In a region, steady and uniform electric and magnetic fields are present. These two fields are parallel to each other. A charged particle is released from rest in this region. The path of the particle will be a 

Figure shows a convex lens of focal length $10 \mathrm{cm}$ lying in a uniform magnetic field $B$ of magnitude $1.2 \mathrm{T}$ parallel to its principal axis. A particle having a charge $2.0\times {10}^{-3}\mathrm{C}$ and mass $2.0\times {10}^{-5} \mathrm{kg}$ is projected perpendicular to the plane of the diagram with a speed of $4.8 {\mathrm{ms}}^{-1}$. The particle moves along a circle with its centre on the principal axis at a distance of $15 \mathrm{cm}$ from the lens. The axis of the lens and of the circle are same. Show that the image of the particle goes along a circle and find the radius of that circle.

The distance between the plates of a parallel plate condenser is $4 \mathrm{mm}$ and potential difference between them is $200 \mathrm{V}.$ The condenser is placed in a magnetic field $B.$ An electron is projected vertically upwards parallel to the plates with a velocity of ${10}^6 \mathrm{m}/\mathrm{s}.$ The electron passes undeviated through the space between the plates. The magnitude and direction of magnetic field $B$ will be -