(a) (i) An object of 5 cm height is placed at a distance of 20 cm from the optical centre of a concave lens…
(a) (i) An object of 5 cm height is placed at a distance of 20 cm from the optical centre of a concave lens of focal length 18 cm. Calculate (1) image distance and (2) the magnification in this case.
(ii) Compare the values of magnification obtained by a concave lens and a convex lens when both the lenses form virtual images.
Solution
(a) Given,
Height of object (h) = 5 cm
Object distance (u) = -20 cm
Focal length (f) = -18 cm
Image distance (v) = ?
(i)
(1) $\dfrac{1}{v}$ - $\dfrac{1}{u}$ = $\dfrac{1}{f} \dfrac{1}{v}$ = $\dfrac{1}{u}$ + $\dfrac{1}{f}$ = 1/-20 + 1/-18 = -[(18+20)/360]
= -38/360
→ v = -360/38 = -9.47 cm
(2) m = v/u = -9.47/-20 = 0.47
(ii) For convex lens : m > 1, for concave lens m < 1