The focal length of a thin lens made from a material of refractive index 1.5 is 15 cm. When it is placed in a liquid of refractive index 4/3, its focal length will be cm.

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(4) 60. Given: n_g=1.5, f=15 cm in air, liquid of nₗ=4/3. Idea: the same piece of glass keeps its shape factor (1/(R₁)-1/(R₂)); only the bracket ((n_g)/(n_medium)-1) changes. In air: 1/(15)=(1.5-1)(1/(R₁)-1/(R₂)), so the shape factor is 2/(15) cm⁻¹. In the liquid: (n_g)/(nₗ)=(1.5)/(4/3)=1.125, so the bracket is 0.125. 1/(f')=0.125×2/(15)=1/(60) f'=60 cm — four times weaker, because the liquid is nearly as dense as the glass.

The focal length of a thin lens made from a material of refractive index 1.5 is 15 cm. When it is placed in a liquid of refractive index 4/3, its focal length will be ________ cm.

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