Critical angle is the angle of incidence at which the angle of refraction in the second medium is 90 degrees. It is the maximum angle of incidence for which the light can still be transmitted through the interface between the two media.

The critical angle can be determined using Snell's law, which relates the angles of incidence and refraction to the refractive indices of the two media:

n1 * sin(theta1) = n2 * sin(theta2)

Where:
- n1 is the refractive index of medium 1
- n2 is the refractive index of medium 2
- theta1 is the angle of incidence in medium 1
- theta2 is the angle of refraction in medium 2

To find the critical angle, we need to find the angle of incidence at which the angle of refraction is 90 degrees. When the angle of refraction is 90 degrees, sin(theta2) is equal to 1.

Therefore, the equation becomes:

n1 * sin(theta1) = n2 * 1

Simplifying the equation:

sin(theta1) = n2 / n1

To find the critical angle, we need to find the angle whose sine is equal to the ratio of the refractive indices of the two media. Taking the inverse sine (or arcsine) of the ratio gives us the critical angle.

Mathematically, the critical angle can be expressed as:

theta_c = arcsin(n2 / n1)

In this case, the refractive indices are given as n1 = 2 and n2 = 1. Plugging these values into the equation, we get:

theta_c = arcsin(1 / 2)

Using a calculator, we can find the value of arcsin(1 / 2) to be approximately 30 degrees.

Hence, the correct answer is option B) 30 degrees.