Which of the following wave has the largest wave speed?

What is the refractive index of the material of a double convex lens having radii of curvature of 5 cm and 10 cm and focal length of $\frac{20}{3} \mathrm{~cm}$

In an interference pattern of Young's double slit experiment, at a point we observe the 12 th order maximum for a monochromatic light source with wavelength $6000 \mathop {\rm{A}}\limits^{\rm{o}} $. What order will be visible here, if the source is replaced by a light of wavelength $4800 \mathop {\rm{A}}\limits^{\rm{o}} $ ?

Two charges are $+10 \mu \mathrm{C}$ and $-10 \mu \mathrm{C}$ are separated by 10 cm . The magnitude of force acting on another charge $5 \mu \mathrm{C}$ placed at the mid-point of the line joining the two charges will be (use, $\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9$ in SI unit)