Physics Investgatory Project

7/28/2019 Physics Investgatory Project

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Physics

INVESTAGATORY PROJECT

Submitted By : Vikash Prasad

XII – A

Roll No. :

Year : 2012 –

2013 Topic : To Determine The Combined Focal Length Of The Lens System

School : Kendriya Vidyalaya No.2



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CERTIFICATE

This is hereby to certify that the project“

To Determine TheCombined Focal Length Of The Lens System” is an original

and genuine investigation work carried out to investigate about

the subject matter and the related data collection and

investigation has been completed solely, sincerely and

satisfactorily by, VIKASH PRASAD of class XII – A as per the

requirement for the CBSE Board Examination for the year 2012 – 13.

Roll No. :

Date :

Concerned Teacher Signature : Examiner Signature :

Principal Signature : School Stamp :



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ACKNOWLEDGEMENT

I wish to express my deep gratitude and sincere thanks to ourchemistry teacher -------------- for her invaluable guidance, constant

encouragement, constructive comments, sympathetic attitude and

immense motivation, which has sustained my efforts at all stages of

this project work. Her valuable advice and suggestions for the

corrections, modifications and improvement did enhance the

perfection in performing my job well.

I would like to express my gratitude for our honorable principal ---

--------------------- for whole hearted co-operation and guidance. I am

also thankful for her encouragement and for all the facilities that

she provided for this project work. I sincerely appreciate this

magnanimity by taking me into her fold for which I shall remain

indebted to her.

I take special pleasure in acknowledging our lab assistant --------------

for his willingness in providing us with necessary lab equipments

and constant support without which this effort would have been

worthless.

Vikash Prasad



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Table of content

Aim

Introduction

Requirements

Procedure

Observations

Calculations

Precautions

Sources Of Errors

Bibliography

Conclusion



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AIM

To determine the combined focal length

of one Concave and one convex lenses

separated by a finite distance



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INTRODUCTION

Many Optical tasks require several lenses in order to achieve an

acceptable level of performance. One such possible approach to lens

combination is to consider each image formed by each lens as the object

for the next lens and so on. This is a valid approach, but it is time

consuming and unnecessary.

In various optical instruments, two or more lenses are combined to

1. Increase the magnification of the image,

2. Make the final image erect w.r.t the object,3. Reduce certain aberrations.

It is much simpler to calculate the effective (combined) focal length and

principal point locations and then use the results in any subsequent

paraxial calculations. Two thin lenses of focal length f 1 and f 2

respectively which are in closed contact, then the effective focal length

of the combination will be given by

1/F= 1/f 1 + 1/f 2

And the total magnification of the lens combination will be given by

M = m1 * m2

If the lenses of focal length are separated by a finite distance d, the focallength F of the equivalent lens is given by

1/F= 1/f 1 + 1/f 2 - d/f 1.f 2



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APPLICATIONS OF COMBINATION OF CONVEX AND CONCAVE LENS

1. CHROMATIC ABBERATION

One common lens aberration is chromatic aberration. Ordinary light is a

mixture of light of many different colours, i.e. wavelengths. Because the

refractive index of glass to light differs according to its colour or wavelength,

the position in which the image is formed differs according to colour, creating

a blurring of colours. This chromatic aberration can be cancelled out by

combining convex and concave lenses of different refractive indices.

2. TELEPHOTO LENSES

Most optical devices make use of not just one lens, but of a combination of

convex and concave lenses. For example, combining a single convex lens

with a single concave lens enables distant objects to be seen in more detail.

This is because the light condensed by the convex lens is once more

refracted into parallel light by the concave lens. This arrangement made

possible the Galilean telescope, named after its 17th century inventor,

Galileo.

Adding a second convex lens to this combination produces a simple

telephoto lens, with the front convex and concave lens serving to magnify

the image, while the rear convex lens condenses it. Adding a further twopairs of convex/concave lenses and a mechanism for adjusting the distance

between the single convex and concave lenses enables the modification of

magnification over a continuous range. This is how zoom lenses work.

REQUIREMENTS



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An optical bench with three uprights (central upright fixed, two

outer uprights with lateral movement)

One convex lens

One Concave lens Two lens holder

Two optical needles

Half metre scale



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PROCEDURE

Take one concave and convex lens.

Find the rough focal length of the two lenses L1(convex) and

L2(concave) individually having focus length of f 1 and f 2 respectively.

Keep the lenses in front of the window and obtain a sharp image of the

object placed at infinity.

Measure the distance between lenses and the image formed with the

help of scale.

Now find the accurate focal length of two lenses L1 and L2 by using the

experimental setup individually and calculate its focal length reading. With left eye closed, see with the right open eye from the other end of

the optical bench. An inverted & enlarged image of the object needle

will be seen. Tip of the image must lie in the middle of the lenses.

Mount the thick optical needle in the fourth upright near the other end

of the optical bench.

Adjust the height of the object needle so that its tip is seen in line with

the tip of the image when seen with the right open eye.

Move the eye towards right. The tips will get separated. The image tip

and the image needle have parallax.

Remove the parallax tip to tip as described.

Combine the two lenses together with the help of two lens holder and

find its accurate combine focal length.



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OBSERVATIONS

Serialno.

Lenses Roughfocal

length(cm)

Radius of curvature(R)

f=R/2(cm)

1. Convex(L1) 9.5 20 102. Concave(L2) 8 18 -9

The separation between the two convex lenses = 7.2 cm

CALCULATIONS

1/F = 1/f 1 + 1/f 2 - d/f 1f 2

1/F = 1/10 + 1/(-9) - 7.2/(10)(-9)

1/F = -1/90 + 7.2/90 = 6.2/90

F = 90/6.2 cm

Therefore, F = 14.516 cm



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PRECAUTIONS

Tips of the object and image needles should lie at the same height

as the centre of the lens.

Parallax should be removed from tip to tip by keeping eye at a

distance at least 30cm away from the needle.

The object needle should be placed at such a distance that only real,

inverted image of it is formed.

SOURCES OF ERROR

The uprights may not be the vertical.

Parallax removal may not be perfect.

BIBLOGRAPHY

Comprehensive Pratical Physics Class XII

NCERT Physics Part –

II Experimental Physics

www.wikipedia.com

http://www.wikipedia.com/





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CONCLUSION

1. The combined focal length of one convex and one

concave lenses having focal length 10 cm and 9 cm

respectively and separated by a distance of 7.2 cm is

14.516 cm.

2. So on combination of the one convex and one concavelens the effective focal length increases and hence its

effective power decreases.

3. The effective nature of the combined lens system is

converging i.e. convex lens since focal length for thesystem comes out to be positive.

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Physics Investgatory Project