Dual Nature of Radiation And Matter - Notes, Topics, Formulas, Books, FAQs

Dual Nature of Radiation and Matter Class 12 Topics (NCERT Syllabus)

1. Introduction

When we see light, hear about X-rays, or study electrons in experiments, we realize that light and matter show dual behavior. Sometimes they behave like waves, and sometimes like particles. This is called the dual nature of radiation and matter. Light shows wave properties such as interference and diffraction, and particle properties such as the photoelectric effect. Similarly, electrons and other particles also behave like waves in certain conditions. This chapter explains this dual nature, which is one of the foundations of modern physics.

2. Electron Emission

The process of releasing electrons from the surface of a metal is called electron emission. Since electrons inside a metal are bound by attractive forces, they need a certain minimum energy (called work function) to escape. Depending on how this energy is supplied, electron emission can be of four types:

1. Thermionic emission – by heating the metal.
2. Photoelectric emission – by shining light on the metal.
3. Field emission – by applying a strong electric field.
4. Secondary emission – by bombarding the surface with fast-moving electrons.

• Electron Emission

3. Photoelectric Effect

The photoelectric effect is the phenomenon in which electrons are emitted from the surface of a metal when light of suitable frequency falls on it. These emitted electrons are called photoelectrons. The effect shows that light behaves like particles called photons, each carrying energy $E=h \nu$, where $h$ is Planck's constant and $\nu$ is the frequency of light.

According to Einstein's photoelectric equation:

$$
K_{\max }=h \nu-\phi
$$

• Photoelectric Effect

4. Experimental Study of Photoelectric Effect

To study the photoelectric effect, a simple experimental setup is used:

• A clean metal plate (like zinc) is exposed to light of varying frequencies.
• The emitted electrons are collected in a vacuum tube with an anode and cathode.
• A potential difference is applied to measure the current due to photoelectrons.

Observations:

• Photoelectric emission occurs only if the frequency of light is greater than a certain minimum value called threshold frequency.
• The photoelectric current is directly proportional to the intensity of incident light (for frequency above threshold).
• The maximum kinetic energy of photoelectrons depends only on the frequency of light, not its intensity.
• The emission of photoelectrons is almost instantaneous.

5. Photoelectric Effect and Wave Theory of Light

According to the wave theory of light, the energy of light is spread uniformly over the wavefront. So, if light of high intensity falls on a metal surface, it should supply enough energy to eject electrons, even if the frequency is low. Also, there should be a time delay before emission, as electrons would need to collect enough energy.

6. Einstein's Photoelectric Equation: Energy Quantum of Radiation

Einstein explained the photoelectric effect using the concept of photons (energy quanta of light). Each photon has energy $E=h \nu$, where $h$ is Planck's constant and $\nu$ is the frequency of light. When a photon strikes a metal surface, its energy is used in two parts:

$$
h \nu=\phi+K_{\max }
$$

• Einstein's Photoelectric Equation
• Einstein's Explanation Of Photoelectric Effect

7. Particle Nature of Light: The Photon

The photoelectric effect showed that light behaves as if made of energy packets called photons. Each photon has:

1. Energy $E=h \nu$ and momentum $p=h \nu / c$.
2. Same energy and momentum for all photons of a given frequency, regardless of light intensity; increasing intensity only increases the number of photons.
3. Speed equal to $c$, the speed of light.
4. Electrically neutral, unaffected by electric or magnetic fields.
5. Collisions with electrons conserve total energy and momentum, though photons can be absorbed or created.

• Photon Theory Of Light