Physics Project on liquid lens

INDEX SL.NO

T I T L E

PAGE.NO

1

Introduction

2

2.

Aim

3

3.

Apparatus

4

4.

Theory

5

5.

Procedure

6

6.

Diagram

8

7.

Observation & calculation

9

8.

Result

12

9.

Precautions & source of error

13

10.

Bibliography

14

1

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 the 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 difficult. Liquid lens experiment can be used to find the optical constants of a lens and also to find the refractive indices of various liquids.

The theory behind the liquid lens is based on the properties of one or more liquid to create magnification within a small amount of space.

The focus of a liquid lens is

controlled by the surface of the liquid. Water normally form a bubble shape when adhered to materials like glass. Thos desirable property of water makes it a very suitable candidate for the production of liquid lens. transparent so as to study its properties.

Essentially the liquid must be

To generate a liquid lens, a liquid is

sandwiched between two pieces of a clear plastic or glass. Glycerin can also be used as a fluid in the liquid lens system. The surface profiles of the liquid determine the focal length of the liquid lens system and how the liquid lens focuses the light rays. If we keep the mirror behind the lens and put the object at the focus of the lens above it, the image of the object will be formed at the same focus where the object is. If it is an extended object, its image will be inverted and the size of the image is same as that of the object. This property has enabled the efficient use of liquid lens to find the refractive index of a fluid by this method. The focal length of the liquid lens can be calculated knowing the focal length of the combination and that of the convex lens, from which the refractive index of the fluid can easily be estimated.

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AIM To determine:1. Optical constants of a convex lens and 2. Refractive index of a liquid lens.

3

APPARATUS 1.

The convex lens

2.

Plane mirror,

3.

The liquid

4.

Glycerine

5.

Retort stand, etc.

4

THEORY Let f be the focal length andR1 and R2 be the radii of curvature of a convex lens. Then, 1 f

=

( n−1 )(

1 1 + ) R1 R2

Hence the refractive index n of the material of the lens is n=

1+

( R 1 R 2) f ( R 1+ R 2)

When the lens is placed over some drops of the given liquid on a planmbination of the vconvex and the e mirror, a plano-concave liquid lens is obtained. If F is the focal length of the combination of the convex lens and the plano-concave liquid lens, the focal length of the liquid lens is given by. Ff

F1= f −F If the first face of convex lens is in contact with the liquid surface, the radius of curvature of the upper surface of the liquid lens is R1. For the liquid lens, R1 = R1 & R2 = ∞ Hence nl = 1+

R1 f1

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PROCEDURE    To find the focal length of the convex lens The convex lens is placed over a plane mirror which is kept horizontally. A bright pointer O is arranged horizontally on the clamp of a retort stand, vertically above the lens. Looking from above, the pointer is moved up or down until the pointer and its inverted image coincides without parallax. The distance x1 and x2 of the pointer from the top of the lens are measured. The average distance[x1+x2]/2 gives the focal length f of the convex lens. The experiment is repeated and the mean focal length is calculated.

   To find the focal length of the liquid lens The lens is then removed, a few drops of the given liquid placed on the plane mirror. The lens is placed on it with the marked first surface of the lens in contact with the liquid. The liquid lens forms a plano-concave lens. The pointer is arranged horizontally above the lens. Looking from above, the pointer is moved up or down until the pointer and its inverted image coincides without parallax. The distances x1 and x2 are measured as before. The average distance[x1+x2]/2 gives the focal length f1 of the combination of the convex lens and liquid lens. The focal length f1 is calculated from the equation F1=

Ff f −F

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 Repeat the experiment by keeping the second surface on water and determine f2 And find R2 andR2 by Using formula 1 f

1 1 + R1 R2 ( n−1 ) ¿

=

)

   Find the focal length f for glycerine Few drops of glycerine is added on a mirror. Lens is placed upon it such that it formed a plano-concave lens. The pointer is arranged horizontally to get a coinciding object and image without parallax. Distances x1 and x2 are noted as before. Focal length is calculated by using formula f=

x 1+ x 2 2

.

7

DIAGRAM

Fig: liquid lens apparatus.

Fig: To find radius of curvature of lens

OBSERVATION AND CALCULATION 8

(i)To find focal length of lens Focal length Sl.no

Distance of pointer from T o p (cm)

o f

l e n s

Top of mirror (cm)

(cm)

1

10.1

11.1

10.6

2

10.2

11.2

10.7

3

9.9

10.9

10.4

4

10

11

10.5

10.3

11.3

10.8

5 Mean = 10.6 cm

(ii) To find the focal length of the combination, 1st surface Focal length Sl.no

Distance of pointer from T o p (cm)

o f

l e n s

(cm) T o p (cm)

o f

m i r r o r

1

15

16

15.5

2

15.2

16.2

15.7

3

14.8

15.8

15.3

4

15.3

16.3

15.8

5

15.4

16.4

15.9

Mean focal length of combination, (cm) = 15.64 cm

9

Focal length of combination , surface 2 Focal Sl.no

Distance of pointer from T o p (cm)

o f

l e n s

T o p (cm)

o f

m i r r o r

length

(cm)

1

15.5

16.5

16.0

2

15.1

16.1

15.6

3

15.3

16.3

15.8

4

15.0

16.0

15.5

5

15.6

16.6

16.1

Mean focal length, = 15.8 cm F1 = 15.64 cm F2 = 15.80 cm

We know,

1 f

=

1 1 + R1 R2 ( n−1 ) ¿

)

R1= (n–1) f1 (R2= = 15.64 ( 1.33 – 1)

∞

)

(n= 1.33 for )

= 15.64 (1.33 – 1) = 5.161 cm Similarly , R2= (1-n) f2 = 15.8 × 0.33 = 5.214 cm

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Also

f1=

Ff f −F

=15.576 cm

(iii) Focal length of liquid lens using glycerin D i s t a n c e Sl.no

T o p

o f

o f

l e n s

(cm)

p o i n t e r T o p

o f

f r o m

Focal length (cm)

m i r r o r

(cm)

1

19.6

20.6

2 0 .

2

19.5

20.5

20.0

3

19.0

20.0

19.5

4

19.2

20.2

19.7

5

19.4

20.4

19.9

Mean focal length of glycerin lens = 19.84 cm

CALCULATIONS

n=

1+

( R 1 R 2) f ( R 1+ R 2)

= 1 + 2.663 = 3.663

nl = 1+

R1 f1

(glycerine)

nl = 1+

R1 f1

(water) = 1+

5.161 19.84

= 1+

5.161 15.64

11

1

= 1+ 0.46

= 1+ 0.33

=1.46

=1.33

Results 1. 2. 3. 4. 5.

Focal length of convex lens = 10.6 cm Radius of curvature of 1st surface = 1 Radius of curvature of 2nd surface = Refractive index of material of lens = Refractive index of liquid= 1.33(water) ; 1.46(glycerine)

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PRECAUTIONS 1. The plane mirror should be clean and must have fully shining surface. 2. The liquid taken should be transparent. 3. The parallax error should be removed tip to tip.

SOURCES OF ERROR 1. Liquid may now be transparent. 2. The parallax error may not be fully removed. 3. Measurements may not be correct.

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BIBLIOGRAPHY 1. Physics ncert textbook . 2. www.experimentalphysics.com 3.www.wikipedia.com 4.Practical physics.

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