is continuously made to fall on a photocell installed at the doorway. A person entering the door interrupts the beam falling on the photocell. The abrupt change in photocurrent is used to start an electric bell ringing. In fire alarm, a number of photocells are installed at suitable places in a building.
In the event of breaking out of fire, light radiations fall upon the photocell. This completes the electric circuit through an electric bell or a siren which starts operating as a warning signal. Photocells are used in the reproduction of sound in motion pictures and in the television camera for scanning and telecasting scenes. They are used in industries for detecting minor flaws or holes in metal sheets.
A photo cell This wavelength is so small that it is beyond any measurement. This is the reason why macroscopic objects in our daily life do not show wave-like properties. On the other hand, in the sub-atomic domain, the wave character of particles is significant and measurable. Consider an electron (mass m, charge e ) accelerated from rest through a potential V .
The kinetic energy K of the electron equals the work done ( eV ) on it by the electric field: K = e V ( . ) Now , K = m v = p m , so that p = m K m e V ( . ) The de Broglie wavelength # of the electron is then # h h h p m K m eV ( . ) Substituting the numerical values of h, m, e , we get .
nm V ( . ) where V is the magnitude of accelerating potential in volts. For a V accelerating potential, Eq. ( .
) gives # = . nm. This wavelength is of the same order as the spacing between the atomic planes in crystals. This suggests that matter waves associated with an electron could be verified by crystal diffraction experiments analogous to X-ray diffraction.
We describe the experimental verification of the de Broglie hypothesis in the next section. In , de Broglie was awarded the Nobel Prize in