Dear students, we have learnt about refraction of light from rare medium to denser medium and vice versa.You know that angle of refraction of light depends on angle of incidence. Greater the angle of incidence, greater will be angle of refraction generally. If we perform an experiment, we may observe that on increasing the measurement of angle of incidence of a ray of monochromatic light passing through denser medium to rare medium, we will reach a value for which angle of refraction will be 90 degree ( right angle ). This angle is termed as Critical angle.
Critical Angle :
The incident angle of a ray of light in denser medium for which angle of refraction of light in rare medium would be right angle ( 90 degree ), is called Critical angle.
n1 = refractive index of rare medium
n2 = refractive index of denser medium
From Snell’s Law, we may write
n2sinC = n1sin90
n2sinC = n 1* 1 ( since, sin90 = 1 )
sinC = n1 / n2
Now think students what happen when a ray of light incident with a angle greater than the critical angle. Whether the ray of light refract or not.
Actually, the ray of light will not refract. It is reflected back into in its own medium. This phenomenon is called as Total Internal Reflection of Light.
Total Internal Reflection of Light :
When a ray of light incident on a surface in denser medium with an angle greater than the critical angle for the medium, then the ray of light will not refract but it is totally relflected back into in ints own medium. This phenomenon of light is called as Total Internal Reflection of Light.
Different questions on the above topics :
1 ) Complete the path of the incident ray in the following figure till it emerges out, if the critical angle is 42 degree and marks the angle wherever necessary. (a) Which phenomenon is responsible for this action of prism ? (b) What is the nature of image in relation to object ? (c) In which device, such a prism is used ? What is the application of that device ?
(a) Total internal reflection and refraction of light are responsible for this action of prism.
(b) The image will be inverted. That means if we want to get the erect image of an inverted object, we must use this kind of prism and this prism is called as erecting prism.
(c) In SLIDE PROJECTOR, this kind of prism is used. It is used to show the photographic slides.
2 ) In the following figure, the ray of light is incident normally on the surface AB.
(a) Complete the ray diagram showing its emergence into air after passing through the prism. (b) Name the phenomenon the ray of light suffers at each face. (c) Write down the value of angle of incidence for each surafce.
Ans : (a)
(b) Name of the phenomenon that the ray suffers at the surface AB is Refraction from air to glass prism.
Name of the phenomenon that the ray suffers at the surface AC is the total internal reflection as incident angle ( 60 degree ) is greater than the critical angle ( 42 degree ).
On the surface BC, the ray of light ( RS ) is incident normally, hence, this is also a case of normal incidence.
3 ) Draw a neat labelled ray diagram to show the total internal reflection of a ray of light normally incident on one face of a 300,900,600 prism. (a) State the angle of incidence at each surface. (b) Name the phenomenon the ray of light suffering at each surface.
(a) Angle of incidence at surface PR = 0 degree ( due to normal incidence )
Angle of incidence at the surface QR = 60 degree
Angle of incidence at the surface PQ = 30 degree.
(b) Name of the phenomenon that the ray of light suffers at the surface PQ is normal incidence.
Name of the phenomenon that ray of light suffers at the surfae QR is Total internal refelction as angle of incidence ( 60 degree ) is greater than the critical angle ( 42 degree ).
Name of the phenomenon that ray of light suffers at the surface PQ is refraction of light through denser medium to rare medium.