The source of electromagnetic wave can be a charge

A light beam of intensity $$20 \mathrm{~W} / \mathrm{cm}^2$$ is incident normally on a perfectly reflecting surface of sides $$25 \mathrm{~cm} \times 15 \mathrm{~cm}$$. The momentum imparted to the surface by the light per second is

An antenna uses electromagnetic waves of frequency $$5 \mathrm{~MHz}$$. For proper working, the size of the antenna should be

Due to Doppler's effect the shift in wavelength observed is $$0.1 \mathop A\limits^o$$ for a star producing wavelength $$6000 \mathop A\limits^o$$. Velocity of recession of the star will be