To Investigate the dependence of the angle of deviation on the angle of Incidence, using a hollow Prism Project PDF Class 12
Introduction
A prism is a transparent optical instrument with flat, polished surfaces that refract, reflect, or disperse light. The angles between its surfaces vary depending on the application. While the traditional prism shape is triangular with a triangular base and rectangular sides, some optical prisms have different geometries.
Prisms can be made from materials such as optical glass, plastic, and fluorite, chosen based on their transparency to specific wavelengths. Their main function is to disperse light into its spectral colours
(the colour of rainbow) due to varying refraction angles for different wavelengths, a principle widely used in spectroscopy.
Besides dispersion, prisms are used for internal reflection, as seen in periscopes and binoculars, where they change light direction without significant loss. Some, like Nicol and Wollaston prisms, split light into different polarizations, useful in optical instruments.
Prisms play a crucial role in cameras, lasers, scientific instruments, and fiber optics, helping manipulate and direct light efficiently in various applications.
Some Important Terms
- Refraction: The phenomena of bending of light from its straight-line path on the surface of separation of two optical media is known as refraction of lights.
- Refractive Index: It is defined as the ratio of velocity of light in vacuum of that in any medium.
- Angle of Incidence: The angle between the incident ray and the normal.
- Angle of Deviation: The angle between the incident ray and the emergent ray.
- Minimum Angle of Deviation: The angle of incidence where the angle of deviation in a prism is minimum is called the minimum deviation position of the prism and that very deviation angle is known as the minimum angle of deviation.
- Dispersion: The phenomena of splitting of white light into its constituent colours when light ray passes through a prism.
Aim
To Investigate the dependence of the angle of deviation on the angle of Incidence, using a hollow prism filled one by one with different transparent fluids.
Apparatus
- Hollow Glass Prism
- Various Liquids like Water, Benzaldehyde, Dil. Sulphuric Acid
- Drawing Pins, Pencil
- Drawing Board
- White Sheet
- Protractor
THEORY
As light moves from one medium to another (e.g., from air into a glass prism), its speed changes, causing it to refract and enter the new medium at a different angle, as explained by Huygens’ principle. The degree of bending depends on the incident angle and the ratio of the refractive indices of the two media, a relationship described by Snell’s law.
The refractive index of many materials, such as glass, varies with wavelength, a phenomenon known as dispersion. This causes different colours of light to refract at different angles, leading to their separation and creating a rainbow-like effect. This property allows a prism to break white light into its constituent spectrum of colours. Unlike diffraction gratings, prisms can disperse light over a wider frequency range without the issue of overlapping spectral orders.
Prisms are also used for internal reflection rather than dispersion. If light inside the prism strikes a surface at a sufficiently steep angle, total internal reflection occurs, meaning all the light is reflected instead of refracted. This makes prisms a practical alternative to mirrors in certain optical applications.
The refractive index of the liquid Is given by the formula:
U= (Sin i)/(Sin r) = (Sin (((a+d))⁄2))/(Sin (a⁄2))
Where, U= Refractive Index of the Liquid.
a= The Angle of Minimum Deviation
d= Angle of Prism
i= Angle of Incidence
r= Angle of Refraction
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.
Diagram
Connections are made as shown in the figure given below, where K is the key, E is the battery, A is the ammeter, R is the rheostat, C is the commutator, and T.G is the tangent galvanometer. The commutator can reverse the current through the T.G coil without changing the current in the rest of the circuit. Taking the average of the resulting two readings for deflection averages out any small error in positioning the T.G coil relative to the earth’s magnetic field H.
Observation
- Benzaldehyde
S. No. | a | i | d |
1 | 60 | 30 | 45 |
2 | 60 | 35 | 42 |
3 | 60 | 37.5 | 40 |
4 | 60 | 39 | 42 |
5 | 60 | 40 |
- Water
S. No. | a | i | d |
1 | 60 | 30 | 25 |
2 | 60 | 35 | 22 |
3 | 60 | 40 | 20 |
4 | 60 | 45 | 22 |
5 | 60 | 50 | 25 |
6 | 60 | 55 | 28 |
- Sulphuric Acid
S. No. | a | i | d |
1 | 60 | 20 | 33 |
2 | 60 | 30 | 30 |
3 | 60 | 35 | 25 |
4 | 60 | 40 | 29 |
5 | 60 | 45 | 30 |
Graphical Representation
Result
The Angle of Deviation initially decreases with an increase in the Angle of Incidence, reaches a minimum value, and then increases with further increase in Incidence Angle.
Conclusion
The experiment investigated the relationship between the angle of incidence and the angle of deviation for Benzaldehyde, Water, and Diluted Sulphuric Acid using a prism. The observations confirm that the angle of deviation initially decreases with an increase in the angle of incidence, reaches a minimum value, and then increases with further increase in incidence angle.
The refractive indices of Benzaldehyde, Water, and Diluted Sulphuric Acid were determined experimentally and found to be very close to their actual values, with only slight variations.
Benzaldehyde:
Experimental = 1.504, Actual = 1.546
Water:
Experimental = 1.306, Actual = 1.33
Dil. Sulphuric Acid:
Experimental = 1.351, Actual = 1.355
The slight deviations between the experimental and actual values could be attributed to measurement inaccuracies, instrumental limitations, or minor variations in experimental conditions such as temperature.
Precautions
- Ensure the hollow prism is clean and free from any air bubbles before filling it with the liquid.
- Angle of incidence should lie between 35-60 degree.
- Pins should be vertically fixed and should lie in same line.
- Distance between 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.
- The angle of incidence should be measured accurately using a protractor.
- The pins should be placed vertically and in a straight line to minimize parallax error.
- The drawing board should be stable, and the white sheet should be properly pinned to avoid movement during the experiment
Project PDF Download Link:
