Physics Investigatory

Physics investigatory
To verify a lens combination with the

specified focal length by using two lenses
from the given set of lenses .
Atomic Energy Central School
Mysuru
CLASS 12
DONE BY : PRAGNA DYAVANUR
INDEX
CERTIFICATE
ACKNOWLEDEMENT
APPLICATIONS OF COMBINATION OF
CONVEX AND CONCAVE LENS
SPHERICAL LENS
EXPERIMENT
Applications of combination of convex and
concave lens
Chromatic aberration
Chromatic aberration, also known as “colour fringing” or “purple
fringing”, is a common optical problem that occurs when a lens is
either unable to bring all wavelengths of colour to the same focal
plane, and/or when wavelengths of colour are focused at different
positions in the focal plane. Chromatic aberration is caused by lens
dispersion, with different colours of light travelling at different speeds
while passing through a lens. As a result, the image can look blurred
or noticeable coloured edges (red, green, blue, yellow, purple,
magenta) can appear around objects, especially in high-contrast
situations.
A perfect lens would focus all wavelengths into a single focal point,
where the best focus with the “circle of least confusion” is located, as
shown below:
Telephoto lenses
A telephoto lens is a type of camera lens designed for taking

photographs of subjects at moderate to far distances. Also known as a
"tele lens" or "long lens", they are a type of long-focus lens which use
a special internal construction to give them a focal length much longer
than the length of the lens itself. This makes them smaller, lighter, and
easier to handle, while still giving excellent long-range capabilities.
Telephoto lenses are commonly used when photographing sports

events, wildlife, and in any other circumstances where the
photographer can't get close to the subject. They are also popular in
portrait and macro photography as they produce a pleasing, natural
perspective free from the distortion caused by using a wide-angle lens
 Telephotos Make Subjects Appear Closer to the Camera

 Telephotos Help Emphasize Blurred Backgrounds
Spherical lens
What is spherical lens?
A lens is a portion of transparent glass bound by two
spherical surfaces.
Lenses are of two types .
Convex lens : a convex lens is thick in the middle and thin at
the edges . It is also called converging lens.
Concave lens
A concave lens is thin in middle and thick at the edges.
Some important terms realted to lens .
Principal axis
The principal axis of a lens is defined as a straight line passing through the
optical centre and the centre of curvature of two surfaces of a lens. In
figure given below, the line C1 C2 represents principal axis of both the
convex lens and concave lens.
Optical Centre
The centre point of a lens which lies on the principal axis of the lens is
called its optical centre. The optical centre is represented by letter C. See
figure given below.
Focal Length
The focal length of a lens is the distance between its optical centre and
principal focus. The focal length of a lens is represented by letter f. In
figure given above, the distance CF is the focal length of the lens.
Principal Focus
The principal focus of a lens is a point on its principal axis to which the
light rays parallel to the principal axis converge (in case of convex lens) or
appear to diverge (in case of concave lens) after passing through it. The
principal focus of a lens is represented by letter F as shown in figure given
below.
Sign convention
All distances measured above the principal axis are taken as positive.
Thus, height of an object and that of an erect image are positive and all
distances measured below the principal axis are taken as negative.
The distances measured in the direction of incident rays are taken as
positive and all the distances measured in the direction opposite to that of
the incident rays are taken as negative.
All distances on the principal axis are measured from the optic centre.
The Sign Convention is as follows:-

U=object distance
V=image distance
F=focal length
D= dioptre
M=magnification
CONVEX CONCAVE
U= '- ve' (negative) U='- ve' (negative)
F='+ve' (Positive) F='-ve' (Positive)
D='+ve'(positive) D='-ve'(positive)
M='-ve' (negative) M='+ve' (positive)

if
m= <1 --> Image is determined
m= >1 --> Image is Enlarged
m= 1 --> Image is Same Size
m= 'positive' --> Image is Virtual and erect
m= 'Negative' --> Image is Real and inverted
Experiment
AIM: To verify a lens combination with the specified focal length by
using two lenses from the given set of lenses .
MATERIALS REQUIRED
A set of thin convex lenses ,a single lens holder,a lens holdera metal rod
and a screen
THEORY
Power of a lens
The reciprocal of focal length in metre is called power of lens in dioptre
(D)
P= 1\f
OR
The power P of a lens is defined as the tangent of the angle by which it
converges or diverges a beam of light falling from unit distant from the
optical centre
COMBINATION OF THIN LENS IN CONTACT
Let us consider two lenses A and B of focal length f1 and f2 placed

