A ray of light hits a block of glass of a r.i 1.74 calculate the critical angle for the material?

The critical angle (assuming the light is travelling from the glass to air with a refractive index of 1) is given by sin^-1(1/1.74)

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{sin}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{1}}}{{\mathtt{1.74}}}}\right)} = {\mathtt{35.079\: \!510\: \!940\: \!524^{\circ}}}$$

See http://www.physicsclassroom.com/class/refrn/Lesson-3/The-Critical-Angle, for example, for more detailed explanation.

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