The light emanating through the interface is bent towards the glass. Once the incident angle is improved sufficiently, the transmitted angle (in air) gets to 90 degrees. It is at this point no light is transmitted into air.
If the incident ray is exactly at the critical angle, the refracted ray is tangent towards the boundary at the point of incidence. If for instance, visible light were going through acrylic glass (with an index of refraction of around 1.50) into air (having an index of refraction of 1.00), the computation would give the critical angle with regard to light from acrylic into air.
In optics, whenever light refracts through denser to rarer medium, based on Snells law of refraction it will deviate far from the common normal towards the mediums. If we do it continually increasing the incidence angle the moment will come once the refracted light will be simply along the surface separating the 2 media. Now if you improve the incidence angle more it will be reflected back in the identical media which is denser. This is referred to as Total internal reflection.
To find the critical angle, simply enter the index of refraction for every medium.
Refractive index of initial media multiplied from the SIN of the incidence angle = refractive index of 2nd medium multiplied from the refracted angle. If refractive index of initial medium is u1 and 2nd one is u2, angle of incidence i, angle of reflection r then eqn is: u1*sin i=u2*sin r nw think about the case of total internal reflection in that situation the refracted light is along the separating surface therefore angle r=90.so SIN 90=1
Critical angle is an angle of incidence within a denser medium for that angle of refraction within the rarer medium is 90°. Therefore, the angle greater than critical angle leads to ray to reflect back within the incident denser medium.
Critical angle of water is 48 degree means, at angle 48 degree the angle of refraction is actually 90 degree i.e. the refracted ray moves over the interface of the media. Beyond this angle the ray reflect back or goes through in total internal reflection.