in contact with each other. An object is placed at O beyond the focus of
the first lens A on the common principal axis. The lens A produces
an image at I1. This image I1 acts as the object for the second lens B. The
final image is produced at I as shown in figure. Since the lenses are thin, a
common optical centre P is chosen.
Let PO = u, object distance for the first lens (A), PI = v, final image
distance and PI1 = v1, image distance for the first lens (A) and also object
distance for second lens (B).
For the image I1 produced by the first lens A,
1/v1 – 1/u = 1/f1 ….... (1)
For the final image I, produced by the second lens B,
1/v – 1/v1 = 1/f2 …... (2)
Adding equations (1) and (2),
1/v – 1/u = 1/f1 + 1/f2 …... (3)
If the combination is replaced by a single lens of focal length F such that it
forms the image of O at the same position I, then
1/v – 1/u = 1/f …... (4)
From equations (3) and (4),
1/F = 1/f1 + 1/f2 …... (5)
This F is the focal length of the equivalent lens for the combination.
The derivation can be extended for several thin lenses of focal
lengths f1, f2, f3 ... in contact. The effective focal length of the combination
is given by,
1/f = 1/f1 +1/f2 +1/f3+..........…... (6)

In terms of power, equation (6) can be written as,
P = P1 + P2 + P3 + …... (7)
Where P is the net power of the lens combination .
PROCEEDURE
1)Calculate the power of the combination of two lenses corresponding to the
required focal length.
2) Select a lens from the given set of lenses whose power is smaller than that of
the combination of lenses to be prepared, (if only convex lenses are provided).
3)Calculate the power of unknown convex lens to be kept in contact with the
lens of known focal length to obtain a combination of lenses of desired focal
length. Select the lens whose power is close to the calculated power from the
given set of lenses.
4) Set up the optical bench on a horizontal table. Adjust the collimator to direct
parallel beam of light along the optical bench. In case collimator is not
available, a plane mirror may be used to direct sunlight along the optical bench
[Fig. A 14.1 (b)] and illuminate a slit with it.
5) Place the two lenses on the uprights such that they are in contact with each
other. An upright that can hold two lenses in contact may also be used or the
same may be improvised by fixing the lenses on grooves carved on a
thermocole sheet.
6) Direct a parallel beam of light on the combination of lenses and obtain a

sharply focussed image of the source of light on a screen placed on the other
side of the lenses. This can be done by adjusting the distance between the
combination of lenses and screen
7)Measure the distance of the screen from both the lenses and record it in a
table.
8) Repeat the activity atleast three times by changing the position of the lens
combination on the optical bench. Record your observations in each case.
OBSERVATION
F =10cm P=10D
f1 f2 Expected Measured
value value
1/f1 + 1/f2
20 20 10 9.8
25 15 9.7 9.8
Calculations
𝑓1𝑓2
F= 𝑓1+𝑓2
Mean distance of screen from lens combination

𝑑1+𝑑2
F=
2
The mean distance of the screen from the lens combination is a
measure of its focal length. Take average of all readings as the focal
length of the combination determined by the experiment
RESULT
Measured value of focal length of lens combination =..9.8.cm
Difference between measured value of focal length and the calculated
focal length =.0.2..cm
The difference between the two could be due to experimental error.
PRECAUTIONS
The object needle should be placed at such a
distance that only real inverted image of it is formed
.
Thin lenses should be taken
Lenses should have same aperture
Bibliography
www.ncert.nic.in
www.physicsadda.in

Physics Investigatory

Uploaded by

Copyright:

Available Formats

Physics Investigatory

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Physics Investigatory

Uploaded by

Copyright:

Available Formats

Physics investigatory

To verify a lens combination with the

DONE BY : PRAGNA DYAVANUR

A telephoto lens is a type of camera lens designed for taking

Telephoto lenses are commonly used when photographing sports

 Telephotos Make Subjects Appear Closer to the Camera

The Sign Convention is as follows:-

U= '- ve' (negative) U='- ve' (negative)

F='+ve' (Positive) F='-ve' (Positive)

M='-ve' (negative) M='+ve' (positive)

Let us consider two lenses A and B of focal length f1 and f2 placed

1/f = 1/f1 +1/f2 +1/f3+..........…... (6)

Where P is the net power of the lens combination .

6) Direct a parallel beam of light on the combination of lenses and obtain a

Mean distance of screen from lens combination

You might also like