When a wave enters into a rarer medium from a denser medium, the property of the wave which remains constant is

The focal length of the objective lens of a telescope is 30 cm and that of its eye lens is 3 cm . It is focussed on a scale at a distance 2 m from it. The distance of objective lens from eye lens to see the clear image is

In case of diffraction, if $a$ is a slit width and $\lambda$ is the wavelength of the incident light, then the required condition for diffraction to take place is

The electric field intensity $E$ at a distance of 3 m from a uniform long straight wire of linear charge density $0.2 \mu \mathrm{~cm}^{-1}$ is