Consider figure for photoemission.
How would you reconcile with momentum-conservation ? Note light (photons) have momentum in a different direction than the emitted electrons.
When incident photon is absorbed completely, its momentum becomes zero. This momentum is gained by atoms of metal and so these atoms get excited. As a result, electrons in them, transit from lower to higher energy levels and along with this, some free electrons get emitted in the form of photoelectrons. Here, total momentum is definitely conserved, irrespective of type of collision.
There are materials which absorb photons of shorter wavelength and emit photons of longer wavelength. Can there be stable substances which absorb photons of larger wavelength and emit light of shorter wavelength.
One day on a spacecraft corresponds to $2$ days on the earth. The speed of the spacecraft relative to the earth is
The spectrum of radiation $1.0 \times {10^{14}}Hz$ is in the infrared region. The energy of one photon of this in joules will be
Light of wavelength $5000\,\,\mathop A\limits^o $ falling on a sensitive surface. If the surface has received $10^{-7}\,J$ of energy, then the number of photons falling on the surface will be