Aim To find out the refractive indices of different liquids using a hollow prism
Apparatus Required
- Hollow glass prism
- Various liquids like water, carbon disulphide, benzaldehyde etc.
- Bell pins
- Drawing board
Theory
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.
The refractive index of the liquid Is given by the formula:
U=sini/sinr=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.
Observation
- Benzaldehyde
| S.no | a º(angle of prism) | i º (angle of incidence) | d º (angle of deviation) |
| 1 | 60 | 30 | 45 |
| 2 | 60 | 35 | 42 |
| 3 | 60 | 37.5 | 40 |
| 4 | 60 | 39 | 42 |
| 5 | 60 | 40 | 45 |
U=sin((60+40)/2)/sin(30) = 1.504
- Water
| S.no | a º(angle of prism) | i º (angle of incidence) | d º (angle of deviation) |
| 1 | 60 | 25 | 30 |
| 2 | 60 | 35 | 22 |
| 3 | 60 | 40 | 20 |
| 4 | 60 | 45 | 22 |
| 5 | 60 | 50 | 25 |
| 6 | 60 | 55 | 28 |
U=sin((60+22)/2)/sin(30) = 1.306
- Diluted Sulphuric Acid
| S.no | a º(angle of prism) | i º (angle of incidence) | d º (angle of deviation) |
| 1 | 60 | 20 | 33 |
| 2 | 60 | 30 | 30 |
| 3 | 60 | 35 | 25 |
| 4 | 60 | 40 | 29 |
| 5 | 60 | 45 | 30 |
U=sin((60+25)/2)/sin(30) = 1.351
Refractive indices at room temperature:
- Benzaldehyde
- Actual: 546
- Experimental: 504
- Water
- Actual: 33
- Experimental: 306
- Dil . Sulphuric acid
- Actual: 355
- Experimental: 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.