What is the Photoelectric Effect
In a metal, free electrons move about easily but cannot normally escape the surface, because any electron attempting to leave is pulled back by the positive ions left behind. To break free, an electron must be supplied a minimum amount of energy — the work function of the metal. One of the three physical processes that can supply this energy (alongside thermionic and field emission) is photoelectric emission: when light of a suitable frequency illuminates a metal surface, electrons are ejected from it.
These light-generated electrons are called photoelectrons, and the phenomenon is the photoelectric effect. NCERT §11.3 develops the topic as a discovery story across three contributors — Hertz, Hallwachs and Lenard — before Einstein later supplied the explanation. NIOS §25.1 frames the same definition compactly: photoelectric effect is the emission of electrons from metals irradiated by light of a frequency greater than a certain characteristic frequency.
| Process | Energy source | Mechanism |
|---|---|---|
| Thermionic emission | Heat | Heating imparts thermal energy to free electrons so they can leave the surface. |
| Field emission | Electric field | A very strong field ($\sim 10^{8}\ \text{V m}^{-1}$) pulls electrons out, as in a spark plug. |
| Photoelectric emission | Light | Light of suitable frequency frees electrons; the focus of this page. |
Hertz's Observations (1887)
The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz (1857–1894) — and notably, not while he was hunting for it. Hertz was investigating the production of electromagnetic waves by means of a spark discharge. In that apparatus a high voltage is applied across a small air gap so that a spark jumps across it; a separate detector loop with its own gap registers the radiated wave by a tiny spark of its own.
Hertz observed that the high-voltage sparks across the detector loop were enhanced when the emitter plate was illuminated by ultraviolet light from an arc lamp. In other words, shining UV light on the metal somehow made it easier for charge to escape the surface and bridge the gap. NIOS §25.1 records the same effect in different words: air in the spark gap became a better conductor when illuminated by ultraviolet rays.
UV light incident on the emitter plate frees charges, making the spark across the gap stronger — Hertz's original 1887 clue.
Hertz did not yet identify what was leaving the surface. As NCERT puts it, light shining on the metal somehow facilitated the escape of free charged particles which we now know as electrons. Some electrons near the surface absorb enough energy from the incident radiation to overcome the attraction of the positive ions, and escape into the surrounding space. The identification of these particles as electrons had to wait until J. J. Thomson's discovery of the electron in 1897.
Hertz discovered the effect, not the electron
A common confusion is to credit Hertz with discovering the photoelectron, or to credit J. J. Thomson with the photoelectric effect. Keep the timeline clean: Hertz observed the effect in 1887; the electron was discovered by J. J. Thomson in 1897. Only after 1897 was it understood that the particles emitted in Hertz's experiment were electrons.
Hertz (1887) → photoelectric effect observed. Thomson (1897) → electron identified.
Hallwachs' and Lenard's Observations
Wilhelm Hallwachs and Philipp Lenard investigated photoelectric emission in detail during 1886–1902, turning Hertz's stray observation into a controlled experiment. Lenard (1862–1947) allowed ultraviolet radiation to fall on the emitter plate of an evacuated glass tube enclosing two electrodes. He found that a current flowed in the circuit, and that the current stopped the instant the ultraviolet radiation was stopped.
The interpretation is direct: ultraviolet light falling on the emitter plate C ejects electrons, which are attracted towards the positive collector plate A by the electric field. The electrons flow through the evacuated tube, producing the photocurrent in the external circuit. Light falling on the emitter therefore drives a current — and the immediate cut-off when the light is removed shows the current is caused by the light, not by anything else.
Lenard's arrangement: UV light through a quartz window ejects electrons from emitter C; the field from the battery drives them to collector A, and the microammeter registers the photocurrent.
Hallwachs, in 1888, took the study further with an electroscope. He connected a negatively charged zinc plate and observed that the plate lost its charge when illuminated by ultraviolet light. Going further: an initially uncharged zinc plate became positively charged under UV light, and a plate that was already positively charged had its positive charge further enhanced. Each of these results points the same way — negatively charged particles were being driven off the zinc plate by the ultraviolet light.
| Initial state of zinc plate | Effect of UV light (Hallwachs, 1888) | What it proves |
|---|---|---|
| Negatively charged | Loses its charge | Negative particles leave the plate |
| Uncharged | Becomes positively charged | Negative particles leave, leaving net positive |
| Positively charged | Positive charge enhanced | More negative particles leave |
Role of the Plate Potential
Lenard's tube was not a passive detector — it carried a battery whose potential could be controlled and even reversed. The two electrodes, emitter C and collector A, were connected so that A could be held at a chosen potential with respect to C. NIOS §25.1 notes the key practical feature: a commutator lets the experimenter reverse the polarity, so plate A can be made either positive or negative relative to C.
