✨ Dual Nature of Radiation & Matter – Quick Intro
🔦 Light acts like a wave. Maxwell’s equations and Hertz’s spark-gap experiments in 1887 nailed this idea down early on. But right when everyone felt comfy with waves, a series of low-pressure gas-discharge studies shook things up and hinted at a particle side too! :contentReference[oaicite:12]{index=12}
1 🔍 Cathode-Ray Adventures
- William Crookes spotted mysterious “cathode rays” zipping from the negative plate (cathode) in 1870. 💨
- J. J. Thomson steered those rays with crossed electric & magnetic fields and measured their charge-to-mass ratio: $$\frac{e}{m}=1.76\times10^{11}\,\text{C kg}^{-1}$$. The rays whizzed along at about \(0.1\!-\!0.2\,c\) (with \(c = 3\times10^{8}\,\text{m/s}\)). :contentReference[oaicite:13]{index=13}
- Same \(e/m\) no matter what gas or cathode you use → the particle is universal. Thomson named it the electron in 1897. :contentReference[oaicite:14]{index=14}
2 ⚡ Quantised Charge – Millikan’s Oil-Drop
Robert Millikan balanced tiny, charged oil drops in an electric field (1913). He showed the charge always came in whole-number chunks of $$e = 1.602\times10^{-19}\,\text{C}$$ and picked up the 1923 Nobel Prize for it. 🏆 :contentReference[oaicite:15]{index=15}
3 💡 Need Electrons Out? – Electron Emission Basics
Metals teem with free electrons, but surface forces hold them back. To escape, an electron must overcome the work function \( \phi_0 \) (energy barrier) of the metal.
- Work function is measured in electron-volts (eV).
- Definition: $$1\;\text{eV}=1.602\times10^{-19}\,\text{J}$$ :contentReference[oaicite:16]{index=16}
- Emission route highlighted here: Thermionic emission 🔥 – heat the metal so electrons get enough kinetic energy to hop out. :contentReference[oaicite:17]{index=17}
🚀 Why This Matters
The evidence above tells a thrilling story:
- Wave experiments → light behaves like a wave.
- Cathode-ray & oil-drop results → charge is granular, electrons are particles.
- Electrons pop out of metals once you supply the work-function energy.
🎯 High-Yield NEET Nuggets
- Precise value & meaning of the charge-to-mass ratio \(e/m\) for electrons. :contentReference[oaicite:18]{index=18}
- Millikan oil-drop experiment and the quantisation of electric charge. :contentReference[oaicite:19]{index=19}
- Definition of work function and its link to electron emission from metals. :contentReference[oaicite:20]{index=20}
- Electron-volt as a convenient atomic-scale energy unit: \(1\;\text{eV}=1.602\times10^{-19}\,\text{J}\). :contentReference[oaicite:21]{index=21}
- Thermionic emission principle used in devices like cathode-ray tubes and vacuum tubes. :contentReference[oaicite:22]{index=22}
Keep these concepts handy, sprinkle in a bit of practice, and you’ll shine on exam day! 😄✨
