My Profile
Active Members
TodayLast 7 Days
more...
Awards & Gifts
Online Exams
Fresher Jobs
Our fresher job section is exclusively for fresh graduates! Find jobs for freshers in major Indian
cities including Bangalore, Chennai, Hyderabad, Pune or Kochi
Resources
Find educational articles, blogs, discussion threads and other resources.
Colleges
Find details about any college in India or search for courses.
Paid Surveys
|
Construction Of Telescope
Posted Date:
06 Aug 2007
Total Responses:
0
|
Posted By: Jose Mathew
Member Level: Platinum Posted Date: 06 Aug 2007
Revenue Score: 
|
|
TOPIC: CONSTRUCTION OF TELESCOPE
INTRODUCTION
A Lens is any transparent object having two non-parallel curved refracting surfaces. The curved refracting surfaces may be spherical, parabolic or cylindrical, but spherical surfaces are the most common. Lenses are nearly always made of glass & are of two types.
1. Convex lens 2) Concave lens
The convex lens is thicker at the centre than at the edger while a concave lens is thinner at the centre than at the edges. A convex lens causes parallel rays of light to converge after refraction opposite.
The properties of lenses made us a way to optical instruments – mainly telescope. Telescope is an optical instrument used to see distant objects clearly and apparently magnified. /when a distant object is viewed through a telescope, it appears closer and therefore larger. A telescope increases the visual angle. It does not make the image bigger than the object. Suppose you are looking at the moon through a telescope. The image of the moon appears to be much bigger when viewed through a telescope because the angle it subtends at the eye is larger. When you look at the moon with unaided eye, the visual angle is small and therefore the moon looks smaller. The most commonly used refracting telescopes are
1) Astronomical telescope 2) Terrestrial telescope
OBJECTIVE
o To study the nature and size of image formed by a convex lens
o To study the structure and working of telescope.
METHOD OF STUDY
1. The size and nature of the image of an object formed by a convex lens depends upon the position of the object in front of the lens.
POSITION, NATURE AND SIZE OF THE IMAGES FORMED BY A CONVEX LENS
POSITION OF
NATURE OF IMAGE
SIZE OF IMAGE
OBJECT
IMAGE
Real/inverted
Erect /Inverted
At infinity At F Real Inverted Highly diminished
Beyond 2F Between F and
2 F Real Inverted Diminished
At 2 F At 2 F Real Inverted Same size
Between F and 2F Beyond 2F Real Inverted Enlarged
At F At infinity - - -
Ray diagrams
P1
Picture
2) Telescope is an optical instrument used to see distant object clearly and apparently magnified. Telescopes are divided into two: Astronomical telescope and Terrestrial telescope.
Astronomical telescope: An astronomical telescope consists of two convex lenses. One is called an ‘eye piece’ which have short focal length and ‘objective’ having larger focal length. These two lenses are fitted at the ends of two co axial tubes. The distance b/w them can be adjusted by means of a rack and pinion arrangement.
Rays from distant object enter the telescope through the objective and form a real inverted and diminished image at the focal plane of the objective.
Picture
P2
If the image formed by the objective is adjusted to be at the focal plane of, the eyepiece, the final image formed will be at infinity. This arrangement is known as the normal adjustment of the telescope.
Magnification produced by the telescope is the ratio of angle subtended by the final image at eye to the angle subtended by the object at eye.
I.e. = Magnification = Angle subtended by the final image / Angle subtended by the object
M = ß / 8
But from ? CIM, tan 8 = IM / IC
And from ? C’IM, tan ß = I M / IC’
For small values of 8 and = ß, tan 8 ˜ 8, tan ß ˜ ß
M = (IM /IC’) / (I M/IC) = (IC / IC’) = (F0 / Fe)
Where F0 and Fe are the focal length of the objective and eye piece.
In this case the distance between the lenses will be L= F0 + Fe, the final image formed at the least distance of distinct vision (D). This type of arrangement is used when the image is observed for a short time. The magnification produced by the telescope, M= (Fo / Fe) (1 + Fe / D)
Terrestrial telescope: In an astronomical telescope the image formed is inverted. We want an erect image. So we use concave lens as eye piece as like of astronomical telescope. By using concave lens as eye piece and convex lens as objective, we get a magnified erect image. The magnification of the telescope is same like the terrestrial telescope.
M= F0 / Fe
This telescope is called Newtonian Telescope.
