True

The critical angle is indeed defined as the angle of incidence at which total internal reflection starts to occur. To understand this concept, let's break it down into the following sections:

1. Refraction and Total Internal Reflection:
When light travels from one medium to another with different refractive indices, it changes its direction. This phenomenon is known as refraction. The angle at which the light ray bends is determined by the refractive indices of the two mediums and is given by Snell's law.

However, there is a limit to the bending of light. If the angle of incidence is such that the refracted ray would be bent away from the normal, it means that the refracted ray would not exist inside the second medium. Instead, the light gets completely reflected back into the first medium. This phenomenon is known as total internal reflection.

2. Critical Angle:
The critical angle is the angle of incidence at which the light ray is refracted along the interface between two mediums and emerges parallel to the boundary. In this scenario, the angle of refraction becomes 90 degrees, which is the maximum possible angle of refraction.

The critical angle is dependent on the refractive indices of the two mediums. It can be calculated using the formula:

Critical angle = sin^(-1)(n2/n1)

Where n1 is the refractive index of the first medium (from which light is incident) and n2 is the refractive index of the second medium.

3. Total Internal Reflection:
When the angle of incidence is greater than the critical angle, total internal reflection occurs. The light ray is fully reflected back into the first medium, and no light is transmitted into the second medium.

This phenomenon is essential in various applications, such as optical fibers, prisms, and mirage formation. It allows for efficient transmission and manipulation of light.

Conclusion:
In conclusion, the statement that the critical angle is defined as the angle of incidence at which total internal reflection starts to occur is true. The critical angle determines the boundary between refraction and total internal reflection, and it is a crucial concept in the field of optics.