Q. 7

# Calculate the radius of curvature of an equi-concave lens of refractive index 1.5, when it is kept in a medium of refractive index 1.4, to have a power of –5D ?

OR

An equilateral glass prism has a refractive index 1.6 in air. Calculate the angle of minimum deviation of the prism, when kept in a medium of refractive index 4 √2/5.

Answer :

We know that the focus of a lens is given by:

where f is the focal length of the lens and R_{1} and R_{2} are the radius of the refracting surface on the side of the object and on the other side respectively.

Let the radius be R.

and

Power of the lens,

Refractive index,

We know that,

OR

**Concept/Formula used:**

__Angle of minimum deviation__:

If is the angle of minimum deviation and is the angle of prism, then

where n is the refractive index of the prism with respect to the medium in which the prism is.

Note that in an equilateral prism,

The refractive index of the prism with respect to the medium is:

If is the angle of minimum deviation and is the angle of prism, then

Substituting the values,

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