Class 12 Project PRISM AND NATURE OF LIGHT

Class 12 Project Of Physics

1. INDEX



SI NO TITLE
1 INTRODUCTION
2 PRISM AND NATURE OF LIGHT
3 HOW DOES A PRISM WORK?
4 REFRACTION
5 PRISM FORMULA
6 EXPERIMENT
7 BIBLIOGRAPHY

2. INTRODUCTION
In optics, a prism is a transparent optical element with flat,
polished surfaces that refract light.The exact angles between
the surfaces depend on the application.The traditional
geometrical shape is that of a triangular prism with a
triangular base and rectangular sides,and in colloquial use
"prism" usually refers to this type. Some types of optical prism
are not in fact in the shape of geometric prisms.Prisms can be
made from any material that is transparent to the wavelengths
for which they are designed. Typical materials include glass,
plastic and fuorite.A prism can be used to break light up into
its constituent spectral colors (the colors of the
rainbow).Prisms can also be used to refect light, or to split
light into components with different polarizations.

3. PRISM AND NATURE OF LIGHT

Before Isaac Newton, it was believed that white light was
colorless, and that the prism itself produced the color.
Newton’s experiments demonstrated that all the colors already
existed in the light in a heterogeneous fashion, and that
"corpuscles" (particles) of light were fanned out because
particles with different colors traveled with different speeds
through the prism. It was only later that Young and Fresnel
combined Newton’s particle theory with Huygen’s wave theory to
show that color is the visible manifestation of light’s
wavelength. Newton arrived at his conclusion by passing the red
color from one prism through a second prism and found the color
unchanged.From this,he concluded that the colours must already
be present in the incoming light- thus the prism did not create
colors, but merely separated colors that are already there.He
also used a lens and a second prism to recompose the spectrum
back into white light.This experiment has become a classic
example of the methodology introduced during the scientifc
revolution. The results of this experiment dramatically
transformed the field of metaphysics, leading to John Locke’s
primary vs secondary quality distinction.Newton discussed prism
dispersion in great detail in his book Opticks.A quantitative
were introduced in the 1980s

4. HOW DOES A PRISM WORK?

Light changes speed as it moves from one medium to another (for
example, from air into the glass of the prism). This speed
change causes the light to be refracted and to enter the new
medium at a different angle (Huygens principle). The degree of
bending of the light’s path depends on the angle that the
incident beam of light makes with the surface, and on the ratio
between the refractive indices of the two media (Snell’s law).
The refractive index of many materials (such as glass) varies
with the wavelength or color of the light used, a phenomenon
known as dispersion. This causes light of different colors to be
refracted differently and to leave the prism at different
angles, creating an effect similar to a rainbow. This can be
used to separate a beam of white light into its constituent
spectrum of colors. Prisms will generally disperse light over a
much larger frequency bandwidth than diffraction gratings,
making them useful for broad-spectrum spectroscopy. Furthermore,
prisms do not suffer from complications arising from overlapping
spectral orders, which all gratings have. Prisms are sometimes
used for the internal refection at the surfaces rather than for
dispersion. If light inside the prism hits one of the surfaces
at a suffciently steep angle, total internal refection occurs
and all of the light is refected. This makes a prism a useful
substitute for a mirror in some situations.

5. Refraction
In a homogenous medium, light travels along a straight line.
But whenever it falls on the surface of another medium, a very
small fraction of it is reflected back and most of the light
passes into the medium, though with a change of direction. This
phenomenon of the bending of light at the surface of separation
of two media is called refraction of light.
Cause of Refraction
The phenomenon of refraction takes place when a beam of light
enters a medium in which light travels with a different
velocity.



Laws Of Reflection:
1. The reflected ray, the incident ray, and the normal at the
point of incidence all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection.

6. Laws Of Refraction:
1.The incident ray, the refracted ray, and the normal at the
point of incidence all lie in the same plane.
2.For any two given media the ratio sin i / sin r is a constant
(where i is the angle of incidence, r is the angle of
refraction). This is also called Snell's Law.

