Glass prism, drawing board, sheet of white paper, drawing pins, protractor, set squares, pins and a meter rod.
Observations and Calculations:
| No. of Obs. | Angle of incidence PNO=angle ‘‘C’ | Critical | Angle of reflection ‘r’ ZPNF |
| 1234 |
Mean value of critical angle = C =
Refractive Index of glass =
= 1 / sinc = Procedure:
First of all, fix a sheet of white paper on the drawing board with the help of drawing pins at its corners. Place a prism on the drawing board with one edge towards you. Draw the boundary of the prism with a pencil. Name the corners of the prism as A, B and C such that ‘A’ should be towards you. Fix a pin in the middle of the side AB after removing prism. Replace the prism in the previous position so that the pin P just touches the face AB. Look through the face AC of the prism to see the image of pin P, then move your eyes from C to A. To bright image of pin P seen first and when you move further towards ‘A’ the image gradually become fainter and fainter. Fix the first pin, R when the image of in P just before it disappears. Fix another pin Q in the line of the image of pin P and pin R. Now the pin R and Q and image of P are in the same line. Remove all the pins, encircle all the pins, join R and Q to meet the face AC at F. Draw PG perpendicular to face BC. Produce PG to H, such that PG = GH. Now join H to F which cuts the face BC at N. Join P and N. Measure the angle Z PNO. The critical angle will be the half of the angle PNF. i.e.
PNO = ∠ NOF == critical angle
Take at least four readings and find its mean value of critical angle C and hence the refractive index of the glass used by using formula.
Refractive Index = = 1/ sin c.
Precautions:
1. The edge of the prism should be towards the observer.
- The faces of the prism should be cleaned.
- The pins should be vertical and well apart.
- The path of ray of light should be shown by arrows.
- Encircle the pin pricks and then joined with a sharp pencil.
- Fix the pin in the line with the point image of the left pin when it disappears.
VIVA VOCE
Q-1 Does the critical angle depend only on the material of the prism?
- No, it also depends on the nature of other medium.
Q-2 Will the critical angle of a given prism remains the same for all colors of light?
A- No, it will have different values for different colors.
Q-3 Describe any one application of total internal reflection?
- Optical fiber used in telecommunication is the most application of total internal reflection.
Q-4 Why do we fix the pin P touching the face of the prism?
- We fix the pin P in the position so that it may serve as an object lying in glass. Any ray of light coming open.it be an incident ray in the denser medium.
Q-5 What is prism?
- It is a retracting medium bounded by plane surfaces which intersect along the parallel Straight lines.
Q-6 What is a basic function of prism?
- The basic function of prism is to disperse white light into component colors and this make a spectrum.
Q-7 What are values of critical angle for glass. Water and diamond?
- Critical angle for glass. water and diamond are 42.49 and 44 respectively.
Q-8 What is refraction?
- When a ray of light passes from one transparent medium to another its velocity and direction Changes. the change in velocity and direction is called refraction.
Q-9 What is the unit of index of refraction?
- It has no unit, because it is a ratio between two similar quantities.\
Q-10 What happens when light travels from rare medium to a denser medium?
A- The speed of light decreases and the refracted ray bend towards normal.
Q-11 Does the critical angle of the material changes with the colour of light?
A- It is greater for red light and smaller for violet light because the frequency of violet light is greater than red light.
Q-12 Does the total internal reflection obeys the ordinary law of reflection of light?
- Yes. the total internal reflection obeys the ordinary law of reflection of light.
Q-13 Why do the precious stone like diamond shine brilliantly?
A- As the critical angle of diamond is small (240) and hence the refractive index is large (2.42). Therefore. the incoming light is totally reflected a number of times with much loss in intensity of light. This makes the faces of diamond bright.
Q-14 Define angle of prism?
A- The angle between two refracting rectangular surfaces opposite to the base.
Q-15 Define the angle of minimum deviation.
A- It is minimum value of angle of deviation.
Q-16 Does the critical angle of prism changed by changing light of difference colors?
A- No. the critical angle only depends upon material of transparent object.