A convex lens and a diverging lens are combined and the effective power of the combination is 1.5 D. If the focal length of the convex lens is 50 cm, then the focal length of the diverging lens is ___ cm

The effective power of combination lens = 1.5 D

The effective power of combined lenses= Sum of the power of individual lenses

The focal length of a convex lens= 50 cm= 0.5 m
Power = $\frac{1}{0.5}$= 2 D
Power of combination = 2 D + P = 1.5 D
Power of diverging lens, P₂= 1.5 - 2 = -0.5 D
The focal length of the diverging lens, f₂= $\frac{1}{-0.5 D}$= -2 m= -200 cm

Therefore, the focal length of the diverging lens will be -200 cm.