Refractive Index:
For a monochromatic light, the ratio of the sine of the angle of
incidence to the angle of refraction is a constant for two given
media in contact.If "i" is the angle of incidence and "r" the
angle of refraction then sin i / sin r = constant.
This constant is called the refractive index. For most purposes
it may be assumed that the refractive index is with represet.
air. When light travels from rarer to denser medium it bends
towards the normal and when it travels from denser to rarer
medium it bends away from the normal. It has been experimentally
determined that refractive index of a substance,

7. µ= c/v.
c=the speed of light in vacuum
v= the speed of light in the substance
Refractive Edge:
The line of interaction of the edges of the planes is known as
the refractive edge of the prism.
Angle of Deviation:
The angle through which the incident ray of light is deviated is
called the angle of deviation. It is the angle between the
emergent ray and the incident ray produced.
Angle of Minimum Deviation: As the value of the angle of
incidence (i) increases, the angle of deviation (d) decreases
till for a particular value of angle of incidence, it attains a
minimum value 'Dm' called the angle of minimum deviation and
then increases again. At this angle (Dm) the incident ray and
the emergent ray are symmetrical w.r.t. the refracting surfaces.
Critical Angle: It is that angle of incidence in the denser
medium for which the corresponding angle of refraction in the
rarer medium is 90 degrees.
µ = l/sinc ,where
µ = Refractive Index
c= critical angle
Relation between refractive index and critical angle according
to Snell's Law:
bµa= sin i/ sin r where i = c and r = 90°
bµa = sinc/ sin90° = sin c
But bµa = 1/ aµb

8. i.e. 1/ aµb = sin c or aµb= 1/sin c



PRISM FORMULA
Let ABC represent a section of the glass prism and let L be a
ray incident at an angle "I" on the first face AB of the prism
at a point "E". NN’ is the normal to this face. The material of
the prism is denser with respect to air, as such the ray would
refract in the direction EF making an angle r with the normal,
reaching the second face AC of the prism at the point F making
an angle e with the normal MM’ . The ray emerges in the
direction FS bending away from the normal making an angle "e"
with the normal. If the incident ray PE be produced forwards to
meet FS (also to be produced backwards) at G then the angle HGF
is called the angle of deviation and is represented by D. Angle
"BAC" is called the refracting angle of the prism and
represented by "A".
From the figure it can be proved:
D = (I + e) - (r1 + r2) (using exterior angle property of a
triangle)

9. A = (r1 + r2)
Therefore A + D = I + e; when angle of deviation D has the
minimum value Dm, the following conditions are fulfilled
I = e and r1 = r2 = r (say)
Applying these conditions in the equation
A = 2r Or r = A/2 A + Dm = 2I I = (A + Dm)/2
Since 1µ2 = sin i/ sin r 1µ = {sin(A + Dm)/2}/{sin A/2}

10. Experiment
AIM:
To find out the refractive indices of different liquids using a
hollow prism and to find the speed of light in given transparent
fluids.
APPARATUS:
* Hollow glass prism
* Drawing board
* Pins
* Meter scale
* Protractor
* Sheets of white paper
* Various liquids
a)Water
b) Vinegar
c)Vegetable Oil

THEORY:
Light is an electromagnetic radiation that is visible to the
human eye usually having a wavelength in the range of 400 nm to
700 nm between the infrared, with longer wavelengths and the
ultraviolet with the shorter wavelength. The speed of light in
vacuum is found to be exactly 299,792,458 m/s. Observable events
that result from the interaction of light and matter are called
optical phenomenon. Refraction is a surface phenomenon due to a

11. change in its transmission medium.


When a ray of light passes from one medium into the other, it
either bends towards the normal or away from the normal in the
second medium. This phenomenon is known as the refraction of
light. A prism is a transparent optical element with flat,
polished surfaces that refract light. Prisms can be made from
any material that is transparent including glass, plastic and
fluorite. A prism can be used to break light up into its
constituent spectral colors. Prisms can also be used to reflect
light, or to split light into components with different
polarizations. For a particular pair of two media and for a
particular wavelength of light (colour) the ratio of the sine of
the angle of incidence and the sine of the angle of refraction
is a constant quantity called the refractive index of the second
medium w.r.t. the first. It is represented by -----
2µ1 = sin i / sin r.
The value of the angle of incidence "i" can be obtained in the
terms of the refracting angle "A" of the prism and the angle of
minimum deviation "Dm" and the angle of refraction "R" can also
be obtained in terms of the refracting angle "A" of the prism.
Thus we find that we can use the above relation derived for
determining the refractive index. The experiment thus consists

