(C) The focal length of a lens is given by the Lens Maker's Formula: $\frac{1}{f} = (\frac{n_l}{n_m} - 1) (\frac{1}{R_1} - \frac{1}{R_2})$.For a conca

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convex lens of focal length $3.5 R$

(C) The focal length of a lens is given by the Lens Maker's Formula: $\frac{1}{f} = (\frac{n_l}{n_m} - 1) (\frac{1}{R_1} - \frac{1}{R_2})$.For a concave lens,$R_1 = -R$ and $R_2 = +R$.Substituting these values: $\frac{1}{f} = (\frac{1.5}{1.75} - 1) (\frac{1}{-R} - \frac{1}{R})$.$\frac{1}{f} = (\frac{6}{7} - 1) (-\frac{2}{R}) = (-\frac{1}{7}) (-\frac{2}{R}) = \frac{2}{7R}$.Therefore,$f = 3.5 R$.Since the focal length $f$ is positive,the lens acts as a convex lens.

Class 12 Physics Ray Optics and Optical Instruments Refraction by Lenses

Medium

$A$ concave lens (refractive index $n_l = 1.5$) has both surfaces of the same radius of curvature $R$. If it is immersed in a liquid of refractive index $n_m = 1.75$,it will act as a:

MHT CET 2025 All MHT CET

A
concave lens of focal length $3.5 R$
B
concave lens of focal length $2 R$
C
convex lens of focal length $3.5 R$
D
convex lens of focal length $2 R$

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Ray Optics and Optical Instruments

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Refraction by Lenses

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Find the magnification for the lens. The focal length is $25 \; cm$.

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In the displacement method,there are two positions of a lens for which we get a clear image on a screen. If the first position of the lens is at $40 \, cm$ from the object and the second position is at $80 \, cm$ from the object,the focal length of the lens is ......... $cm$.

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The converging or diverging ability of a lens or mirror is called

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If a lens is moved towards the object from a distance of $40 \,cm$ to $30 \,cm$,the magnification of the image remains the same (numerically). The focal length of the lens is ......... $cm$.

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For a convex lens,the distance of the object is taken on the $X$-axis and the distance of the image is taken on the $Y$-axis. The nature of the graph so obtained is:

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$A$ concave lens (refractive index $n_l = 1.5$) has both surfaces of the same radius of curvature $R$. If it is immersed in a liquid of refractive index $n_m = 1.75$,it will act as a:

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Find the magnification for the lens. The focal length is $25 \; cm$.

In the displacement method,there are two positions of a lens for which we get a clear image on a screen. If the first position of the lens is at $40 \, cm$ from the object and the second position is at $80 \, cm$ from the object,the focal length of the lens is ......... $cm$.

The converging or diverging ability of a lens or mirror is called

If a lens is moved towards the object from a distance of $40 \,cm$ to $30 \,cm$,the magnification of the image remains the same (numerically). The focal length of the lens is ......... $cm$.

For a convex lens,the distance of the object is taken on the $X$-axis and the distance of the image is taken on the $Y$-axis. The nature of the graph so obtained is:

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