does not explain the assertion appropriately.

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Question 81 Mark

Assertion : There is no atmosphere on moon
Reason : Escape velocity at the surface of moon is low.

Answer

(a) At the surface of moon $v_{-}>v_{-}$hence molecules escape out before reaching their $r m s$ velocity that's why there is no atmosphere present.

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Question 91 Mark

Assertion : A star which appears blue will be much hotter than the sun
Reason : lt is based on Wien's law

Answer

(a) According to Wien's law, $\lambda_m T=b=$ constant. As $\lambda_m$ for the star is blue, which is less than $\lambda_m$ for sun, which is yellow, therefore temp. $T$ of star will be much higher than the temperature of the sun.

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