Magnification By Lenses

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An illuminated object 2 cm tall is placed before a convex lens that produces a real image 6 cm tall on a screen, so what is the magnification?

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Magnification for a lens, as defined in NCERT, is the ratio of image height to object height, $m=\frac{h_i}{h_o}$, and it also equals $\frac{v}{u}$; the sign carries information about orientation. A real image from a single convex lens is always inverted, so its height is taken as negative under the standard convention. Here $h_o=+2$ cm and $h_i=-6$ cm because the image is inverted, giving $m=\frac{-6}{2}=-3$. The negative sign confirms inversion and the magnitude 3 shows the image is three times taller than the object. The value $+3$ is wrong because it ignores the inversion that always accompanies a real image from a convex lens. The value $-0.33$ is wrong because it inverts the ratio, dividing object by image height. The value $+0.33$ compounds both the inversion and reciprocal errors. As stated in NCERT Class 12, Chapter 9, real images formed by convex lenses are inverted, hence the negative magnification. A consistency check confirms that a magnitude greater than one correctly signals an enlarged image, matching the 6 cm versus 2 cm heights.

Magnification By Lenses

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