When rays travel from a denser medium to a less dense medium, light bends away from the normal. For a particular incident angle, refracted angle $2=90.$ So$,$ when the refracted angle becomes $90,$ the corresponding incident angle is called critical angle.

When the angle of incidence is greater than the critical angle, $100\%$ of the light intensity is reflected. This is called total internal reflection because all the light is reflected.

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$1.$ Find the critical angle of glass and water combination.

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$2.$ What is the critical angle for the interface between Mystery A and glass? To be internally reflected, the light must start in which material? Show your calculation.

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$3.$ What is the critical angle for the interface between Mystery B and glass? To be internally reflected, the light must start in which material? Show your calculation.