To make an astronomical telescope using two converging
lenses, the focal lengths of the two converging lenses and the length of the
telescope must be equal.
Concept
An astronomical telescope is an instrument used to observe
objects in the sky, such as the moon and stars. It is a telescope made using
two lenses. One lens should be made with a smaller diameter and the other with
a larger diameter. The smaller lens is called the eyepiece and the larger one
is called the object piece. The separation between these two lenses should be
equal to the sum of their focal lengths.
Materials needed
Two convex lenses, one with an optical centre of 2.5
centimetres and a focal length of 10 centimetres, and the other with an optical
centre of 2.8 centimetres and a focal length of 50 centimetres. Two cards or
plastic tubes that fit together. Materials needed to make these include
scissors and glue.
Make the parts of the telescope out of two cards or two
plastic tubes. When the total value of the focal length of the two convex
lenses reaches the length of this telescope, you will see the object clearly.
One should fit inside the other easily. The diameter of one
tube should be slightly less than the diameter of the eyepiece lens, and the
other should be slightly less than the diameter of the object piece lens. The
side where the smaller lens is placed is the eyepiece lens and the side where
the larger lens is placed is the object piece lens.
Wrap it with small diameter paper to make it fit well. The length of each tube should be two-thirds of the distance between the lenses to produce an accurate image.
Take two convex lenses and glue them to the ends of the
tubes. If the lenses are smaller than the ends, attach the lenses to a
cardboard ring.
Point the telescope towards a distant object and move the
tubes in and out until you can see the image accurately.
In this case, the total length will be equal to the sum of
the focal lengths of the two lenses. You will see the absorbed image of the
object.
Note: We can see stars or the moon through this telescope but we should not
use it to look at the sun because the powerful rays of the sun can damage our
eyes.