Energy Levels in Atoms and Line Spectra problem 10

As the light passes through the cloud of cool gas, electrons in the gas atoms interact with the light photons and gain energy. This energy gained causes the electrons to be excited and move to higher energy levels. The photon energy is equal to the difference in energy levels of the electrons. Therefore, when the electrons de-excite and move back to lower energy levels, photons are emitted in all directions, creating a number of dark lines.

Photons of light incident on the light-dependent resistor (LDR) give energy to the electrons in its valence band; these electrons, therefore, become excited and move into the conduction band, leaving holes in the valence band. These electrons and holes are charge carriers, which therefore increase the current in the LDR, hence a decrease in the resistance of the LDR.

So the higher the light intensity, the more the photons, so more electrons in the conduction band. This, therefore, leads to more electrons and holes, therefore higher current in the LDR and hence, lower resistance.