A double convex lens of focal length 20 cm is made of glass of refractive index 3/2. When placed completely in water $({_aμ}_w= 4 / 3)$, its focal length will be

80 cm 15 cm 17.7 cm 22.5 cm

80 cm

$\frac{f_w}{f_a}=\frac{(μ_g-1)}{\frac{μ_g}{μ_w}-1}⇒\frac{f_w}{f_a}=4$ $∴f_w=4×f_a=4×20=80cm$