Takshashila International School
2023-2024
To find refractive indices
of water and oil
(Transparent)
Submitted by:
Manav Joshi
Class: XII
Roll no.-
submitted to:
Mr.Rajesh Ghodraj
CERTIFICATE
This is to certify that Mr. Manav joshi of
Class XII Science roll no.
Has
satisfactorily complete his physics
investigatory project as prescribed by the
course during the Academic Year
2023-2024
Sign of
Internal Examiner
Sign of
External Examiner
Sign of
Principal
Declaration
I hereby declare that the investigatory project
entitled
“To find the refractive indices of water and
oil” has been carried out by my own efforts
and fact arrived at my observation under the
guidance and encouragement of subject
teacher
“Mr. Rajesh Ghodraj” [PGT Physics]
Sign of Student
Name- Manav Joshi
Standard- 12th Science
Roll No. -
Acknowledgement
At the outsiet,
I express our gratitude to the
Almighty Lord for the divine,
guidance and wisdom showeredon
me to undertake this project.
I am immensely grateful to my
Chemistry teacher Mr. Rajesh
Ghodraj for the guidance and
suggestions and the help to make
this project a success.
My Parents also played a key role
in shaping up this project nicely and I
convey my special Thanks to them as
well.
Sign of student
Index
SR. NO.
1
CONTENTS
OBJECTIVE
PAGE NO.
1
2
APPARATUS
2
3
THEORY
2
4
PROCEDURE
3
5
OBSERVATION
6
6
CALCULATION
7
7
RESULT
8
8
PRECAUTION
8
9
SOURCES OF ERROR
9
10
BIBLIOGRAPHY
10
History Further information
History of the telescope The telescope is more a discovery
of optical craftsmen than an invention of a scientist The
lens and the properties of refracting and reflecting light
had been known since antiquity and theory on how they
worked were developed by ancient Greek philosophers,
preserved and expanded on in the medieval Islamic
world, and had reached a significantly advanced state by
the time of the telescope’s invention in early modern
Europe. But the most significant step cited in the
invention of the telescope was the development of lens
manufacture for spectacles, first in Venice and Florence in
the thirteenth century, and later in the spectacle making
centers in both the Netherlands and Germany. It is in the
Netherlands in 1608 where the first recorded optical
telescopes (refracting telescopes) appeared. The invention
is credited to the spectacle makers Hans Lippershey and
Zacharias Janssen in Middelburg, and the instrumentmaker and optician Jacob Metius of Alkmaar. Galileo
greatly improved [citation needed] on these designs the
following year, and is generally credited as the first to use
a telescope for astronomy. Galileo’s telescope used Hans
Lippershey’s design of a convex objective lens and a
concave eye lens, and this design is now called a Galilean
telescope. Johannes Kepler proposed an improvement on
the design that used a convex eyepiece, often called the
Kepler Ian Telescope.
Surface resolvability
The smallest resolvable surface area of an object, as
seen through an optical telescope, is the limited
physical area that can be resolved. It is analogous to
angular resolution, but differs in definition: instead of
separation ability between point-light sources it refers
to the physical area that can be resolved. A familiar
way to express the characteristic is the resolvable
ability of features such as Moon craters or Sun spots.
Expression using the formula is given by the sum of
twice the resolving power R over aperture diameter
multiplied by the objects diameter multiplied by the
constant all divided by the objects apparent diameter.
Resolving power R is derived from the wavelength
using the same unit as aperture; where 550 nm to
mm is given by. The constant is derived from
radians to the same unit as the objects apparent
diameter; where the Moons apparent diameter of
radians to arcsecs is given by:
Angular resolution
Ignoring blurring of the image by turbulence in the
atmosphere (atmospheric seeing) and optical
imperfections of the telescope, the angular
resolution of an optical telescope is determined by
the diameter of the primary mirror or lens
gathering the light (also termed its “aperture“). The
Rayleigh criterion for the resolution limit is the
wavelength and is the aperture. For visible light in
the small-angle approximation, this equation can be
rewritten. Here, R denotes the resolution limit in arc
seconds and is in millimeters. In the ideal case, the
two components of a double star system can be
discerned even if separated by slightly less than R.
