Physics · Dual Nature Of Radiation And Matter · NEET
The glass/quartz tube is evacuated so that emitted electrons travel freely from plate C to plate A. If air were present, the electrons would collide with gas molecules, lose energy, or get absorbed, and the ammeter would read a wrong or zero current. A vacuum keeps the current a clean measure of how many photoelectrons leave the emitter.
Ordinary glass blocks most ultraviolet (UV) light. Since many metals need UV to release electrons, a transparent quartz window is sealed on the tube because quartz lets UV pass through. So the quartz window is there to let short-wavelength (UV) light reach the emitter plate C.
Plate C is the emitter (also called the cathode). It is the thin photosensitive plate that the light hits, so it emits the photoelectrons. Plate A is the collector (anode) that catches them. Easy way to remember: C = Cathode = Comes out; A = Anode = Attracts/catches.
A commutator is a switch that can reverse the polarity of the plates. It lets you make collector A positive with respect to C (to attract and collect electrons) or negative with respect to C (to repel them). This is exactly how the stopping potential is measured — you make A negative until even the fastest electrons are turned back.
Monochromatic means light of a single frequency. Using one fixed frequency lets you study how frequency and intensity each affect the current separately, without mixing effects. Short wavelength (high frequency) is needed because only light above the threshold frequency can eject electrons at all.
The microammeter in the circuit measures the very small photoelectric current, which tells you the number of electrons reaching A per second. The voltmeter measures the potential difference between C and A, which is varied using the battery. Plotting current against this voltage gives the key photoelectric graphs.
Photons with energy 5 eV are incident on a cathode C in a photoelectric cell. The maximum energy of emitted photoelectrons is 2 eV. When photons of energy 6 eV are incident on C, no photoelectrons will reach the anode A if the stopping potential of A relative to C is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An evacuated glass or quartz tube, a photosensitive emitter plate C, a metal collector plate A, a transparent quartz window, a monochromatic light source, a battery with a commutator to set the plate polarity, a voltmeter to read the plate voltage, and a microammeter to read the tiny photoelectric current.
The emitter (plate C, the cathode) is the light-sensitive plate that gives out electrons when light of high enough frequency falls on it. The collector (plate A, the anode) is the plate that catches those electrons when it is kept positive, completing the circuit.
The intensity is varied by moving the light source closer to or farther from the emitter plate C. Bringing the source closer increases intensity; moving it away decreases intensity, without changing the frequency.
Only the electrons that both absorb a photon and escape the surface, then reach the collector, form the current. This is a small fraction of all electrons, so the current is only microamperes and needs a sensitive microammeter.
Yes. Using the commutator to make collector A negative with respect to C and slowly increasing this reverse voltage until the current just becomes zero gives the stopping potential, which measures the maximum kinetic energy of the photoelectrons.
