I am assuming you know the relation obtained between slit width, distance of screen from slits, distance of interference pattern obtained on the screen from the center and the wavelength of monochromatic light used in Young's Double Slit experiment.
λ =
![\frac{y*d}{D} = \frac{3.55*10^{-3}*1.25*10^{-4} }{2.25} = 1.97*10^{-7} m](https://tex.z-dn.net/?f=%20%5Cfrac%7By%2Ad%7D%7BD%7D%20%3D%20%20%5Cfrac%7B3.55%2A10%5E%7B-3%7D%2A1.25%2A10%5E%7B-4%7D%20%20%7D%7B2.25%7D%20%3D%201.97%2A10%5E%7B-7%7D%20m)
λ ~ 1.97 ×10⁻⁷m
It’s around the g force so it’s gonna be around 54 km/h
The first one. The E and B chatacteristic are perpendicular to eachother. The direction of the wave can be found by the right hand rule.