$(a)$ Why is the solar cooker box covered with a plane glass plate?
$(b)$ Why is energy of water flowing in a river considered to be an indirect form of solar energy?
$(c)$ How is the fission of ${}^{235}U$ nucleus brought about?

(N/A) The plane glass plate traps heat inside the box by preventing the escape of infrared radiation,which has a longer wavelength. This phenomenon is known as the greenhouse effect.
$(b)$ Solar energy causes the evaporation of water from oceans and other water bodies to form clouds. These clouds lead to precipitation (rain and snow) in mountainous regions. This water then flows into rivers,making the kinetic energy of river water an indirect form of solar energy.
$(c)$ The fission of ${}^{235}U$ is initiated by bombarding the nucleus with a slow-moving neutron. The ${}^{235}U$ nucleus absorbs this neutron to become ${}^{236}U$,which is highly unstable and immediately undergoes fission into smaller nuclei,releasing energy.