Optics — Physics STD 11 Science — Question

ii. The linear magnification is the ratio of the size of the image to the size of the object.

iii. When the distances of the object and image formed are very large as compared to the focal lengths of the instruments used, the magnification becomes infinite. Whereas, the magnifying power being the ratio of angle subtended by the object and image, gives the finite value.

iv. For example, in case of a compound microscope,
$M _{\min }=\frac{D}{f}=\frac{25}{5}=5$ and $M _{\max }=1+\frac{D}{f}=6$
Hence image appears to be only 5 to 6 times bigger for a lens of focal length $5 cm$.
For $M _{\min }=\frac{D}{f}=5, V =\infty$
$\therefore$ Lateral magnification $(m)=\frac{v}{u}=\infty$
Thus, the image size is infinite times that of the object, but appears only 5 times bigger.