Answer :

The phenomenon that occurs when the light comes across the object and it obstructs is known as diffraction.

The essential condition for diffraction of light is given by,

• The wavelength of the light should always be comparable to the size of the object.

• In certain cases, it may occur is the size of the object is less than the wavelength of light.

(ii)

Let a set of parallel rays from lens L_{1} allows to fall on the slit which then works as secondary source using Huygens’s principle. The resultant intensity is maximum.

For secondary minima:

Consider the right-angle triangle ,

For first minima, n=1

Thus, for nth minima,

For secondary maxima:

The path difference at the point is given by,

…(n=1)

For n=2,

General formula for nth maxima is,

(iii) In right angle triangle COP,

As θ is very small, sin θ=tan θ

For central maxima,

(iv) The size of the central band is given by,

Thus, size of central band reduces to half.

**OR**

(i) The schematic ray diagrams of astronomical telescope are,

(ii) There are two types of aberrations of reflecting telescope:

Spherical aberration:

It is the defect in the lens that result in the focusing of the beam of light on the point after passing through the spherical surface.

Chromatic aberration:

It is the defect in the lens which results in the formation of the blurred and coloured image.

(iii)

• The resolving power of the telescope increases if the frequency of the incident light is increased.

• The resolving power of the telescope does not change if the focal length of the objective is increased.

• The resolving power of the telescope halved if the aperture of the objective is halved.

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