(c) $y = a\sin{\frac{2\pi}{T}t \Rightarrow \frac{a}{\sqrt{2}} = a\sin{\frac{2\pi}{T}.t}}$ $\Rightarrow \sin{\frac{2\pi}{T}t = \frac{1}{\sqrt{2}} = \sin\frac{\pi}{4} \Rightarrow \frac{2\pi}{T}t = \frac{\pi}{4} \Rightarrow t = \frac{T}{8}}$
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.