Understanding Light as a Wave and a Particle.pptx

Understanding Light as a Wave and a Particle

1. THEORIES OF LIGHT & ITS PROPERTIES Understanding Light as a Wave and a Particle

2. Light is a form of energy that allows us to see the world. It is a type of electromagnetic radiation that travels through space. Scientists have developed various theories to explain its nature, building on past discoveries. Energy can be transferred through waves or particles, leading to different interpretations of light. WHAT IS LIGHT?

3. Speed of light describe how we see in dimly lit conditions Wave-particle duality Criteria for Light Theories Theories should explain: Reflection & refraction Diffraction & interference Energy quantization

4. CORPUSCULAR THEORY (NEWTON, 1704) Light is made of tiny particles (corpuscles) with negligible mass. Emitted by luminous sources (Sun, candle, lamp, etc.). Properties of Corpuscles: Perfectly elastic, moving in straight lines. Carry kinetic energy and travel at high speed. Initially thought to travel faster in denser media (later proven wrong). Vision occurs when corpuscles strike the retina. Different corpuscle sizes account for different colors of light. States that light is made up of tiny particles called ‘corpuscles’ (little particles) that always travel in a straight line.

5. WAVE THEORY (HUYGENS, 1678) Light propagates as a wave. Light travels through a hypothetical medium called "ether". Each point on a wavefront acts as a new source of disturbance (Huygens' Principle). Light energy is distributed equally in all directions. Explains phenomena like color spectrum, diffraction, and polarization proposed that every point that a luminous disturbance meets turns into a source of the spherical wave itself. The sum of the secondary waves, which are the result of the disturbance, determines what form the new wave will take.

6. Light is an electromagnetic wave. Light waves possess electrical and magnetic properties. Light can travel through a vacuum. Unified electricity, magnetism, and light. Equation: Maxwell’s equation describe electromagnetic waves: ELECTROMAGNETIC WAVE THEORY (MAXWELL, 1865) states that light is an electromagnetic disturbance that propagates through space as waves, consisting of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of propagation. These waves do not require a material medium for transmission and travel at the speed of light in a vacuum.

7. Einstein’s Theory Einstein used Planck's concept of quantization to explain the photoelectric effect The photoelectric effect is the ejection of electrons from certain metals when exposed to light Einstein's theory showed that light also behaves as if it is made of particles, later called "photons" QUANTUM THEORY (PLANCK & EINSTEIN, 1905) states that energy is quantized, meaning that different atoms and molecules can emit or absorb energy only in discrete quantities known as "quanta." Specifically, the energy of electromagnetic radiation is directly proportional to its frequency Formula: E =h ν where: E= Energy v= Planck’s Constant (6.626 x 10^-34 joule-seconds. ) Planck's quantum theory Energy can only be absorbed or emitted in multiples of a quantum, a small unit of energy The energy of a quantum is proportional to the frequency of radiation Planck's constant is a fundamental constant that describes the sizes of quanta Planck's theory applies to all forms of radiation.

8. QUANTUM ELECTRODYNAMICS (QED) (DIRAC, FEYNMAN, 20TH CENTURY) Light consists of discrete packets of energy called photons. Photons exhibit both wave-like and particle-like behavior. QED successfully explains phenomena such as the anomalous magnetic moment of the electron and scattering processes involving light and matter. Paul Dirac's C ontribution: Formulated a Hamiltonian that describes electrons moving close to the speed of light, integrating quantum theory with special relativity. His work laid the groundwork for QED by quantizing electromagnetic radiation and its interaction with matter. Richard Feynman's Contribution: Devel oped a visual representation of particle interactions through Feynman diagrams, illustrating how photons and electrons interact. Introduced the concept of probability amplitudes to calculate the likelihood of various interactions in QED.

9. PROPERTIES OF LIGHT Reflect ion: Bouncing of light on surfaces. Refraction: Bending of light through different media. Diffraction: Light spreading after passing through a small opening. Dispersion: The separation of light into different colors due to varying refraction. Interference: Overlapping waves can reinforce or cancel out each other. Photoelectric Effect: Emission of electrons when light strikes a surface. Polarization: Restriction of light waves to vibrate in a single direction. Electromagnetic Spectrum: Light spreading after passing through a small opening. Reflection Refraction Diffraction Interference Photoelectric Effect Polarization Electromagnetic Spectrum

10. Wavelength: Distance between two wave peaks; determines color (e.g., blue ~450 nm, red ~700 nm). Frequency: Number of wave peaks per second; higher frequency = more energy. Speed: Travels at 3 × 10⁸ m/s in a vacuum, c onstant for all wavelengths. LIGHT AS A WAVE Wave Behaviors: Reflection: Light bounces off surfaces (angle of incidence = angle of reflection). Refraction: Light bends when passing through different media (Snell’s Law). Diffraction: Light spreads when passing thr ough small openings or around obstacles. Interference: Overlapping waves create bright (constructive) and dark (destructive) patterns. Polarization: Light waves can vibrate in a single plane, used in sunglasses & photography.

11. LIGHT AS A PARTICLE E instein’s Photon Theory (1905): Light consists of discrete energy packets called photons. Photoelectric Effect: Light can eject electrons from metal surfaces, proving it has particle-like properties. Energy of a Photon: Given by E=hf (where h is Planck’s constant and f is frequency). Key Evidence for Particle Nature: Photoelectric effect (Einstein’s Nobel Prize-winning discovery). Compton scattering (X-ray photons transferring energy to electrons). Light travels in straight lines in some cases (e.g., shadows). FORMULA: E=hf h = Planck’s constant (6.626×10−34 Joule-seconds ) f = frequency

12. SUMMARY Historical Development Newton’s C orpuscular Theory (1704) → Light as particles Huygens’ Wave Theory (1678) → Light as waves Maxwell’s Electromagnetic Wave Theory (1865) → Light as EM waves Planck & Einstein’s Quantum Theory (1905) → Light as photons Quantum Electrodynamics (20th Century) → Unified wave-particle dualit y Experimental Evidence Newton’s Prism Experiment (Corpuscular Theory) Young’s Double-Slit Experiment (Wave Theory) Michelson Experiment (EM Wave Theory) Photoelectric Effect Experiment (Quantum Theory) Comparison of Theories Early theories were limited in explaining all light behaviors. Modern quantum theories incorporate both particle & wave properties. Quantum Electrodynamics (QED) is the most complete and accurate theory. Impact on Science Led to the development of lasers, fiber optics, and quantum mechanics. Helped in advancements in communication, medicine, and technology. Provided a deeper understanding of light-matter interactions. Key Features and Limitations

13. Who proposed the wave theory of light? A. Newton B. Huygens C. Maxwell D. Einstein 2. Which theory explains the phot o el ect ric effect? A. Corpuscular Theory B. Wave Theory C. Quantum Theory D. Electromagnetic Wave Theory 3. What phenomenon supports light’s wave nature? A. Reflection B. Diffraction C. Photoelectric Effect D. Straight-line motion MULTIPLE CHOICE QUESTIONS 4. Which scientist proposed that light has electromagnetic properties? A. Einstein B. Planck C. Maxwell D. Huygens 5. What is the modern theory explaining light’s behavior? A. Corpuscular Theory B. Wave Theory C. Quantum Electrodynamics D. Maxwell’s Theory