Most Asked Questions
12 min read

Ray Optics And Optical Instruments

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1
JEE Mains

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.

2

Image formation for lenses with one side silvered

A plano convex lens of focal length has its plane surface silvered. An object is placed from the lens on the convex side. The distance of the image from the lens is:

or,
or,
or,
Now, this image acts as the object for the lens.
So,

or,
or,
One side of the bi-concave lens is silvered. Now it will behave like
A
Convex mirror of
B
Concave mirror of
C
Convex mirror of focal length
D
Concave mirror of focal length
3

Angle of deviation

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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 ,
4

combination of lenses

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Problem:
A convex lens, of focal length , a concave lens of focal length , and a plane mirror are arranged as shown. For an object kept at a distance of from the convex lens, the final image, formed by the combination, is a real image, at a distance of :
Solution:
Location of image formation after first refraction from the lens,.
This gives
Next refraction from concave lens,
Hence
Since this will be formed behind the mirror, this will be final image.It is at a distance of from the concave lens.
An upright object is placed at a distance of in front of a convergent lens of focal length . A convergent mirror of focal length is placed at a distance of on the other side of the lens. The position and size of the final image will be
A
from the convergent mirror, same size as the object
B
from the convergent mirror, same size as the object
C
from the convergent mirror, twice the size as the object
D
from the convergent mirror, twice the size as the object
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JEE Advanced

Apparent depth and real depth

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Real Depth is actual distance of an object beneath the surface, as would be measured by submerging a perfect ruler along with it.

Apparent depth in a medium is the depth of an object in a denser medium as seen from the rarer medium. Its value is smaller than the real depth.
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Transmission of light from a denser medium to a rarer medium at different angles of incidence

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When a ray of light enters from a denser medium to a rarer medium,it bends away from the normal, for example, the ray .The incident ray is partially reflected () and partially transmitted() or refracted, the angle of refraction (r ) being larger than the angle of incidence (i ). As the angle of incidence increases, so does the angle of refraction, till for the ray , the angle of refraction is .The refracted ray is bent so much away from the normal that it grazes the surface at the interface between the two media. This is shown by the ray If the angle of incidence is increased still further (e.g.,the ray ), refraction is not possible, and the incident ray is totally reflected.
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Problems on Total internal reflection in Prism

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Example: At what angle should a ray of light be incident on the face of a prism of refracting angle so that it just suffers total internal reflection at the other face. Find the refractive index of the prism.Refractive index of prism is 1.524.

Solution:

A ray monochromatic light is incident on the face of prism near vertex at an incident angle of (see figure). If the refractive index of the material of the prism is , which of the following is (are) correct?
This question has multiple correct options
A
The ray gets totally internally reflected at face CD
B
The ray comes out through face AD
C
The angle between the incident ray and the emergent ray is 90
D
The angle between the incident ray and the emergent ray is 120
8

Speed of object and speed of image

Suppose while sitting in a parked car, you notice a jogger approaching towards you in the side view mirror of . If the jogger is running at a speed of , how fast the image of the jogger appear to move when the jogger is away?
Solution:
From the mirror equation, we get:

For convex mirror, since , then for ,
Since the jogger moves at a constant speed of , after , the position of the image (for ) is .
The shift in the position of image in is

Therefore, the average speed of the image when the jogger is between and from the mirror, is .
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10

Lens Maker's Formula

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Consider a convex lens (or concave lens) of absolute refractive index  to be placed in a rarer medium of absolute refractive index . Considering the refraction of a point object on the surface , the image is formed at  who is at a distance of .

 (as the lens is thin)

It follows from the refraction due to convex spherical surface

The refracted ray from A suffers a second refraction on the surface and emerges along BI. Therefore I is the final real image of O.

Here the object distance is

is very small

Let

(Final image distance)

Let  be radius of curvature of second surface of the lens. It follows from refraction due to concave spherical surface from denser to rarer medium that

Adding

or

But and

Thus we get=


11

Combination of thin lenses in contact

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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)
Adding equations (1) and (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.
12

combination of lenses

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Problem:
A convex lens, of focal length , a concave lens of focal length , and a plane mirror are arranged as shown. For an object kept at a distance of from the convex lens, the final image, formed by the combination, is a real image, at a distance of :
Solution:
Location of image formation after first refraction from the lens,.
This gives
Next refraction from concave lens,
Hence
Since this will be formed behind the mirror, this will be final image.It is at a distance of from the concave lens.
A biconvex lens of focal length 15 cm is in front of a plane mirror. The distance between the lens and the mirror is 10 cm. A small object is kept at a distance of 30 cm from the lens. The final image is :
A
virtual and at a distance of 16 cm from the mirror
B
real and at a distance of 16 cm from the mirror
C
virtual and at a distance of 20 cm from the mirror
D
real and at a distance of 20 cm from the mirror