This is taken into account by the Dawes limit.
Focal length and focal ratio
The focal length of an optical system is a
measure of how strongly the system converges
or diverges light. For an optical system in air, it
is the distance over which initially collimated
rays are brought to a focus. A system with a
shorter focal length has greater optical power
than one with a long focal length; that is, it
bends the rays more strongly, bringing them to
a focus in a shorter distance. In astronomy, the
f-number is commonly referred to as the focal
ratio notated as N. The focal ratio of a telescope
is defined as the focal length of an objective
divided by its diameter or by the diameter of an
aperture stop in the system. The focal length
controls the field of view of the instrument and
the scale of the image that is presented at the
focal plane to an eyepiece, film plate, or CCD.
The light gathering power
The light-gathering power of an optical
telescope, also referred to as light grasp or
aperture gain, is the ability of a telescope to
collect a lot more light than the human eye. Its
light-gathering power is probably its most
important feature. The telescope acts as a light
bucket, collecting all of the photons that come
down on it from a far away object, where a
larger bucket catches more photons resulting in
more received light in a given time period,
effectively brightening the image. This is why
the pupils of your eyes enlarge at night so that
more light reaches the retinas. The gathering
power compared against a human eye is the
squared result of the division of the aperture
over the observer’s pupil diameter with an
average adult having a pupil diameter of 7mm.
Younger persons host larger diameters,
typically said to be 9mm, as the diameter of the
pupil decreases with age.
Magnification
The magnification through a telescope magnifies a
viewing object while limiting the FOV. Magnification is
often misleading as the optical power of the telescope,
its characteristic is the most misunderstood term used
to describe the observable world. At higher
magnifications the image quality significantly reduces,
usage of a Barlow lens—which increases the effective
focal length of an optical system—multiplies image
quality reduction. Similar minor effects may be present
when using star diagonals, as light travels through a
multitude of lenses that increase or decrease effective
focal length. The quality of the image generally
depends on the quality of the optics (lenses) and
viewing conditions—not on magnification.
Magnification itself is limited by optical
characteristics. With any telescope or microscope,
beyond a practical maximum magnification, the image
looks bigger but shows no more detail. It occurs when
the finest detail the instrument can resolve is
magnified to match the finest detail the eye can see.
Magnification beyond this maximum is sometimes
called empty magnification.
Observing through a telescope there are many
properties of optical telescopes and the
complexity of observation using one can be a
daunting task; experience and experimentation
are the major contributors to understanding
how to maximize one’s observations. In
practice, only two main properties of a
telescope determine how observation differs:
the focal length and aperture. These relate as to
how the optical system views an object or range
and how much light is gathered through an
ocular eyepiece. Eyepieces further determine
how the field of view and magnification of the
observable world change. Observable world
Observable world describes what can be seen
using a telescope, when viewing an object or
range the observer may use many different
techniques. Understanding what can be viewed
and how to view it depends on the field of view.
Viewing an object at a size that fits entirely in
the field of view is measured using the two
telescope properties—focal length and aperture,
with the inclusion of an ocular eyepiece with
suitable focal length (or diameter). Comparing
the observable world and the angular diameter
of an object shows how much of the object we
see. However, the relationship with the optical
system may not result in high surface
brightness. Celestial objects are often dim
because of their vast distance, and detail may be
limited by diffraction or unsuitable optical
properties.
Astronomical Telescope Experiment
Experiment: To set up an astronomical telescope and find
its magnifying power. Astronomical telescope consists of
two converging lenses. One is the objective lens O of a
long focal length fo. The other is the eye lens E of short
focal length fe. A distant object is seen through it by
keeping the objective lens towards that object. For
simplicity, assume that the axis of the telescope EO points
towards the base A of the distant object AB situated far
beyond the figure. The objective lens makes a real,
inverted and diminished image A’B’ of that object. As the
rays enter the eye lens, A’B’ functioning as the new object,
its virtual magnified image A’B’ is formed. Thus, you
observe fine details in A’B’ by the eye lens. The image A’B’
is at the focus of lens O and also is approximately at the
focus of lens E. Therefore, separation between the lenses
is OE = fo = fe Magnifying power of the telescope is angle
subtended by the image A"B" at E divided by the angle
subtended by the object AB at O. m = fo/fe In order to
observe the image of distant object through the telescope,
your eye should not be too close to the eye lens E. This lens
makes a real image of lens O at I. It is just beyond the
outer focal point Fe of the lens E. All light rays entering
through O and passing through lens E, also pass through
this image. This is called the exit pupil of the telescope.
