A biconvex lens of refractive index \( 1.5 \) has a focal length of \( 20 \, \text{cm} \) in air. Its focal length when immersed in a liquid of refractive index \( 1.6 \) will be:
\( +16 \, \text{cm} \)
\( -16 \, \text{cm} \)
\( -160 \, \text{cm} \)
\( +160 \, \text{cm} \)