(C) The focal length of a convex lens is $f_1 = +20 \ cm$ and the focal length of a concave lens is $f_2 = -20 \ cm$.When placed in contact,the equiva

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(C) The focal length of a convex lens is $f_1 = +20 \ cm$ and the focal length of a concave lens is $f_2 = -20 \ cm$.When placed in contact,the equivalent focal length $F$ is given by $\frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2}$.Substituting the values: $\frac{1}{F} = \frac{1}{20} + \frac{1}{-20} = 0$,which implies $F = \infty$.$A$ combination with infinite focal length acts as a plane glass plate.$A$ plane glass plate forms a virtual,erect image of the same size as the object at the same distance behind the plate.

Class 12 Physics Ray Optics and Optical Instruments Combination of Lenses

Medium

$A$ concave lens and a convex lens have the same focal length of $20 \ cm$ and are placed in contact. This combination is used to view an object $5 \ cm$ long kept at $20 \ cm$ from the lens combination. As compared to the object,the image will be:

A
Magnified and inverted
B
Reduced and erect
C
Of the same size and erect
D
Of the same size and inverted

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Class 12 Questions

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Ray Optics and Optical Instruments

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Combination of Lenses

$A$ lens maker has to design an achromatic combination of effective power $-0.5 \, D$. The two types of glasses to be used (say $A$ and $B$) have dispersive powers in the ratio $\omega_A : \omega_B = 3 : 5$. Then the lens made from glass $A$ will have a focal length of ....... $cm$.

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Two lenses of power $+12 \ D$ and $-2 \ D$ are placed in contact. The combined focal length of the combination will be ....... $cm$.

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$A$ convex lens of focal length $30 \ cm$ is placed in contact with a concave lens of focal length $20 \ cm$. An object is placed at $20 \ cm$ to the left of this lens system. The distance of the image from the lens in $cm$ is . . . . . .

DifficultJEE MAIN 2025

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Two thin convex lenses of focal lengths $20 \; cm$ and $25 \; cm$ are placed in contact. The effective power of the combination is:

Medium

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An object is located $4 \ m$ from the first of two thin converging lenses of focal lengths $2 \ m$ and $1 \ m,$ respectively. The lenses are separated by $3 \ m$. The final image formed by the second lens is located from the source at a distance of (in $m$):

DifficultWBJEE 2015

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$A$ concave lens and a convex lens have the same focal length of $20 \ cm$ and are placed in contact. This combination is used to view an object $5 \ cm$ long kept at $20 \ cm$ from the lens combination. As compared to the object,the image will be:

Similar Questions

$A$ lens maker has to design an achromatic combination of effective power $-0.5 \, D$. The two types of glasses to be used (say $A$ and $B$) have dispersive powers in the ratio $\omega_A : \omega_B = 3 : 5$. Then the lens made from glass $A$ will have a focal length of ....... $cm$.

Two lenses of power $+12 \ D$ and $-2 \ D$ are placed in contact. The combined focal length of the combination will be ....... $cm$.

$A$ convex lens of focal length $30 \ cm$ is placed in contact with a concave lens of focal length $20 \ cm$. An object is placed at $20 \ cm$ to the left of this lens system. The distance of the image from the lens in $cm$ is . . . . . .

Two thin convex lenses of focal lengths $20 \; cm$ and $25 \; cm$ are placed in contact. The effective power of the combination is:

An object is located $4 \ m$ from the first of two thin converging lenses of focal lengths $2 \ m$ and $1 \ m,$ respectively. The lenses are separated by $3 \ m$. The final image formed by the second lens is located from the source at a distance of (in $m$):

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