What is total internal reflection?
Total internal reflection is commonly used in designing the diamonds and fibre optics. The common examples of total internal reflection are mirage and rainbow.
Total internal reflection is defined as a phenomenon in which a light wave hits the boundary of a medium and makes an angle which is greater than the critical angle.
What is a critical angle?
The critical angle is the specific angle in which the light rays travel from a denser medium to rare medium at an angle which is a straight line across the boundary. Hence, the critical angle is defined as the angle of incidence in a medium in which the angle of refraction in the air is 90°. To denote a critical angle, c is used.
Conditions that are necessary for total internal reflection to take place are:
- Lightray should travel from denser medium to rarer medium.
- The angle of incidence formed in the denser medium must be greater than the critical angle of the medium.
- Should follow the law of reflection.
Total internal reflection experiment
There is an experiment which explains the total internal reflection.
The objective of this experiment is to show the effect of an increase in the angle of incidence on the angle of refraction when the light travels from denser medium to rarer medium.
Materials that are used in this experiment are white paper, protractor, a pair of compasses, semicircular glass black, and a ray box with a single slit.
Procedure
- Draw a circle on the white paper using a degree scale and place the semicircular glass block on top of this circle.
- Consider the vertical plane of the set-up such that the light ray is direct through the semicircular glass block.
- It can be observed that the angle of incidence has increased which means that there is a change in the refracted light ray.
- When the angle of refraction reaches 90°, the angle of incidence is measured.
- These steps are repeated so as to see what happens to the light ray.
By performing this experiment following are the observations that are made:
- In a denser medium, when the angle of incidence is lesser than the critical angle, the light ray gets refracted away from the normal to the less dense medium.
- The angle of incidence is smaller than the angle of refraction when the light ray is in a denser medium.
- When the angle of refraction is equal to 90°, the angle of incidence is known as the critical angle.
For total internal reflection to take place, the angle of incidence in the denser medium must be greater than the critical angle.
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