With gradual increase in frequency of an a.c. supply, the impedance of an LCR series circuit

An alternating e.m.f. is given by $e = e_0 \sin\omega t$. In what time the e.m.f. will have half its maximum value, if 'e' Starts from zero?

A step down transformer has turns ratio 20:1. If 8V are applied across $0.4\ \Omega$ secondary then the primary current will be

A simple microscope is used to see the object first in blue light and then in red light. Due to the change from blue to red light, what is the effect on its magnifying power?