Physics · Dual Nature Of Radiation And Matter · NEET
In 1887 Hertz was studying sparks in his electromagnetic wave setup. He noticed that the spark across the detector gap jumped more easily when the metal plate was hit by ultraviolet (UV) light from an arc lamp. The UV light helped electrons escape the metal, so a spark formed at a lower voltage. Hertz only reported the effect; he did not explain it.
Hallwachs (1888) connected a zinc plate to an electroscope. A negatively charged zinc plate lost its charge under UV light. An uncharged plate became positive, and a positive plate became even more positive. This only makes sense if UV light throws out negative particles (electrons) from the metal. So the plate is left with less negative or more positive charge.
Hallwachs watched charge change on a single plate with an electroscope. Lenard (1902) used two electrodes in an evacuated tube. When UV light fell on the emitter plate C, a current flowed to collector plate A. When the light stopped, the current stopped at once. Lenard's setup measured current in a circuit; Hallwachs measured charge on a plate.
The current comes only from electrons that UV light knocks out of the emitter plate. No light means no electrons are ejected, so no charge crosses the tube and the current drops to zero immediately. This shows the emission is directly caused by the light, not stored or delayed.
No. Hertz, Hallwachs and Lenard only observed and recorded the facts. They found that below a certain minimum frequency (threshold frequency) no electrons come out, no matter how bright the light is. The explanation came later from Einstein's photon idea. This is a common NEET point: these three observed, Einstein explained.
Hallwachs and Lenard found that if the frequency of the incident light is below a certain minimum value, no electrons are emitted at all, even with very bright light. This minimum is the threshold frequency, and it depends on the metal. Above it, electrons come out. This early clue could not be explained by wave theory.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Heinrich Hertz first observed it in 1887 while studying electromagnetic waves. He saw UV light made sparks jump more easily, but he did not explain the cause.
In 1888 Hallwachs used a zinc plate and electroscope to show that UV light makes a metal lose negative charge (or gain positive charge), proving negative particles are emitted.
Philipp Lenard showed that UV light on the emitter plate causes a current to flow between two electrodes in a vacuum tube, and the current stops the instant the light is stopped.
It is the minimum frequency of incident light needed to eject electrons. Below it, no electrons are emitted even with bright light. It depends on the metal used.
NEET often asks who observed versus who explained the photoelectric effect, and about the threshold frequency clue. These qualitative facts appear in match-the-column and assertion-reason questions.
