🚀 Big Idea
When light of the right frequency hits a metal, it can kick electrons out of the surface. These light-driven electrons are called photoelectrons :contentReference[oaicite:0]{index=0}.
🕰️ A Quick History Tour
- 1887 – Hertz: Noticed UV light made sparks in his detector loop jump more easily :contentReference[oaicite:1]{index=1}.
- 1886-1902 – Hallwachs & Lenard:
- Built an evacuated glass tube with two plates: emitter C and collector A.
- UV light on the emitter started a current; switching the light off stopped it immediately :contentReference[oaicite:2]{index=2}.
- Discovered the need for a threshold frequency \( \nu_0 \) below which no electrons came out, no matter how bright the light was :contentReference[oaicite:3]{index=3}.
🔧 Classic Experimental Setup
The go-to arrangement is an evacuated quartz tube with:
- A thin photosensitive plate C (emitter).
- A collector plate A that can be kept positive or negative with respect to C using a battery and commutator.
- A voltmeter (V) to read the plate voltage and a micro-ammeter (μA) to track the tiny photocurrent :contentReference[oaicite:4]{index=4}.
- Monochromatic light passing through a quartz window to strike C.
🔍 What the Setup Reveals
- Instant kick-off: Current begins the moment the light is on and drops to zero the moment it’s off :contentReference[oaicite:5]{index=5}.
- Intensity effect: Brighter light sends out more photoelectrons, giving a bigger current, only if the light’s frequency is above \( \nu_0 \) :contentReference[oaicite:6]{index=6}.
- Frequency effect: Below \( \nu_0 \) nothing happens, even with high intensity; above \( \nu_0 \) electrons fly out easily :contentReference[oaicite:7]{index=7}.
- Collector voltage:
- Positive A pulls more electrons across, raising the current.
- Negative A can repel the slower electrons; at a large enough negative voltage (called the stopping potential) the current falls to zero.
🗝️ Key Terms
| Term | Friendly meaning |
|---|---|
| Photoelectron | Electron freed by light ✨ |
| Threshold frequency \( \nu_0 \) | Smallest frequency that can nudge electrons out |
| Threshold wavelength \( \lambda_0 \) | The corresponding longest wavelength (\( \lambda_0 = c/\nu_0 \)) |
| Stopping potential | Reverse voltage needed to stop the current completely |
📈 How Different Knobs Change the Photocurrent
- Intensity knob: Raises or lowers the saturation current but leaves the stopping potential almost untouched (because electron energy depends on frequency, not on how bright the beam is!).
- Frequency knob: Higher frequency (\( \nu > \nu_0 \)) means each electron can leave with more kinetic energy, so you need a larger stopping potential to halt them.
- Voltage knob:
- Positive voltage → current climbs and quickly levels off at a saturation current.
- Negative voltage → current falls; at the stopping potential the curve touches zero.
🎯 High-Yield NEET Nuggets
- Threshold concept: No emission occurs for \( \nu < \nu_0 \), no matter how intense the light.
- Instantaneous response: Emission starts without any time lag (< 10–9 s) when light is on.
- Kinetic-energy link: Maximum kinetic energy (checked via stopping potential) grows linearly with the light frequency.
- Current vs. intensity: At a fixed frequency \( \nu > \nu_0 \), raising intensity cranks up the photocurrent but leaves the kinetic energy unchanged.
- Material dependence: Alkali metals (Na, K, Cs, Rb) eject electrons with visible light, whereas Zn, Cd, or Mg need ultraviolet :contentReference[oaicite:8]{index=8}.
🌟 Take-away
Light doesn’t just warm a surface; above a magic frequency it can whack electrons straight out. The effect happens in a flash, scales with brightness for current, and proves that light’s energy comes in bite-sized chunks. 🌈⚡
