Astronomical Telescope: It is an optical instrument which is used to see heavenly bodies.
Construction: An Astronomical telescope consists of two lens system. The lens which is kept towards object is called lens or objective and the lens which is kept towards eye is called eye lens. The distance between two lens is adjusted by rack and pinion arrangement.
(i) When final image is formed at Infinity: when a parallel beam of light rays from distant object falls on objective then its real and inverted image PQ is formed on the side of the objective.
Now eye lens is so adjusted that PQ lies at the focus of eye lens, then final image is
formed at infinity.
Magnifying power of astronomical telescope
(ii) When the final image is formed at least distance of district vision.In this position eye lens is so adjusted that PQ lies between optical centre and focus of eye lens then final image is formed at the least distance of district vision.
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west of the geographic north. The north tip of the magnetic needle of a dip circle placed in the plane of magnetic meridian points above the horizontal. The horizontal component of the earth’s field is measured to be 
Specify the direction and magnitude of the earth’s field at the location.At a certain location in Africa, a compass points 
and 
carrying currents of 
and 
in the same direction are separated by a distance of 
. Estimate the force on a 
section of wire A.
wire carrying a current of 
is placed inside a solenoid perpendicular to its axis. The magnetic field inside the solenoid is given to be 
. What is the magnetic force on the wire?
and making an angle of 
with the direction of a uniform magnetic field of 
. What is the magnitude of the field 
at a point 
from the wire?
-turns carrying current 
.
is at distance 
from the wire. Find the force acting on the electron if its velocity is directed towards the wire.