Explain partial reflection, critical angle and total internal reflection with the help of a neat labelled diagram.

Explanation

i. When a ray of light enters a rarer medium from a denser medium, it gets partially reflected i.e., part of the light gets reflected and comes back into the denser medium obeying the laws of reflection. This is called partial reflection.
ii. The rest of the light gets refracted and goes into the rarer medium. As light is going from denser to rarer medium, it bends away from the normal i.e., the angle of incidence (i) is smaller than the angle ofrefraction (r).
iii. According to Snell's law, the refractive index of the material is constant. Hence, on mcreasmg the angle of incidence the angle of refraction (r) increases.
iv. For a particular value of i for which, the value of r becomes equal to `90^(@)`, is called the critical angle.
For i = critical angle `=i_(c), r=90^(@)`
`""_(2)n_(1)=(sin i_(c))/(sin 90^(@))=sin i_(c) ..... (because sin 90^(@) = 1)`
v. For angles of incidence larger than the critical angle, the angle of refraction is larger than `90^(@)`. Such rays return to the denser medium as shown in the figure. Thus, all the incident light gets reflected back into the denser medium. This is called total internal reflection.

Partial reflection, Critical angle and total internal reflection