BACK GROUND INFORMATION:
It has been observed that speed of light changes when it passes from one medium to another.
In a denser medium the speed of light is less than the speed in a rarer medium.
When ray of light enters from one medium to another medium, it bends from its original path; this bending of rays is called refraction. Refraction has two laws:
The incident ray, refracted ray and the normal at the point of incident all lie in the same plane.
2nd Law (Snell’s Law):
For two particular media, the ratio of the sine of the angle of incident to the sine of the angle of refraction is constant. This is called Snell’s Law.
n = sini/sinr
This constant “ n” is called refractive index of the medium. Greater the value of “ n” greater would be the bending effect of light. Refractive index is also calculated as: i) n = (Speed of light in air)/(Speed of light in medium) i.e. n = c/v and, ii) n = (Real Depth)/(Apparent Depth)
Glass slab, drawing board, drawing pins, sheets of paper, cello tape, protractor and a pencil.
1. First of all, I fixed a sheet of paper on the drawing board with cello tape.
2. I placed a rectangular glass slab horizontally and marked its boundary ABCDA.
3. Then, I removed the glass slab and draw an incident ray MX oblique to the interface AB.
4. I draw the normal NN’ at the point of incidence X and fixed two points p & Q vertically on the incident ray MX about 5cm apart.
5. Again, I placed the slab within the boundary ABCDA .
6. Looking through the opposite side of the glass slab, I fixed two more pins R and S in such a way that all the four pins appeared to lie along a straight line.
7. After this, I removed the slab and pins. I marked and encircled the pins pricks.
8 I joined R & S and extended the line upto the side CD. Let it met CD at point Y.
9. Now, I joined X and Y and obtained the line XY which was the refracted ray. 10. Lastly, I measured the angle MXN and YXN’. These were the angle of incidence ( i ) and angle of refraction (r) respectively. 11 . I took two more observations with different values of angles of incidence.
It shows that incident ray, the refracted ray and the normal, all lie on the same. Hence, first law is verified. We observe that in all the three readings, the value of the ‘n’ is constant, so that second law is also verified. Cautions: Pins must be fixed vertically. Use sharp pencil for drawing work. The direction of the ray should be marked with an arrow head. The pins should be about 5cm apart.