2. Telescope is used to provide angular magnification of the dista
nt objects.
It is of two types:
(1) Retracting type telescope
(2) Reflecting type telescope
4. ASTRONOMICAL TELESCOPE:
• It consists of two lenses called objective a
nd eye piece.
– The objective has a large focal len
gth and a much large aperture wh
ereas eye piece has small apertu
re and is of short focal length.
• Light from a distant object enters the obj
ective and a real image is formed in the t
ube at its second focal point.
• The eye piece magnifies this image produ
cing a final inverted image.
Also known as Keplerian Telescope
5. 1. Linear magnification
Height of Image image distance
Magnifying Power = =
Height of Object object distance
2.Angular magnification
Angle subtended at the eye by the image formed at the distance of distinct vision
Magnifying Power =
Angle subtended at the eye by the object formed at the distance of distinct vision
6. Magnifying Power of Astronomical Telescope:
CASE 1: When final image is formed at infinity (Normal adjustment).
• The position of the eye piece is
adjusted, such that the image of
objective lens lies at the focus of
the eye piece forming its final
inverted
Magnification will be high as fo >> fe.
Length of the tube = fo + fe.
7. Magnifying Power of Astronomical Telescope:
CASE 2: When final image is formed at least distance of distinct vision
The position of the eye piece is
adjusted such that image of the
objective lens is taken within focal
length of eyepiece forming its image
at near point.
For small angles,
The magnifying power of a telescope is greater, when
the image is formed at least distance of distinct vision
than in the case of normal adjustment
8. TERRESTRIAL TELESCOPE
• With compare to an astronomical
telescope, it has one more additional
lens called erecting lens to make final
image erect and used to see distant
terrestrial (earthly) object.
• The magnifying power will remain
same as in an astronomical
telescope. The length of the
telescope will increase by 4f as
compared to astronomical
telescope
9. Galilean Telescope
• Similar to terrestrial telescope with two advantage
1. Its tube length is smaller
2. It does not require an inverting image to produce erect image
• It is made by combination of convex and concave lens.
• The convex lens acts as objective and concave lens works as ey
epiece.
• Convex lens forms image on its focal length for distant objects
