NO, the reverse is not true, as the incident light or beam falls at grazing angle it continues to pass almost undeflected, and thus is not altered in direction.

So, the concept of total internal reflection holds good only when the angle of incidence is just less than 90 degrees, or falling beam just does not graze or is not parallel to surface, as it should change medium fot TIR to occur.

 

 

 

The critical angle is the angle of incidence above which total internal reflection occurs. The angle of incidence is measured with respect to the normal at the refractive boundary. The critical angle ?_c is given by:

where n₂ is the refractive index of the less dense medium, and n₁ is the refractive index of the denser medium. This equation is a simple application of Snell's law where the angle of refraction is 90°.

 

If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence.

 

 

 

When light is incident upon a medium of lesser index of refraction, the ray is bent away from the normal, so the exit angle is greater than the incident angle. Such reflection is commonly called "internal reflection". The exit angle will then approach 90° for some critical incident angle ?_c , and for incident angles greater than the critical angle there will be total internal reflection.

The critical angle can be calculated from Snell's law by setting the refraction angle equal to 90°. Total internal reflection is important in fiber optics and is employed in polarizing prisms.

For any angle of incidence less than the critical angle, part of the incident light will be transmitted and part will be reflected. The normal incidence reflection coefficient can be calculated from the indices of refraction. For non-normal incidence, the transmission and reflection coefficients can be calculated from the Fresnel equations.

i'm just adding my views to an already answered query

 

 

 

bahut accha socha baalak!....

 

So you r thinking that if u pass ray parallel to the surface, it will pass straightway without going into glass slab. You r partially correct.

When we say that the ray is grazing on the surface, the situation actually is little complex.

when the ray is incident exactly at critical angle then the refracted ray is just 90deg to normal. Pay ur attention to 'exactly', i mean to say that as soon as the angle is just more than critical angle(even .0000001deg) then the ray would undergo Total Internal reflection. And when it is just less than critical angle(even 0.0000000001 deg) then the ray is not 90deg to the surface.

 

By heisenberg uncertainty principle there will always b some error, i.e. one can not make rays perfectly grazing the surface.

 

Practically a perfect angle of 90.000000000000000000000000000000 deg cant b achieved. So the almost 90 deg ray will always find some point of incidence.

 

 

 

But let us assume somehow one succeeds in making incident rays grazing the interface, then we want only one point of incidence on the glass slab. But suppose glass surface is perfectly smooth and plane such that the ray is just above the surface. Then we will need only one point of glass just one point above the surface so that the ray gets a surface to strike. And then it will trace the same path.

So practically the principle of optical reversibility is always true.