what is meant by critical angle?
The critical angle is the minimum angle of incidence inside a medium at which a light ray is reflected back into the the medium. When light is incident on the surface of the medium past the critical angle the light is totally reflected back into the medium.
The critical angle of reflection is another key concept in the study of light refraction and is illustrated below in Figure 4. When light passes through a medium of high refractive index into a medium of lower refractive index, the incident angle of the light waves becomes an important factor.