CBSE Class 12 · 2007 · Geometrical OpticsShort Answer

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Define the term `resolving power' of an astronomical telescope. How does it get affected on
(i) increasing the aperture of the objective lens,
(ii) increasing the wavelength of the light used? Justify your answer in each case.

Solution

\textbf{Resolving power} of a telescope is its ability to form separate images of two close objects; it is measured by the reciprocal of the smallest angular separation

d θ = \frac{1.22 λ}{D}

, so

R P = \frac{D}{1.22 λ}

(i)

R P \propto D

: increasing the aperture \emph{increases} the resolving power.
(ii)

R P \propto 1/ λ

: increasing the wavelength \emph{decreases} the resolving power.

CBSE Class 12 Board · 2007 · Subjective

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