Final answer:
The critical angle is the smallest incident angle at which light is no longer refracted into water, but instead is totally internally reflected. This can be calculated using Snell's law, where the sine of the critical angle is the ratio of the indices of refraction for water and glass.
Step-by-step explanation:
The smallest value of the incident angle θa for which none of the ray refracts into water, and instead exhibits total internal reflection, is known as the critical angle. To find this critical angle, we can apply Snell's law (n1 × sin(θa) = n2 × sin(θb)), where θa is the incident angle and θb is the refracted angle when θb is 90°, the angle of refraction is at the maximum and therefore indicates the critical angle condition. Using the indices of refraction for glass (n1) and water (n2), we can solve for the critical angle which will indicate the threshold above which light will not refract into water but instead be totally internally reflected.