## Ray Optics And Optical Instruments

- Get to know usually which areas are frequently asked in the CBSE exam
1

## Angle of deviation

Angle of deviation () is the angle between emergent ray and incident ray.
For a single refracting surface,
For a prism,
where is the angle of the prism.
For angle of minimum deviation, is minimum and

For small ,
A ray of light passes through an equilateral glass prism, such that the angle of incidence is equal to the angle of emergence. If the angle of emergence is times the angle of prism. The refractive index of the glass prism is:
A
1.5
B
1.414
C
1.616
D
1.7
2

## Refractive Index of Prism using the angle of minimum deviation

Example: If the angle of minimum deviation produced by an equilateral prism is 30 , then find the refractive index of material of prism.

Solution:
given,

=
The angle of minimum deviation produced by a prism is . Calculate the refractive index of the material of the prism.
A
1.33
B
1.53
C
1.63
D
1.44
For prism of refractive index , the angle of minimum deviation is equal to the angle of the prism. The angle of the prism is:
A
B
C
D
3

## Limit of resolution of optical instruments

In determining the limit of resolution of optical instruments like a telescope or a microscope, for the two stars to be just resolved,

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

The resolving power of telescope of aperture for light of wavelength is ...............
A
B
C
D
4

## Combination of thin lenses in contact

Consider two lenses A and B of focal length and placed in contact with each other.
An object is placed at O on  the common principal axis. The lens A produces an image at and this image acts as the object for the second lens B. The final image is produced at as shown in figure.
PO = u, object distance for the first lens (A),
PI = v, final image distance and
, image distance for the first lens (A) and also object distance for second lens (B).
For the image produced by the first lens A,
.... (1)
For the final image I, produced by the second lens B,
... (2)
... (3)
If the combination is replaced by a single lens of focal length F such that it forms the image of O at the same position I, then
... (4)
From equations (3) and (4),
...... (5)
This F is the focal length of the equivalent lens for the combination.
Three lenses in contact have a combined focal length of . When the third lens is removed the combined focal length becomes . The third lens is :
A
a converging lens of focal length 30 cm
B
a converging lens of focal length 60 cm
C
a diverging lens of focal length 30 cm
D
a diverging lens of focal length 60 cm