Define magnifying power of a telescope. Write its expression.
A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eye piece.
a) Magnifying power of telescope is the ratio of the angle subtended at the eye by the image to the angle subtended at the unaided eye by the object.
Mathematically, we can write
where, fo is the focal length of the objective, fe is the focal length of the eye-piece and D is the least distance of distinct vision.
b) Using, the lens equation for objective lens,
Hence, magnification due to the objective lens is given by,
Now, using lens formula for eye-piece, we get
Therefore, magnification due to eyepiece me = 
Hence, total magnification, 
So, size of final image = 