12. in finding the value of the refracting angle "A" of the prism
and the value of the angle of minimum deviation Dm. The
refractive index of the liquid Is given by the formula:
µ = {sin(A + Dm)/2}/{sin A/2}
For finding the value of Dm a curve is plotted between angles of
incidence (i) and their respective angles of deviations (d).
PROCEDURE
A) For finding the angle of prism
* Take a piece of white paper, fix it on a drawing board using
board pins.
* Place the hollow glass prism on the sheet and carefully draw
its outline. Draw a normal and carefully draw its outline.
* Draw a normal and an incident ray at an angle of 35 degrees
with the normal on side AB of the prism.
* Fix two pins P1 and P2 on the incident ray which are at least
5 cm apart.
* Fill the prism with water and place it over its outline.
Observe the refracted ray that comes after refraction from the
face AB of the prism.
* Fix two more pins P3 and P4 to cover the image of P1 and P2.
* Obtained angles r1 and r2 and add them to obtain the angle of
the prism.
B) For finding the angle of minimum deviation
* Fix a white sheet of paper on a drawing board using board pins
* Place a hollow glass prism on the sheet and carefully draw
its outline. Draw a normal and an incident ray of angle of
incidence 35 degrees on the side AB of the prism.

13. * Fix two pins P1 and P2 on the incident ray at least 5 cm
apart.
* Fill the hollow prism with water and place it over its drawn
outline. Observe the refracted ray which comes after refraction
by placing two more pins P3 and P4 covering P1 and P2.
* Extended the incident and refracted ray to obtain the angle of
deviation, D.
* Repeat the above procedure taking other liquids and the angles
of incidence as 40° , 45° , 50° , 55° and 60°. Note the lowest
obtained value of angle of deviation as the angle of minimum
deviation, Dm .
*Using the value of the angle of prism (A) and the angle of
minimum deviation (Dm), calculate the value of the refractive
index of the liquids by using the equation given in the theory.
*Select suitable scales to represent the angle of incidence
along the X-axis and angle of deviation along the Y-axis and
plot a graph. The graph gives the value of Dm, which is the
minimum most point of the parabola.

WATER
S.No Angle of Incidence Angle of Deviation
1. 35° 25°
2. 40° 24°
3. 45° 23°
4. 50° 25°
5. 55° 27°
6. 60° 28°

14. VINEGAR
S.No Angle of Incidence Angle of Deviation
1. 35° 26°
2. 40° 25°
3. 45° 23.3°
4. 50° 25°
5. 55° 27°
6. 60° 28°

15. VEGETABLE OIL
S.No Angle of Incidence Angle of Deviation
1. 35° 49°
2. 40° 39°
3. 45° 34°
4. 50° 36°
5. 55° 39°
6. 60° 40°



OBSERVATIONS:

CALCULATIONS:
A) Refractive index of liquids
Angle of prism (A) = 60°
Formula used: µ= {sin ((A + Dm)/2}/{sin (A/2)}
Water:
Dm=23°
Therefore µ = sin 41.5 /sin 30 = 0.6626 /0.5 =1.3252
Vinegar:
Dm=23.5°

16. Therefore µ = sin 41.25 /sin 30 = 0.6593 /0.5 =1. 3186
Vegetable Oil:
Dm=34°
Therefore µ = sin 41.25/ sin 30 = 0.6593 /0.5 =1. 3186

Sl no Liquid Speed of light v= c /n
(m/s)
Speed of light (m/s)
1.
Water 3×108 /1.3252 2.26×108
2.
Vinegar 3×108 /1.3186 2.27×108
3.
Vegetable oil 3×108 /1.4626 2.05×108

RESULT
The refractive indexes of the four liquids were found to
be as follows:-
* Water, µ = 1.3252
* Vinegar, µ = 1.3186
* Vegetable Oil, µ = 1.4628
The speeds of light in the four liquids were found to be
as follows:-
* Water, v=2.26×108 m/s
* Vinegar, v=2.27×108 m/s
* Vegetable oil, v=2.05×108 m/s




PRECAUTIONS

17. * The position of the prism should not be disturbed on the white
sheet.
* There should be no parallax between the pins P1, P2 and their
images P3, P4.
* The angles should be measured carefully.
* The curve of the graph should be smooth.
SOURCES OF ERROR
* Pin pricks may be thick
* Measurement of angles may be wrong

BIBLIOGRAPHY
 Physics Class XII NCERT Textbook
 Google images
 LAB MANNUAL CLASS XII
 www.google.com
 https://www.quantumstudy.com/
 https://mycurvefit.com/