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Published on Sep 03, 2020


A cylindrical glass trough, water in a fug, a circular plastic disc which can float in water, a illuminating torch bulb which can be placed at the bottom centre of the trough .


Faraday's apparatus, a galvanometer, two magnets of different strengths, a battery, a tapping key

Theory :

When light travels from a denser to a raser medium and incident in denser medium at an angle more than critical angle 'C', then it is totally internally reflected.

For height 'h' and circle radius 'r'

Tan C=r/h or r=h tan C, no light will go out.

For a greater height light will be received outside the water


• Put empty glass trough on a table.

• Fit an illuminated tiny torch bulb at the centre of the trough. Bulb illuminating will be seen.

• Pour some water in the trough and float the circular plastic plate in it, keeping its centre in vertical line with trough centre. Illuminated water surface will be visible.

• Pour slowly more and more water. Area of illuminated water surface will go on decreasing.

• Go on pouring water till no illuminated water surface is visible.

You will find that

r/h= tan C

for water C =48.6° and tan C = 1.13428.


Illuminated water surface totally disappeared when

h=r/tan C i.e., r=h tan C

The result is according to the theory.

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