Refraction Of Light


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       Total internal reflection is a very interesting phenomenon which takes place when light rays pass from an optically denser medium (e.g. water, glass) into optically rarer medium (e.g. air). Now, we shall discuss the concept of total internal reflection by taking three different cases. These are:

Case 1

       Consider a point object O is placed in a tank full of water. Now, a ray of light OA starting from the point object O, bends away from the normal AN along the direction AB in the air. As the refracted ray AB is going away from the normal so the angle of refraction r is greater than the angle of incidence i.

Case 2

       Now, as we increase the angle of incidence i, the angle of refraction r also increase. By increasing the angle of incidence up to a particular value which is called the critical angle (i = c), the angle of refraction r becomes equal to 900, and the refracted ray don’t go into air but gores along the horizontal water surface in the direction CD. So, critical angle may be defined as the angle of incidence in the denser medium for which the angle of refraction in rarer medium is 900. The critical angle is represented by letter ‘c’.

Principal Axis Concave Lens

       The value of critical angle for different substances is different, as given in table

Table of Critical Angle of different substances

S No. Name of the substance Critical angle
1. Diamond 240
2. Common glass 420
3. Water 490

Case 3

       Now, if the angle of incidence i is greater than the critical angle (i.e. i > c) then the ray of light OE is reflected back into the water along the direction EF, without going into the air. This phenomenon is called total internal reflection.

       We can now define the total internal reflection as the phenomenon of reflection of light into a denser medium when the angle of incidence of a light traveling in a denser medium is greater than the critical angle of the medium.

Essential Conditions for Total Internal Reflection

       There are two conditions which are essential for total internal reflection. These are:

  1. The light should travel from a denser medium to a rarer medium.

  2. The angle of incidence of light traveling in denser medium should be greater than the critical angle of the medium.

Test Your Understanding and Answer These Questions:

  1. Define critical angle.

  2. What is total internal reflection? Explain with the help of a diagram.

  3. Write the values of critical angle for diamond, glass and water?

  4. What will be angle of refraction for a ray of light which goes from optically denser medium to optically rarer medium having angle of incidence equal to critical angle?

  5. What are the essential conditions for total internal reflection to take place?