Page 273 - text book form physics kssm 2020

Chapter 6
Light and Optics
Telescope
File
Made up of two convex lenses. Objective lens has a long
focal length, f and eyepiece lens has a short focal length,
o
f . Distance between objective lens and eyepiece lens is Normal adjustment of telescope
e
f + f . can be done by adjusting the
o
e
distance between the objective
Parallel rays from a distant object can be focused at the lens and the eyepiece lens,
focal plane of the objective lens to form the fi rst image, L = f + f . This enables the fi nal
e
o
I which is real, inverted and diminished. Th e fi rst image, image to be formed at infi nity for
1
I acts as the object for the eyepiece lens. Eyepiece lens a comfortable vision.
1
forms the fi nal image, I which is virtual, magnifi ed and Magnifi cation of telescope at
2
inverted compared to the object (Figure 6.40). normal adjustment, M is
f
o
Normally, image I is located at infi nity. Th is is known as M = .
f
2
normal adjustment. e

f f
o e

Observer
F , F
o e
I
1

Virtual image Eyepiece lens
at infinity (L )
Objective lens 2
(L )
1

Figure 6.40 Formation of image by a telescope

267
6.5.2 267
6.5.2