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 fluorite. 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 reflect light, or to split light into components with different polarizations.
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 Huygens' 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 colors 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 scientific 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. [1] He also introduced the use of more than one prism to control dispersion.[2] Newton's description of his experiments on prism dispersion was qualitative, and is quite readable. A quantitative were introduced in the 1980s.
THEORY
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 reflection at the surfaces rather than for dispersion. If light inside the prism hits one of the surfaces at a sufficiently steep angle, total internal reflection occurs and all of the light is reflected. This makes a prism a useful substitute for a mirror in some situations. Angle of Minimum Deviation The minimum value of angle of deviation, is called angle of minimum deviation. It is represented by the symbol Dm. For same angle of deviation (D) there are two values of angle of incidence. One value equals ’i’ and
other value equals ‘e’ As angle ‘i’ is increased from a small value, ‘e’ decreases from large value and angle of deviation decreases. When angle of deviation is minimum(Dm), then, ‘i’ and ‘e’ becomes equal. AIM To find out the refractive indices of different liquids using a hollow prism APPARATUS REQUIRED Hollow glass prism Various liquids like water, spirt and coconut oil. Board pins Drawing board
PROCEDURE Fix a white sheet of paper on the drawing board with help of drawing pins. Keep the prism and mark the outline of it as ABC. Drop a normal PQ on the side AB.
Draw the angle of incidence in accordance with the normal PQ and place 2 pins so that they appear to be in the straight line. Place the prism filled with given sample of liquid ,on the marked outline ABC. Now take the pins and place them on the side AC so that all the 4 pins appear to be in same line. Remove the prism and draw the line joining the points so obtained. Mark the diagram as shown in the figure.
Repeat this with different liquids and different angle of incidence.
Observation Benzaldehyde S.n a º(angle i º (angle o of prism) of incidence) 1 60 30 2 60 35 3 60 37.5 4 60 39
d º (angle of deviation) 45 42 40 42
5
60
40
45
U=sin((60+40)/2)/sin(30) = 1.504 Water S.n a º(angle i º (angle o of prism) of incidence) 1 60 30 2 60 35 3 60 40 4 60 45 5 60 50 6 60 55
d º (angle of deviation) 25 22 20 22 25 28
U=sin((60+22)/2)/sin(30) = 1.306 Dil. Sulphuric Acid
S.n a º(angle i º (angle o of prism) of incidence) 1 60 20 2 60 30 3 60 35 4 60 40 5 60 45
d º (angle of deviation) 33 30 25 29 30
U=sin((60+25)/2)/sin(30) = 1.351
Conclusion Refractive indices at room temperature: Benzaldehyde oActual: 1.546
oExperimental: 1.504 o Water oActual: 1.33 oExperimental: 1.306 o Dil . Sulphuric acid oActual: 1.355 oExperimental: 1.351
Precautions Angle of incidence should lie b/w 35-60 degree.
Pins should be vertically fixed and should lie in same line. Distance b/w two points should not be less than 10mm. Same angle of prism should be used for all observation. Arrow head should be marked to represent emergent and incident ray.
Record your observation.