Introduction to Modern Physics

Modern Physics Introduction

• "Modern" physics refers to the 20th century
• By the end of the 19th century, most laws of physics were known, including planetary motion and electromagnetism
• However, classical physics struggled to explain phenomena at the atomic level

Atoms and Classical Physics

• Classically, electrons orbit the nucleus similar to planets around the sun
• Electromagnetic theory predicts orbiting electrons would constantly radiate energy, leading to a collapse into the nucleus
• Consequently, the classical model suggests atoms cannot exist

Problems with Newton's Laws

• Newton's laws successfully describe large-scale objects like planets but are inadequate for explaining atomic-level phenomena
• Newton's laws were based on observations of macroscopic objects, not atoms
• Physical laws have limitations and need refinement as knowledge advances

Newton's Laws Fail at High Velocities

• Einstein demonstrated that mass is not constant but depends on velocity
• As velocity approaches the speed of light, mass increases indefinitely
• Velocity cannot exceed the speed of light

The Failure of "Old" Physics

• Several effects couldn't be explained by pre-20th-century physics
• The discoveries led to revolutionizing our understanding of light, matter, particles, and waves
• New concepts revolutionized technologies like semiconductors, leading to personal computers and cell phones

The Photoelectric Effect - Photons

• Light can cause electrons to be emitted from a metal surface
• Photoemission occurs only if the light's wavelength is below a threshold value, regardless of intensity
• Colour (wavelength) is critical; blue light can produce photoemission, while red light cannot

Details of a Photocell

• Photocells consist of a light-sensitive metal plate (negative) and a collector (positive) inside an evacuated tube
• Light striking the metal plate releases electrons through photoemission, creating a current
• An ammeter measures this photocurrent

Photocells as Safety Devices

• Photocells used in safety systems (e.g., automatic garage doors)
• Infrared light beams are interrupted when a person or object moves into the beam, triggering the system to stop

No Classical Explanation for the Photoelectric Effect

• Electromagnetic theory suggests that increasing light intensity would increase electron emission
• However, the wavelength of light affects photoemission, not just intensity
• Classical theory fails to explain this effect

Einstein Receives the 1921 Nobel Prize

• Einstein proposed light as particles (photons), an idea that explained the photoelectric effect
• Photons are packets of energy moving at the speed of light
• Light exhibits wave-particle duality

Photoelectric Effect - Photons (2)

• A photon's energy depends on its wavelength or frequency; higher frequencies (shorter wavelengths) correspond to more energy
• Energy of a photon (E) = Planck's constant (h) x frequency (f)

The Photon Concept Explains the Photoelectric Effect

• A certain energy is required to release an electron from a metal
• Photoemission occurs when an electron absorbs enough energy from a photon
• The number of photons does not determine emission if the photons do not have the specified energy level.

Blue and Red Photons - Example

• A photon with a shorter wavelength has a higher frequency and more energy
• For example, a 350 nm photon has twice the energy of a 700 nm photon.

The Quantum Concept

• The photon concept contrasts sharply with classical physics
• Energy in classical physics is continuous, but in modern physics, it's quantized, coming in discrete packets (photons)
• The photoelectric effect demonstrates energy absorption in discrete amounts

Video Recorders and Digital Cameras

• Electronic cameras use the photoelectric effect to convert light into electric charges
• A megapixel sensor array captures the charges and records their intensity in computer memory

Niels Bohr Explains Atoms in 1913

• Danish physicist Niels Bohr used quantum concepts to explain atomic structure
• Electrons orbit the nucleus at specific energy levels; any transitions means energy is absorbed or emitted, in particular cases photons
• Atoms emit light in discrete lines (characteristic wavelengths), not in continuous bands

Line Spectra of Atoms

• Line spectra (unique patterns of light) can identify different atoms
• These have specific patterns for each type of atom much like fingerprints

The Bohr Atom

• Electrons orbit the nucleus in specific, quantized energy levels
• If electrons change energy levels, light is emitted or absorbed, corresponding to the difference
• The electron can absorb or release photons to change between energy levels

Line Spectra of Atomic Hydrogen

• The Bohr model successfully predicted the spectral lines in hydrogen atoms

Emission and Absorption

• When an electron jumps from a higher to a lower energy level, a photon is emitted
• Conversely, an electron absorbs a photon to jump to a higher level
• These phenomena are fundamental to atomic processes

Emission and Absorption (2)

• Illustrations depict transitions of electrons and corresponding energy level changes

Quantum Mechanics

• Niels Bohr's work was crucial to the development of Quantum Mechanics
• Quantum Mechanics replaced Classical Mechanics as a fundamental theory describing phenomena at the atomic level
• This new theory recognizes limitations in the predictability of certain quantities, notably quantized energies/particles

This quiz explores the fundamental concepts of modern physics, focusing on atomic theory and the limitations of classical physics. It covers the inadequacies of Newton's laws when applied to atomic phenomena and the implications for understanding mass and energy. Test your knowledge on the transition from classical to modern physics.