Question

The magnifying power of an astronomical telescope can be increased if we:
A. increase the focal length of the objective
B. increase the focal length of the eye-piece
C. decrease the focal length of the objective
D. decrease the focal length of the objective and at the same time increase the focal length of the eye piece

Answer 1

Hint: Astronomical telescope is used to see the small objects or celestial bodies clearly. Telescope uses lenses of smaller sizes to focus the light from the small image into our eyes. The magnifying power of the telescope will tell us how big or small the image of the objective will be when we will be using the particular telescope.

Formula used:
$m = \dfrac{{ - {f_o}}}{{{f_e}}}\left( {1 + \dfrac{{{f_e}}}{D}} \right)$
Where $D = $least distance of the clear vision.
$m = $magnifying power
${f_o} = $focal length of the objective.
${f_e} = $focal length of the eye-piece.

Complete step-by-step answer:
The above is the formula for the magnifying power of the astronomical telescope, with the help of that we measure the and calculate the magnifying power of the astronomical telescope.
So, the magnifying power of an astronomical telescope:
$m = \dfrac{{ - {f_o}}}{{{f_e}}}\left( {1 + \dfrac{{{f_e}}}{D}} \right)$
Where $D = $least distance of the clear vision.
$m = $magnifying power
${f_o} = $focal length of the objective.
${f_e} = $focal length of the eye-piece.
Now on seeing the options we see that the option (A) says that the increasing of focal length of the objective will increase the magnifying power of the astronomical telescope. This is a correct statement because the magnifying power is directly proportional to the focal length of the objective.
Hence, we can also say that if the focal length of the objective decreases then the magnifying power of the astronomical telescope also decreases.

So, the correct answer is “Option A”.

Note: The astronomical telescope is best suited for getting the clear image of celestial bodies both one limitation of this astronomical telescope is that the image formed by an astronomical telescope is inverted that means we will see upside down image of the object we are aiming to see.
Another type of the telescope is there which is called a terrestrial telescope, it has one additional lens other than the small lens that is used in the astronomical telescope which makes this upside-down image into the right side up.