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CBSE NOTES CLASS 12 PHYSICS

CHAPTER 10 WAVE OPTICS

Newton’s corpuscular theory

Wave theory - nature of electromagnetic waves

Wave front

Huygens principle

Refraction of a plane wave from rarer to denser medium

Refraction of a plane wave from denser to rarer medium

Reflection of a plane wave by a plane surface

Behaviour of a plane wave front with different surfaces

The Doppler effect

Superposition principle

Coherent sources of light

Interference of light

Young’s double slit experiment

Fringe width in double slit experiment

Diffraction of light

Single slit experiment

Double slit vs single slit patterns

Interference vs diffraction due to single slit

Constraints for diffraction due to single slit

Viewing the diffraction pattern

Energy is conserved during interference and diffraction

When can we consider the light beam to be parallel beam in single slit experiment?

Resolving power of an objective lens

Fresnel distance

Polarisation

Polarisation by transmission

Polarisation by scattering

Polarisation by reflection - Brewster’s law

Law of Malus

Polaroid and uses of polaroids

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 10 WAVE OPTICS

Resolving power of an objective lens

The resolving power of an objective lens is measured by its ability to differentiate two lines or points in an object. The greater the resolving power, the smaller the minimum distance between two lines or points that can still be distinguished.

A parallel beam of light falling on a convex lens, because of diffraction, instead of getting focused to a point gets focused to a spot of finite area.

The pattern on the focal plane consists of a central bright region surrounded by concentric dark and bright rings.

???

The radius of the central bright region is approximately

ro=1.22λf2a=0.61λfa

Although the size of the spot is very small, it affects the limit of resolution of optical instruments like a telescope or a microscope.

For the two stars to be just resolved,

fΔ  ro=0.61λfa   Δ = 0.61λa

Thus Δθ will be small if the diameter of the objective is large. It is for this reason that for better resolution, a telescope must have a large diameter objective.

For a microscope the object is placed slightly beyond f, so that a real image is formed at a distance v. The magnification – ratio of image size to object size – is given by,

m = vf

Also, Df ≈ 2tan β, where 2β is the angle subtended by the diameter of the objective lens at the focus of the microscope.

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When the separation between two points in a microscopic specimen is comparable to the wavelength λ of the light, the diffraction effects become important. The image of a point object will again be a diffraction pattern whose size in the image plane will be

 = v1.22λD  

Two objects whose images are closer than this distance will not be resolved and will be seen as one. The corresponding minimum separation, dmin, in the object plane is given by

dmin= 1.22λD ×vm

=1.22fλD 

=1.22λ2tanβ   1.22λ2 sin β

If the medium between the object and the objective lens has a refractive index n

dmin= 1.22λ2sin β

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