When the collector A is held positive with respect to the emitter C, the negatively charged photoelectrons are attracted to it, and their arrival constitutes the photocurrent read by the microammeter. This controllability is what made the apparatus powerful: Hallwachs and Lenard could study how the photocurrent varied with (a) the collector plate potential, (b) the frequency of incident light, and (c) the intensity of incident light.
The full set of current–voltage graphs (saturation current, stopping potential, and the frequency dependence) is built up in Experimental Study of the Photoelectric Effect.
Threshold Frequency and Photosensitive Metals
The single most important qualitative result from this period was the existence of a cut-off in frequency. Hallwachs and Lenard observed that when ultraviolet light fell on the emitter plate, no electrons were emitted at all when the frequency of the incident light was smaller than a certain minimum value — the threshold frequency, denoted $\nu_0$. Crucially, this minimum frequency depends on the nature of the material of the emitter plate; it is not a universal constant.
This metal-dependence explains why different substances respond to different colours of light. Metals such as zinc, cadmium and magnesium have a high threshold frequency, so they respond only to short-wavelength ultraviolet light. Alkali metals — lithium, sodium, potassium, caesium and rubidium — have a lower threshold frequency and are sensitive even to visible light.
| Metal group | Examples | Light that causes emission |
|---|---|---|
| High threshold frequency | Zinc, cadmium, magnesium | Only short-wavelength ultraviolet light |
| Low threshold frequency (alkali) | Lithium, sodium, potassium, caesium, rubidium | Sensitive even to visible light |
Two further qualitative features were recorded that became the bedrock of the chapter. Above the threshold frequency, the emission is instantaneous — it starts without any apparent time lag, in a time of the order of $10^{-9}\ \text{s}$ or less, even if the incident radiation is very dim. And below the threshold frequency, no amount of intensity will produce emission.
Threshold frequency is metal-specific; emission is instantaneous
Two facts examiners exploit. First, $\nu_0$ is a property of the metal, so the same light may eject electrons from caesium but not from zinc. Second, photoelectric emission is instantaneous — increasing intensity adds more electrons per second but never speeds up or delays the onset. If a question asks whether a dim source causes a "time delay," the answer is no, provided $\nu > \nu_0$.
No emission for $\nu < \nu_0$, however intense the light. For $\nu > \nu_0$, emission begins in $\sim 10^{-9}\ \text{s}$.
Summary of the Discovery Facts
The discovery sequence in NCERT §11.3 hands you a small but high-yield set of facts: who observed what, in which year, and the qualitative conclusions that classical wave theory would later fail to explain. The table below collects them so the chronology and the conclusions stay separate in memory.
| Contributor | Period | Key observation |
|---|---|---|
| Heinrich Hertz | 1887 | Sparks across the detector loop enhanced when the emitter was illuminated by UV light. |
| Wilhelm Hallwachs | 1888 | A charged zinc plate lost negative charge / gained positive charge under UV; negative particles emitted. |
| Philipp Lenard | 1886–1902 | UV on the emitter of an evacuated tube produced a photocurrent that stopped when the light stopped; studied dependence on potential, frequency and intensity. |
Hertz and Lenard — the essentials
- Photoelectric effect = emission of electrons (photoelectrons) from a metal illuminated by light of suitable frequency; one of three electron-emission processes.
- Hertz (1887) discovered it by accident: UV light on the emitter enhanced the spark across the gap.
- The electron was identified only later, by J. J. Thomson in 1897.
- Lenard built the evacuated two-electrode tube; a photocurrent flowed and stopped instantly with the light.
- Hallwachs (1888) used a charged zinc plate to confirm negative particles were emitted.
- A commutator could reverse the plate potential; with collector positive, photoelectrons are collected as photocurrent.
- Below the metal-specific threshold frequency $\nu_0$, no emission occurs however intense the light; above it, emission is instantaneous ($\sim 10^{-9}\ \text{s}$).