The angle of prism (111) 1he index of refraction of the material of the prism and (iv) Wavelength

of the light.

What is the angle of minimum deviation?

The angle of minimum deviation (Dm) is the least value of the angle of deviation for a particular

value of angle of incidence. The angle of deviation is a minimum when the incident and emer

gent rays are equally incident to their respective faces.

What is critical angle?

When light enters from a denser to a rarer medium, the value of angle incidence for which the

angle of refraction is 90°, is called "critical angle" (C). It is 42° for glass and 24° fodr diamond. If

the less denser medium is air, the refractive index of the more denser medium is :nsin C

Does the critical angle of a material vary with the colour of light?

Sir

Yes: The critical angle changes with colour (wavelen gth) of light. It is greater for red light and

smaller for violet light.

Why do we fix the pin P' touching the face of the prism?

We fix the pin "P" in this position so that it may serve as an object lying in glass. Any ray of

light coming from it will be an incident ray in the denser medium.

Why do you take PQ = QR in this experiment?

Because, the image is formed at the same distance behind as the object is infront of the re-

flecting surface of the prism

Why does the image of the object pin P disappear at a certain position?

Because, at this position, the incident light is refracted and not reflected back.

Does the critical angle depend only on the nature of the medium?

No. It also depends on the nature of the other medium.

How does the refractive index of a refracting medium change with the colour of light?

It is greater for light of smaller wavelength. The refractive index for violet colour (of smaller

wavelength) is greater than for the red colour (of large wavelenght). It is different for sodium or

mercury light.

When light passes from a denser to a less dens (rarer) medium, total intermal reflection occurs for

all rays whose angles of incidence are greater than the critical angle.

What is total internal reflection?

parkling of diamond, mirarge, optical fibres, fish-eye view and use of right-angled isosceles

orisms as totally reflecting prisms in binoculars and periscopes.

Give some examples of total internal reflections.

No the transmitted beam gradually becomes weaker and its intensity diminishes to zero as the

angle of reflection reaches 90.

n total internal reflection, does the intensity of the transmitted beam remain constant.