(B) The power $P$ of a lens is given by $P = \frac{100}{f(cm)}\, D$. For the convex lens,the focal length $f_1 = +40\, cm$. Therefore,the power $P_1 =

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(B) The power $P$ of a lens is given by $P = \frac{100}{f(cm)}\, D$. For the convex lens,the focal length $f_1 = +40\, cm$. Therefore,the power $P_1 = \frac{100}{40} = +2.5\, D$. For the concave lens,the focal length $f_2 = -25\, cm$. Therefore,the power $P_2 = \frac{100}{-25} = -4.0\, D$. The power of the combination of lenses in contact is given by $P = P_1 + P_2$. $P = 2.5\, D + (-4.0\, D) = -1.5\, D$.

Class 12 Physics Ray Optics and Optical Instruments Combination of Lenses

Medium

An eye specialist prescribes spectacles having a combination of a convex lens of focal length $40\, cm$ in contact with a concave lens of focal length $25\, cm$. The power of this lens combination in diopters is

IIT 1997 All IIT

IIT 1982 All IIT

A
$+ 1.5$
B
$- 1.5$
C
$+ 6.67$
D
$- 6.67$

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Ray Optics and Optical Instruments

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Combination of Lenses

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Two convex lenses of focal length $20\,cm$ each are placed coaxially with a separation of $60\,cm$ between them. The image of the distant object formed by the combination is at $...........\,cm$ from the first lens.

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If two $+5D$ lenses are mounted at some distance apart,the equivalent power will always be negative if the distance is

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Two thin lenses have a combined power of $+9 \text{ D}$. When they are separated by a distance of $20 \text{ cm}$,their equivalent power becomes $+\frac{27}{5} \text{ D}$. The power of both the lenses in dioptre are respectively:

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An eye specialist prescribes spectacles having a combination of a convex lens of focal length $40\, cm$ in contact with a concave lens of focal length $25\, cm$. The power of this lens combination in diopters is

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In the figure shown here,what is the equivalent focal length (in $cm$) of the combination of lenses (Assume that all layers are thin)?

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