NCERT Class 12 Physics Physics Part-II: Chapter 3 — 1 INTRODUCTION
This chapter, "Dual Nature of Radiation and Matter," begins by establishing the wave nature of light through Maxwell's equations and Hertz's experiments. It then delves into the historical discoveries of X-rays and electrons in the late 19th century, stemming from experiments with discharge tubes. The text details the discovery and properties of cathode rays, their identification as negatively charged particles by J.J. Thomson, and his determination of their specific charge. The chapter also discusses the photoelectric effect, where metals emit electrons when irradiated by ultraviolet light, and thermionic emission, where heating metals releases electrons. The concept of work function, the minimum energy required for electron emission, is introduced, measured in electron volts (eV). The universality and fundamental nature of electrons as constituents of matter are highlighted, along with Millikan's oil-drop experiment confirming charge quantization. This chapter lays the groundwork for understanding the dualistic behavior of light and matter.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Physics |
| Book | Physics Part-II |
| Chapter | Chapter 3 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 785 |
Learning outcomes
- Understand the historical context of wave-particle duality.
- Learn about the discovery and properties of electrons.
- Define and explain electron emission and work function.
- Recognize the significance of J.J. Thomson's and R.A. Millikan's experiments.
Vocabulary
| Word | Meaning |
|---|---|
| Electromagnetism | Branch of physics concerned with the relationship between electricity and magnetism. |
| Cathode rays | Streams of fast-moving negatively charged particles (electrons) emitted from the cathode of a discharge tube. |
| Specific charge | The ratio of electric charge to mass (e/m) of a particle. |
| Electron | A stable subatomic particle with a negative elementary electric charge. |
| Photoelectric effect | Emission of electrons from a material when light shines on it. |
| Thermionic emission | Emission of electrons from a heated surface. |
| Work function | The minimum thermodynamic work (energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. |
| Electron volt (eV) | A unit of energy commonly used in atomic and nuclear physics, equal to the energy gained by an electron accelerated by a potential difference of 1 volt. |
| Quantised | Having or relating to a system in which quantities such as energy occur in discrete units or 'packets'. |
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Practice questions
- What established the wave nature of light towards the end of the 19th century? Answer: Maxwell's equations of electromagnetism and Hertz's experiments.
- Who discovered X-rays and electrons, and in which years? Answer: Roentgen discovered X-rays in 1895, and J. J. Thomson discovered the electron in 1897.
- What are cathode rays? Answer: Cathode rays are streams of fast-moving negatively charged particles (electrons) that appear to be coming from the cathode in a discharge tube.
- What is the specific charge of an electron? Answer: The presently accepted value of the specific charge (e/m) of an electron is 1.76 × 10^11 C/kg.
- Define the work function of a metal. Answer: The work function is the minimum energy required by an electron to escape from the surface of a metal.
Practice MCQs
Q1. Who confirmed the hypothesis that cathode rays consisted of streams of negatively charged particles?
Explanation: J. J. Thomson confirmed the hypothesis that cathode rays consisted of streams of fast-moving negatively charged particles.
Q2. What is the approximate speed range of cathode ray particles relative to the speed of light?
Explanation: Cathode ray particles were found to travel with speeds ranging from about 0.1 to 0.2 times the speed of light.
Q3. The value of e/m for cathode ray particles was found to be independent of:
Explanation: The value of e/m was found to be independent of the nature of the material/metal used as the cathode or the gas introduced in the discharge tube.
Q4. What phenomenon occurs when certain metals are irradiated by ultraviolet light and emit negatively charged particles?
Explanation: This phenomenon is known as the photoelectric effect, where metals emit electrons when exposed to ultraviolet light.
Q5. What is the unit of work function generally denoted by f0?
Explanation: The work function is generally denoted by f0 and measured in electron volts (eV).
Q6. Who performed the oil-drop experiment to measure the charge on an electron?
Explanation: R. A. Millikan performed the pioneering oil-drop experiment for the precise measurement of the charge on an electron.
Frequently asked questions
What is the significance of Maxwell's equations and Hertz experiments?
They strongly established the wave nature of light towards the end of the 19th century.
What led to the discovery of electrons?
Experimental investigations on the conduction of electricity through gases at low pressure in discharge tubes led to the discovery of electrons.
What did J. J. Thomson determine about cathode ray particles?
J. J. Thomson determined their speed and specific charge (charge to mass ratio, e/m).
What is the work function of a metal?
It is the minimum energy required by an electron to escape from the surface of the metal.
What is an electron volt (eV)?
An electron volt is a unit of energy equal to the energy gained by an electron when accelerated by a potential difference of 1 volt (1 eV = 1.602 × 10–19 J).
What did Millikan's oil-drop experiment establish?
Millikan's experiment established that electric charge is quantized, meaning it exists in discrete integral multiples of an elementary charge.
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NCERT Class 12 Physics — Physics Part-II — Chapter 3 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.