SELECT * FROM question_mgmt as q WHERE id=2769 AND status=1 SELECT id,question_no,question,chapter FROM question_mgmt as q WHERE courseId=3 AND subId=8 AND chapterId=124 and ex_no='1' AND status=1 ORDER BY CAST(question_no AS UNSIGNED)

Question:

The 6563 Å H2 line emitted by hydrogen in a star is found to be red shifted by 15 Å. Estimate the speed with which the star is receding from the Earth.

Answer:

Given that,

Wavelength of H2 line emitted by hydrogen, λ = 6563 Å = 6563 × 10-10 m.

Star’s red-shift can be obtain by, (λ' - λ) = 15 Å = 15 x 10-10 m

We know that, Speed of light, c = 3 x 108 m/s

Let the velocity of the star receding away from the Earth be v.  Since the star is receding away, hence its velocity v is negative.

The red shift is related with velocity as: λ' - λ = v/c λ

∴ v = c/λ x (λ' - λ) = 3 x 108 x 15 x 10-10 / 6563 x 10-10 = 6.87 x 105 m/s

Therefore, the speed with which the star is receding away from the Earth is -6.87 × 105 m/s.

 


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