#18Q3) Waves and Optics
Determination of the refractive index of the material of a prism by the critical angle method
Required Apparatus
An equilateral prism, a drawing board, drawing pins, a white sheet of paper, a few optical pins, a ruler, and a protractor.
Scientific Theory
If is the critical angle on an interface separating two media, then the refractive index of the dense medium relative to the rare medium,
Experimental Method
- Fix the sheet of paper on the drawing board using drawing pins.
- Place the prism on the paper and mark its edges on the paper in pencil.
- Fix a pin (O) vertically touching one face (AB) of the prism.
- Looking through the face BC towards the face AC observe the image of pin O. Move your eye from end C of the face BC towards B until the image of the pin just begins to disappear and at this position, fix two pins (P and Q) vertically to be in line with the disappearing image (and far apart from each other).
- Now remove the prism and the pins and make the following constructions on the paper in accordance with the steps given:
- Draw the line OXI normal to AC so that and locate the image I on it.
- Join the points where the pins P and Q were fixed and extend the line to meet BC at R.
- Join R and I to intersect AC at S.
- Join OS.
- Measure angle OSR.
Important Points
1
- Since angle OSR , calculate the value of and then the value of as shown in the theory.
2
- By placing a wet microscope slide on face AC and trapping a water layer it is possible to determine the critical angle for a water / glass interface.
3
- The pin O should be fixed to be in contact with the face AB in order to prevent refraction on face AC and cause errors. By removing the head of the pin it can be fixed to be in complete contact with the surface of the face AB.
4
- Discuss the course of action to be taken to find accurately the critical angle .