Pupil of your eye must coincide with this image in order
to receive all the light coming through objective and the
eye lens. This enables you to see all the objects that the
telescope is capable of seeing at one time.
Materials required An optical bench with three lens uprights, objective lens (f = 50 cm to 80 cm, diameter - 50
mm), eye lens (f = 5 cm to 10 cm diameter = 20 mm to 50
mm), circular cardboard diaphragm (O.D. - 50 mm,
central hole diameter - 15 mm), a scale with bold marks,
meter scale
How to Perform Experiment:
(A) Setting up the Telescope 1. Find the focal length
of the objective Lens fo, by focussing the image of a
distant bright object on a screen, or on a wall of your
laboratory and measuring its distance from the lens.
Similarly, find the focal length of eye lens fe. These are
only approximate values. 2. Calculate approximate
distance between the two lenses, f0 + fe, for telescope
making. 3. Fix the eye lens in one upright and keep it at
the 10 cm mark on the optical bench. 4. Mark a small
cross (×) in the centre of the objective lens. Fix it on
another upright. Adjust the height of its centre above
optical bench equal to that of the eye lens. Then keep it on
the optical bench at a distance f0 + fe from the eye lens. 5.
Fix the diaphram D in the third upright. Adjust the height
of its centre above optical bench equal to that of the eye
lens. Then keep it on the optical bench at a distance
slightly more than fe from the eye lens on the side
opposite to the objective lens. You should now see the
image of cross mark on objective lens made by eye piece
at the centre of the diaphram. Make fine adjustments in
the position of diaphram vertically, horizontally and
along length of the optical bench. Thus you locate the exit
pupil of the telescope. 6. Now point this telescope to any
distant object. Keep your eye at the hole in diaphram D
and look at inverted image of the object. You will have to
move the diaphram a little forward. You may also have to
adjust the position of lenses O and E a little in order to
focus a sharp image of the object.
(B) Finding the Magnifying Power:
7. Keep the scale with bold marks vertical in front of the
telescope at a distance of at least 10 m. If your laboratory
is not long enough, do this part of experiment in the
corridor. 8. Adjust the position of eye lens so that the final
virtual image of the scale is roughly at the same distance
as the scale seen directly, For this adjustment you may
look by one eye (say the right eye) into the telescope and
by the other eye look directly at the scale. When proper
adjustment is done, you see the scale and its magnified
image together, as if stuck to each other. 9. Your scale
with bold marks is such that it can be seen clearly by your
left eye at a distance of upto 20 m. Observe on it the size
of the enlarged image of one smallest division seen
through the telescope by the right eye. Ratio of the size of
this enlarged image to size of the division gives the
magnifying power of the telescope. 10. Repeat the
observation of step (9) for two divisions of the scale, three
divisions of the scale, and so on. Thus obtain a few more
measured values of magnifying power. Find the mean of
all these values.
Sources of Errors:
1.
fo and fe have been measured only approximately.
2. Expression m = f0/fe is valid only for the case
when object-and its final virtual image are both at
infinity. But it is not so in the experiment. 3. Lenses
used in the experiment are not achromatic. Thus
image seen in the telescope made in the experiment
is not quite sharp as it would be in a standard
telescope using achromatic lenses, Thus magnifying
power cannot be found quite accurately.
APPLICATION:
The device used to form magnified images of distant
objects. The telescope is undoubtedly the most
important investigative tool in astronomy. It provides a
means of collecting and analyzing radiation from
celestial objects, even those in the far reaches of the
universe.
BIBLIOGRAPHY
1. WEBSITES :
 www.wikipedia.org
 www.google.com
 www.yahoo.com
2. BOOKS :
Comprehensive Practical Manual for class XII