# How could I possibly know the radius of that circle?

Light coming from an underwater source emerges from the surface in a circle. If the water is 1.0 m deep and its index of refraction is 1.333, how big is this circle of light?

Is this related to refraction of light? If so, how could I possibly know the radius of that circle?

Consider Snell's law of refraction $n_1sin\theta_1 = n_2sin\theta_2$. When going from a medium of higher refractive index to one of lower refractive index (like going from water [n=1.33] to air [n=1]) there is a maximum angle of incidence that will allow refraction. Light hitting at a greater angle will not refract, but instead will reflect back into the water. Find that maximum angle (called the critical angle) and use it to find the size of the circle.