Photo Cell

The PHOTOCELL is a practical device that puts the photoelectric effect to direct use, converting incident light energy into an electrical (current) signal. Its construction differs slightly from the flat-plate laboratory set-up of Fig. 14.2: inside an evacuated glass or quartz bulb sits a semi-cylindrical photosensitive metal plate E, acting as the cathode, together with a wire LOOP collector C acting as the anode, positioned roughly along the axis of the semi-cylinder. (In some designs, the photosensitive coating is instead applied as a thin film directly on the inner wall of the glass bulb, rather than as a separate curved plate.) The electrodes connect to an external circuit containing a high-tension battery B (supplying the accelerating potential) and a sensitive microammeter $\mu A$ to register the resulting current.

When light of a wavelength the cathode material responds to falls on E, photoelectrons are emitted and drawn across the gap to the anode C by the applied electric field; the resulting photocurrent, read off the microammeter, directly reflects whether (and, within limits, how much) light is falling on the device. …