Or
An astronomical telescope can be converted to a terrestrial telescope by using an erecting lens. The erecting lens is a convex lens placed b/w the objective and eyepiece such that the inverted image formed by the objective is at 2F of the erecting lens. This erecting lens produces an erect image at 2F on the other side. This erect image will act as the virtual object of the eyepiece and hence the final image formed by the eyepiece is erect. Thus in a terrestrial telescope the image formed is erect but the length of the telescope increases by a value 4F.
How to make a telescope?
We have one convex lens as objective of large focal length 35 cm and one concave lens as eyepiece of smaller focal length 5 cm. we want to make these lenses fix on a chart with gum. For this we should roll the chart and fix objective one end. Make another role of chart slightly smaller radius and fix eyepiece on one end. Arrange the smaller radius chart to inside the larger radius. So we can adjust the focus of the telescope.
P3
COLLECTING DATA
The image is formed on the right side of lens. The formation of image is as follows:
Distance b/w object and lens (u) Distance b/w image and lens(v) Size of image
AT infinity 35 – At F Highly diminished
Beyond 2 F -80 64- b/w F & 2 F Diminished
At 2 F – 72 69- At 2F Same size
Between F and 2 F- 58 91- Beyond 2 F Enlarged
At F -35 - No image -
Focal length of convex lens = (F) = (UV) / (U +V)
= (79 X 64) / (79 + 64) = 35.3 cm ˜ 35 cm.
About the telescope - we decided to construct a telescope of magnification 7. So we use lens of 5 cm focal length as eyepiece and lens of 20 cm length as objective. But we get only magnification 4. So we calculated the focal length of objective as we get magnification 7.
M= FO / Fe
7= Fo / Fe Fe= 5cm
.’ . Fo = 5 x 7 = 35 cm
We can know more about the telescope by a problem.
Problem 1
A small telescope has an objective lens of focal length 35 cm and an eye piece 5 cm. what is the magnifying power of the telescope for viewing distant objects when
a) The telescope is in normal adjustment (i.e. when the final i8mage is at infinity)
b) The final image is formed at the least distance of distinct vision 25 cm.
Solution:
a) M= FO / Fe = 35 / 5 = 7
b) M=Fo / Fe (1+Fo/D) =7 (1+35/25) =7x 2.4 =16.8
Problem 2
a) For the telescope described in the above question, what is the separation b/w the objective lens and the eye piece?
b) If the telescope is used to view a 2M tall boy 200 M away. What is the height of the image of the boy formed by the objective lens?
c) What is the height of the final image of boy if it is formed at 35 cm?
Solution:
a) Fo + Fe = 35 + 5 = 40 cm
b) Angle subtended by the boy = 2 / 200 = 1 / 100 rad
Angle subtended by the image produced by the Objective, h / Fo = h / 35
Equating two equations,
h =100/35=2.85cm
c) Magnification of the eyepiece=35/5+1= 8
Height of the final image = 2 x 8 = 16 cm
ANALYSIS AND CONCLUSION
? As the object is moved from infinity towards the lens, the size of the image increases
? All the real images formed corresponding to different positions of the object is inverted.
? Only the virtual images are erect.
? The magnification of telescope depends upon the focal length of lens.
? The focal length of eyepiece should be small and objective should be high.
? If we use two convex lenses the image should be inverted.
? If we use one concave lens & other convex lens we get erect image.
? When power of objective decreases the magnification will be high.
? When power of eyepiece increases the magnification increases.
SUGGESTION IF ANY
? We made a telescope of smaller magnification If we use objective of large focal length and eyepiece of small focal length, then magnification will be very high and we can make a good telescope.
? If any one make telescope they should use achromatic combination of lens to reduce achromatic aberration.
REFERENCE
? A TEXT BOOK OF PHYSICS- NEW JYOTHI PUBLICATIONS,.
Authors – A Jaya Prakash M.Sc. M.Phil
- H Neelakantan M.Sc
? Principles of Physics – S. Chand and Company Ltd
Authors – V.K Metha
- Rohit Metha
? Physics Text Book For Class XII- NCERT
Authors- Aravind Kumar
APPENDIX
o What is the Word meaning of telescope?
Tele-something far off, scope – instrument to see
o Who invented telescope?
Jan tipper shey in 1608
o Who observed stars by telescope first?
Galileo 1609
o Where the object should place to get a virtual image by convex lens?
Object b/w F and C.
By
Kiran placid.
|
Project Feedbacks
|
| Author: Raphael Mutiso | Member Level: Bronze | Revenue Score: | Cool Project!
|
|
|
| Post Feedback |
|
|
You must Sign In to post a feedback.
|
|
|
|
|
|