光電効果
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Q1: What is the photoelectric effect and why does it occur?
The photoelectric effect is the emission of electrons from a metal when light shines on it. Individual photons absorbed by electrons in the metal transfer energy to them. If this energy exceeds the work function—the minimum energy needed to liberate an electron from the metal—the electrons are ejected as photoelectrons. This process demonstrates the quantum nature of light.
Q2: Why does only ultraviolet light cause photoelectric effect in zinc, not visible light?
Zinc has a work function of 4.3 electron-volts, corresponding to a threshold frequency of 10^15 Hertz and threshold wavelength of 300 nanometers in the ultraviolet range. Visible light photons lack sufficient energy to exceed this threshold, while UV light photons possess the required energy to eject electrons from zinc's surface.
Q3: How is photon energy related to light frequency in the photoelectric effect?
Photon energy is directly proportional to light frequency, expressed by the equation E = hf, where h is Planck's constant and f is frequency. This relationship explains why threshold frequency exists: only photons with frequency high enough to produce energy exceeding the work function can eject electrons from a metal.
Q4: What role does the electroscope play in demonstrating the photoelectric effect?
An electroscope detects and measures the presence and relative amount of electric charge on the zinc plate. When UV light ejects electrons from a negatively charged zinc plate, the plate discharges and the electroscope needle collapses. This visible response confirms electron emission and demonstrates that only high-frequency light can cause photoelectric discharge.
Q5: How do night vision devices use the photoelectric effect?
Night vision devices use photoelectric effect principles where photons strike a thin film of alkali metal or semiconductor material, ejecting photoelectrons. These electrons are accelerated by an electrostatic field, multiplied through secondary emissions, and directed at a phosphor-coated screen. The screen converts electrons back into photons, forming visible images in near-total darkness.
Q6: Why must the zinc plate surface be polished with sandpaper before the experiment?
Polishing with sandpaper removes zinc oxide from the metal surface, which is essential for the experiment's success. The oxide layer would block direct contact between light photons and free electrons in the zinc, preventing efficient photoelectric effect and making electron transfer difficult.
Q7: What is a photosensitive electrical switch and how does it apply the photoelectric effect?
A photosensitive electrical switch is an optoelectronic device that uses the photoelectric effect to control electrical current. Blocking or unblocking a light beam on a metal surface turns the electrical current OFF or ON by controlling the presence or absence of photoelectrons, providing a practical application of photoelectric principles.