Question

1. A telescope has a focal ratio of 10 (typically stated as f/10). If the aperture of the objective mirror or lens is 6 inches (152.4mm), what is the focal length of the lens? 2. What is the light-gathering power of the above telescope's objective when compared to a human eye dilated to 8mm?

Answer 1

1.

Focal length : The distance(usually measured in millimeters) from the lens or mirror to the image that it forms.In most telesope the focal length is equal to the length of the tube.some telescopes use extra lenses and/or mirrors to create a long effective focal length i a short tube.

Scope focal ratio: It is defined as the () ratio of mirror's focal length and its aperture. So the scope ratio can be defined as , scope ratio = (focal length/ aperture).

so focal length = scope ratio *aperture.

Now the given values are , scope ratio =10

aperture =152.4 mm = 152.4*10^-3 m

So focal length of the telescope is = 10*152.4*10^-3

= 1524 mm.

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