Total internal reflection
As the angle between the ray (in the glass) and the normal increases, the partial reflection becomes stronger. Between 41° and 42° the partial reflection becomes a total internal reflection

The critical angle can be found by putting angle i equal to 90° in the equation for the refractive index of glass in air....

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When light emerges from glass of refractive index 1.50 in to air, the critical angle (between the ray and the normal in the glass) is just less than 42°. Total internal reflection takes place for all angles greater than 42°.

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