Question

An astronomical refractive telescope has an objective of focal length 20 𝑚 and an eyepiece of focal length 2 𝑐𝑚.

• A. The image formed is inverted.
• B. The length of the telescope tube is " 20.02 𝑚.
• C. The magnitude of magnification is 1000.
• D. An objective of a larger aperture will increase the brightness and reduce chromatic aberration of the image.

Answer 1

Answer: (A), (B), (C)

The magnitude of magnification is 1000.

Step 1: Draw a labelled diagram.


Step 2: Find length, magnification, and nature of image.

Given, $f_o=20mand{f}_e=2cm=0.02m$

In normal adjustment,

length of telescope tube $L=f_o+f_e=20+0.02=20.02m$

Magnification,

$m=-\frac{f_o}{f_e}=-\frac{20}{0.20}=-1000$

A refracting telescope forms an inverted image as it can be seen from the ray diagram. Light emanating from far away objects passes through the objective and converge at the second focus of the objective which is also the first focus of the eyepiece. The image formed is inverted and real. Now taking this real image as the object, we get a virtual image far away formed by the eyepiece. As virtual images have the same vertical orientation as the object, the image formed is inverted. Chromatic aberration can be reduced by increasing the focal length of the lens where possible

Final Answer: (a), (b), (c)