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QUESTION:

A telescope has an OBJECTIVE LENS of focal length 150 cm and an eyepiece of focal length 5 cm. If a 50 m tall tower at a DISTANCE of 1 km is observed through this telescope in normal SETTING, the angle formed by the image of the tower is θ , then θ is close to

ANSWER:

  • θ is close to 60°.

GIVEN:

  • Focal length of telescope(F) = 150 cm

  • Focal length of eyepiece(f) = 50 cm

  • Distance from the tower(d) = 1000m

  • Height of the tower = 50m

TO FIND:

  • CLOSEST VALUE OF θ

FORMULA:

Magnification from mirror:

Magnifying\:\:distance(m)= F/f

EXPLANATION:

d = 1000 m

F = 150 cm

f = 50 cm

Magnifying\:\:distance =150 / 5= 30\\ \\ \\ Magnifying\:\:distance = \dfrac{\tan\beta}{\tan\alpha } = 30\\ \\  \\\tan \alpha = \dfrac{f}{d}\\ \\ \\ \tan \alpha  =  \dfrac{50}{1000}  \\  \\ \\ \tan \alpha  =  \dfrac{1}{20} \\  \\  \\ \tan \beta  = m\times  \tan \alpha  \\  \\  \\ \tan \beta  = 30\times \dfrac{1}{20} \\ \\ \\ \tan\beta  =  \dfrac{30}{20}  \\ \\   \\ \tan \beta  =  \dfrac{3}{2}\:\:radians \\  \\ \\  \beta =tan^{-1}\bigg( \dfrac{3}{2}\bigg) \\ \\ \\ \beta \approx 60^{ \circ}

\bold{\large{NOTE 1 : \tan \alpha= Angle\:\:formed\:\:by\:\:the \:\:eyepiece}}

\bold{\large{NOTE 2 :\tan \beta =Angle\:\:formed\:\:by\:\:the \:\:image}}

OTHER FORMULA:

m= H/h= -v/ u\\ \\ \\H= Height\:\:of\:\: image\\ \\\\h= Height\:\:of\:\:object\\ \\ \\u= Object\:\:distance\\ \\ \\v= Image\:\:distance



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