# Download Physics Notes for Class 12 Chapter 9 Ray Optics and Optical Instruments PDF

Title Physics Notes for Class 12 Chapter 9 Ray Optics and Optical Instruments Lens (Optics) Angular Resolution Reflection (Physics) Refractive Index Optics 697.1 KB 19
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Power of a Lens

The reciprocal of the focal length of a lens, when it is measured in metre, is called power of a

lens.

Power of a lens, (P)= 1/f(metre)

Its unit is diopter (D).

The power of a convex (converging) lens is positive and for a concave (diverging) lens it is

negative.

Focal Length of a Lens Combination

(i) When lenses are in contact 1/F – 1/f1 + 1/f2

Power of the combination P = P1 + P2

(ii) When lenses are separated by a distance d

1/F = 1/f1 + 1/f2 – d/f1f1

Power of the combination

P = P1 + P2 – dP1P2

Linear Magnification

m = I/O = v/u

For a small sized object placed linearly along the principal axis, its axial (longitudinal)

magnification is given by

Axial magnification = – dv/du = (v/u)
2

=(f/f+u)
2
= (f-v/f)

2

Focal Length of a Convex Lens by Displacement Method

Focal length of the convex lens f = (a
2
– d

2
) / 4a

where, a = distance between the image pin and object pin and

d = distance between two positions of lens.

The distance between the two pins should be greater than four times the focal length of the

convex lens, i.e., a > 4f.

Height of the object O = √I1I2

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It can be reduced by using

 lens of large focal lengths

 plano-convex lenses

 crossed lenses

 combining convex and concave lens

(ii) Chromatic Aberration Image of a white object formed by lens is usually coloured and

blurred. This defect of the image produced by lens is called chromatic aberration.

Scattering of Light

When light passes through a medium in which particles are suspended whose size is of the

order of wavelength of light, then light on striking these particles, deviated in different

directions. These phenomena is called scattering of light.

According to the Lord Rayleigh, the intensity of scattered light

I ∝ 1/λ4

Therefore, red colour of light is scattered least and violet colour of light is scattered most.

Daily Life Examples of Scattering of Light

1. Blue colour of sky.
2. Red colour of signals of danger. /
3. Black colour of sky in the absence of atmosp,here
4. Red colour of the time of sun rise and sun set.
5. The human eye is most sentitive to yellow colour

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