A cube of side length 1 mm is placed at distance 30 cm from the pole of a concave mirror of focal length 20 cm. Its one side is parallel optical axis as shown in the figure. Find the magnification in the volume of cube

Here, u=-30cm;   f= -20cm;∵1v+1u=1f ⇒1v−130=−120 ⇒1v=130−120=2−360 ∴v=−60 cm∴Transverse magnification is m=−vu=−2And longitudinal magnification is m1=−v2u2=−4The magnification in sides parallel to optical axis is m1 and the magnification in sides perpendicular to optical axis is m.∴l'l=−v2u2         And l'=−4l=−4 mmAnd b'b=m=−2b'=−2×1=−2 mm∴Volume of image = l'×b'×b'                                       =4×22=16 mm3                                                 Thus, the magnification in the volume is 16.