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What is focal plane of a lens?

Important Questions on Ray Optics and Optical Instruments

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A convex lens of focal length

25 cm

produces images of the same magnification

2

, when an object is kept at two positions

x_{1}

and

x_{2}

(x_{1} > x_{2})

from the lens. The ratio of

x_{2}

and

x_{1}

EASY

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

Two thin lenses are in contact and the focal length of the combination is

80 cm

. If the focal length of one lens is

20 cm

, then the power of the other lens will be

MEDIUM

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A planoconvex lens has thickness of

4 cm .

When placed on a horizontal table, with the curved surface in contact with it, the apparent depth of the bottom most point of the lens is found to be

3 cm .

If the lens is inverted such that the plane face is in contact with the table, the apparent depth of the centre of the plane face is found to be

\frac{25}{8} cm .

The focal length of the lens is (assume thickness of lens to be negligible)

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A converging mirror is placed on the right hand side of a converging lens as shown in the figure. The focal length of the mirror and the lens are $20 cm$ and $15 cm$ respectively. The separation between the lens and the mirror is $40 cm$ and their principal axes coincide. A point source is placed on the principal axis at a distance $d$ to the left of the lens. If the final beam comes out parallel to the principal axis, then the value of $d$ is:

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MEDIUM

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

Two identical glass

(μ_{g} = \frac{3}{2})

equiconvex lenses of focal length,

f

are kept in contact. The space between the two lenses is filled with water

(μ_{w} = \frac{4}{3})

. The focal length of the combination is

EASY

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

Where should an object be placed on the axis of a convex lens of focal length

8 cm

so as to achieve magnification of

- 4

? (Distances are measured from optic centre of the lens)

EASY

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A convex lens shown in the figure is made up of two types of transparent materials. A point sources of light is placed on its principal axis. If reflections from the boundaries between layers are ignored, the lens will form
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MEDIUM

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

What focal length should the reading spectacles have for a person for whom the least distance of vision is

50 cm

? (The normal distance of vision is

25 cm

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A diverging lens with magnitude of focal length

25 cm

is placed at a distance of

15 cm

from a converging lens of magnitude of focal length

20 cm

. A beam of parallel light falls on the diverging lens. The final image formed is:

EASY

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

Two similar thin equi-convex lenses of focal length

f

each are kept coaxially in contact with each other, such that the focal length of the combination is

F_{1} .

When the space between the two lenses is filled with glycerin (which has the same refractive index (

μ = 1.5

) as that of glass), then the equivalent focal length is

F_{2} .

The ratio

F_{1} : F_{2}

will be

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A lens forms real and virtual images of an object when the object is at

u_{1}

and

u_{2}

distances respectively. If the size of the virtual image is double that of the real image, then the focal length of the lens is (Take the magnification of the real image as

' m'

)

MEDIUM

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A convex lens of focal length

f

is placed some where in between an object and a screen. The distance between object and screen is

x

. If numerical value of magnification produced by lens is

m

, focal length of lens is

EASY

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

Two convex lenses $A$ & $B$ placed in contact form the image of a distant object at $P .$ If the lens $B$ is moved to the right a little, the image will

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MEDIUM

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A convex lens is in contact with a concave lens. The magnitude of the ratio of their focal length is

\frac{5}{3} .

If their equivalent focal length is

45 cm,

the individual focal lengths of concave and convex lens are respectively.

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A convex lens is put

10 c m

from a light source and it makes a sharp image on a screen, kept

10 c m

from the lens. Now a glass block (refractive index 1.5) of

1.5 c m

thickness is placed in between the light source and the lens. To get the sharp image again, the screen is shifted by a distance

d .

Then

d

is:

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

Sunlight of intensity

1 .3 kW m^{- 2}

is incident normally on a thin convex lens of focal length

20 cm

. Ignore the energy loss of light due to the lens and assume that the lens aperture size is much smaller than its focal length. The average intensity of light, in

kW m^{- 2}

, at a distance

22 cm

from the lens on the other side is __________.

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A thin convex lens

L

(refractive index

= 1.5

) is placed on a plane mirror

M

. When a pin is placed at

A

, such that

O A = 18 c m,

its real inverted image is formed at

A

itself, as shown in figure. When liquid of refractive index

μ_{l}

is put between the lens and the mirror, the pin has to be moved to

A^{'},

such that

O A^{'} = 27 c m,

to get its inverted real image at

A'

itself. The value of

μ_{l}

will be

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A thin convex lens of focal length

f

is put on a plane mirror as shown in the figure. When an object is kept at a distance

a

from the lens-mirror combination, its image is formed at a distance

\frac{a}{3}

in front of the combination. The value of

a

is:

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

A plano-convex lens is made of a material of refractive index n. When a small object is placed 30 cm away in front of the curved surface of the lens, an image of double the size of the object is produced. Due to reflection from the convex surface of the lens, another faint image is observed at a distance of 10 cm away from the lens. Which of the following statement(s) is (are) true?

HARD

Physics>Optics>Ray Optics and Optical Instruments>Refraction at Spherical Surfaces and by Lenses

Consider a system of thin lenses and mirror, as shown in the diagram. Refractive indices of the media are given as

μ_{1}, μ_{2}

and

μ_{3}

and the radii of curvature of interfaces are given as

R_{1}, R_{2} and R_{3}

, then, the magnitude of focal length of the system is(take

μ_{1} = μ, μ_{2} = 2 μ, μ_{3} = 3 μ

and

R_{1} = R, R_{2} = R_{3} = 2 R

)

What is focal plane of a lens?

Important Questions on Ray Optics and Optical Instruments

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