Equivalent focal length of two lenses in contact having power -15D and +5D will be (a) -20 \mathrm{~cm} (b) -10 \mathrm{~cm} (c) +10 \mathrm{~cm} (d) +20 \mathrm{~cm}
Equivalent focal length of two lenses in contact having power -15D and +5D will be (a) -20 \mathrm{~cm} (b) -10 \mathrm{~cm} (c) +10 \mathrm{~cm} (d) +20 \mathrm{~cm}

Answer: Option (b)-10 \mathrm{~cm}