The property of light that explains the formation of coloured images due to thick lenses is

For an aperture of $5 \times 10^{-3} \mathrm{~m}$ and a monochromatic light of wavelength $\lambda$, the distance for which ray optics becomes a good approximation is 50 m , then $\lambda=$

An electron and a positron enter a uniform electric field $E$ perpendicular to it with equal speeds at the same time. The distance of separation between them in the direction of the field after a time ' $t$ ' is

( $\frac{e}{m}$ is specific charge of electron)

A charge $q$ is placed at the centre ' $O$ ' of a circle of radius $R$ and two other charges $q$ and $q$ are placed at the ends of the diameter $A B$ of the circle. The work done to move the charge at point $B$ along the circumference of the circle to a point $C$ as shown in the figure is