Concept:Total internal reflection requires light to travel from a denser medium to a less dense medium at an incidence angle beyond the critical angle.
Explanation:For total internal reflection to occur, two conditions must be satisfied simultaneously.
First, the light wave must be travelling from an optically denser medium to an optically less dense medium.
This means the refractive index of the first medium must be greater than the refractive index of the second medium, i.e.
n1>n2.
Second, the angle of incidence in the denser medium must be greater than the critical angle for the pair of media.
The critical angle is the angle of incidence for which the angle of refraction would be
90∘.
If the angle of incidence is less than the critical angle, ordinary refraction occurs.
If the angle of incidence is exactly equal to the critical angle, the refracted ray grazes along the boundary.
Only when the angle of incidence exceeds the critical angle does the ray reflect entirely back into the denser medium.
These two conditions are both necessary and sufficient for total internal reflection.
Thus, the correct combination is represented by the conditions that describe light moving from a denser to a rarer medium and the incidence angle being greater than the critical angle.
Answer:The necessary and sufficient conditions are: light travels from an optically denser medium to an optically less dense medium, and the angle of incidence is greater than the critical angle.