Photoelectric effect: observations of Hertz, Hallwachs and Lenard
Light of high enough frequency knocks electrons out of a metal surface. Below a threshold frequency, no electrons come out, however bright the light.
Hertz's observation (1887)
Heinrich Hertz was making electromagnetic waves with a spark discharge. He noticed that sparks jumped across the detector gap more easily when the emitter plate was lit by ultraviolet light from an arc lamp. Light falling on the metal seemed to help charged particles leave its surface. This was the first observation of the photoelectric effect, though Hertz did not explain it.
Observations of Hallwachs and Lenard (1886 to 1902)
- 1Lenard: he used an evacuated glass tube with two metal plates. When ultraviolet light fell on the emitter plate, a current flowed in the circuit.
- 2When the ultraviolet light was stopped, the current stopped at once. So light was freeing charged particles from the plate.
- 3Hallwachs: he joined a negatively charged zinc plate to an electroscope. In ultraviolet light, the plate lost its charge.
- 4An uncharged zinc plate became positively charged in ultraviolet light. A positively charged plate became even more positive.
- 5★Conclusion: light makes negatively charged particles leave the metal. These were later shown to be electrons.
- 6Zinc, cadmium and magnesium responded only to ultraviolet light. Alkali metals (lithium, sodium, potassium, caesium, rubidium) responded even to visible light.
- 7Below a certain threshold frequency, no electrons came out, however intense the light.
★ key step
Key words
| Term | Meaning |
|---|---|
| Photosensitive surface | A metal surface that emits electrons when light falls on it |
| Photoelectrons | Electrons emitted because of light |
| Photoelectric current | The current formed by the photoelectrons |
| Threshold frequency ν₀ | Minimum frequency below which no emission happens; it is different for each metal |
| Threshold wavelength λ₀ | Longest wavelength that can cause emission: λ₀ = c/ν₀ |
Worked example: threshold wavelength
The threshold frequency of a metal is 6 × 10¹⁴ Hz. Find its threshold wavelength. Will light of wavelength 600 nm eject electrons from it? (c = 3 × 10⁸ m/s)
- λ₀ = c/ν₀ = (3 × 10⁸ m/s) ÷ (6 × 10¹⁴ Hz) = 5 × 10⁻⁷ m = 500 nm.
- Light of 600 nm has a longer wavelength than 500 nm.
- Its frequency is (3 × 10⁸) ÷ (6 × 10⁻⁷) = 5 × 10¹⁴ Hz, which is below ν₀.
λ₀ = 500 nm; 600 nm light will not eject electrons, however bright it is.
Older automatic doors and burglar alarms used photocells. When a person blocked the light beam, the photoelectric current stopped, and the circuit opened the door or rang the alarm.
2 marks: “What did Hallwachs observe when ultraviolet light fell on (a) a negatively charged zinc plate, (b) an uncharged zinc plate? What did he conclude?” (a) It lost its charge. (b) It became positively charged. Conclusion: negatively charged particles (electrons) are emitted from zinc in ultraviolet light.
1 marks: “Define threshold frequency.” The minimum frequency of incident light below which no photoelectrons are emitted from a given metal.
2 marks: “Why do alkali metals such as caesium show the photoelectric effect with visible light, while zinc needs ultraviolet light?” Alkali metals have low work functions, so visible-light photons have enough energy. Zinc has a higher work function and needs the higher-frequency ultraviolet photons.
Hertz saw sparks, Hallwachs saw charge, Lenard saw current, all with ultraviolet light on metal.