Two lenses of power $-15\ D$ and $+5\ D$ are in contact with each other. The focal length of the combination is

একটি  লেন্সের ক্ষমতা 1  dioptre  হলে  এর ফোকাস দৈর্ঘ্য কত?

নিচের কোনটি একটি উত্তল লেন্সের জন্য u ~ v লেখচিত্র নির্দেশ করে।

A concave mirror has a focal length of 25 cm. At which of the following distance should a person hold his face from this concave mirror so that it may act as a shaving mirror ?

An angular magnification ( magnifying power ) of 30X is desired using an object 1.25 cm and an eyepiece of focal length 5 cm. If compound microscope is to be set up then the separation between the objective lens and the eyepiece.