2.1 Basic principles of light and electromagnetic radiation
4 min read•august 9, 2024
Light and electromagnetic radiation are the foundation of biophotonics. These waves carry energy in packets called photons, with properties like and determining their behavior. Understanding these basics is key to grasping how light interacts with biological tissues.
From to gamma rays, the electromagnetic spectrum covers a wide range of energies. This section dives into wave characteristics, photon properties, and how light behaves when interacting with matter. These concepts set the stage for exploring more complex optical phenomena in biological systems.
Electromagnetic Waves
Electromagnetic Spectrum and Wave Characteristics
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- Electromagnetic spectrum encompasses all types of electromagnetic radiation
- Ranges from low-energy radio waves to high-energy gamma rays
- Includes visible light, microwaves, X-rays, and
- Wavelength measures the distance between two consecutive wave crests or troughs
- Inversely proportional to frequency
- Varies from kilometers (radio waves) to picometers (gamma rays)
- Frequency represents the number of wave cycles passing a fixed point per second
- Measured in Hertz (Hz)
- Directly proportional to photon energy
- Speed of light remains constant in vacuum at approximately 299,792,458 meters per second
- Represented by the symbol 'c' in equations
- Slows down when passing through different media (glass, water)
Relationships and Calculations
- Wave equation relates wavelength, frequency, and speed of light
- , where c is the speed of light, λ is wavelength, and f is frequency
- Energy of a photon can be calculated using the Planck-Einstein relation
- , where E is energy, h is Planck's constant, and f is frequency
- Wavelength and frequency are inversely related
- As wavelength increases, frequency decreases and vice versa
- Visible light spectrum ranges from about 380 nm (violet) to 700 nm (red)
Photon Properties
Energy and Quantum Nature
- Photon energy depends on the frequency of the electromagnetic wave
- Higher frequency corresponds to higher energy
- Gamma rays have the highest energy, radio waves the lowest
- Quantization of energy occurs in discrete packets called photons
- Energy cannot be divided into smaller units than a single photon
- Explains phenomena like the photoelectric effect
- Photon momentum relates to its wavelength
- , where p is momentum, h is Planck's constant, and λ is wavelength
- Demonstrates wave-particle duality of light
Polarization and Coherence
- Polarization describes the orientation of electromagnetic wave oscillations
- Linear polarization occurs when waves oscillate in a single plane
- Circular polarization results from two perpendicular waves 90° out of phase
- Used in various applications (3D movies, LCD screens, polarized sunglasses)
- Coherence refers to the degree of correlation between wave properties
- Temporal coherence measures correlation over time
- Spatial coherence measures correlation across space
- Crucial for applications like holography and interferometry
- Degree of polarization affects light intensity after passing through polarizers
- Malus's law: , where I is transmitted intensity, I_0 is initial intensity, and θ is angle between polarizer axes
Wave-Matter Interactions
Refraction and Optical Properties
- occurs when light changes speed while passing between media
- Causes bending of light at the interface between two materials
- Governed by : , where n is refractive index and θ is angle of incidence/refraction
- Refractive index measures how much a material slows down light
- Defined as the ratio of speed of light in vacuum to speed in the medium
- Affects phenomena like dispersion (splitting of white light into colors)
- happens when light strikes an interface at an angle greater than the critical angle
- Utilized in fiber optic communications and prism binoculars
Diffraction and Interference Phenomena
- describes the bending of waves around obstacles or through openings
- More pronounced when wavelength is comparable to obstacle/opening size
- Explains why we can hear around corners but not see around them
- Single-slit diffraction pattern consists of a central maximum with alternating bright and dark fringes
- Angular position of minima given by , where m is order number, λ is wavelength, and d is slit width
- results from the superposition of two or more waves
- Constructive interference occurs when waves are in phase, amplifying the signal
- Destructive interference happens when waves are out of phase, canceling each other
- Double-slit experiment demonstrates both interference and diffraction
- Produces a pattern of bright and dark fringes on a screen
- Spacing between fringes depends on slit separation and wavelength of light
- Fundamental to understanding wave-particle duality in quantum mechanics
Key Terms to Review (16)
Diffraction: Diffraction is the bending and spreading of waves, such as light or sound, when they encounter an obstacle or pass through a narrow opening. This phenomenon illustrates the wave nature of light, revealing important insights into how light behaves in various environments and how it interacts with different materials.
Frequency: Frequency is defined as the number of occurrences of a repeating event per unit of time, specifically in the context of electromagnetic waves, it represents how many wave cycles pass a point in one second. This concept is crucial for understanding the behavior of light and electromagnetic radiation, as it directly relates to energy levels and color perception in visible light. Higher frequencies correspond to shorter wavelengths and greater energy, while lower frequencies relate to longer wavelengths and less energy.
Infrared radiation: Infrared radiation is a type of electromagnetic radiation with wavelengths longer than visible light, typically ranging from about 700 nanometers to 1 millimeter. It plays a crucial role in various natural and technological processes, such as heat transfer and remote sensing. Understanding infrared radiation is essential for applications in fields like communication, thermal imaging, and biophotonics, where it is used to study biological systems through non-invasive techniques.
Interference: Interference is the phenomenon that occurs when two or more waves overlap and combine, resulting in a new wave pattern. This can lead to either constructive interference, where waves amplify each other, or destructive interference, where waves cancel each other out. Interference is essential in understanding how light behaves, especially in the context of various optical phenomena, and is a fundamental principle of wave behavior in electromagnetic radiation.
Maxwell's Equations: Maxwell's Equations are a set of four fundamental equations that describe how electric and magnetic fields interact and propagate. These equations form the foundation of classical electromagnetism, explaining phenomena such as the behavior of light as an electromagnetic wave. They encapsulate the relationship between electric charges, currents, and the fields they generate, bridging the understanding of electricity and magnetism with the nature of light.
Photometry: Photometry is the science of measuring visible light, particularly in terms of how humans perceive it. This field involves quantifying light intensity, illuminance, and luminous flux, which are essential for applications that require precise control and measurement of light, such as in therapeutic techniques or optical systems. The principles of photometry are critical for understanding how light interacts with biological tissues and how to effectively deliver light in medical treatments.
Planck's Law: Planck's Law describes the electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature. This law is fundamental in understanding how light and energy are emitted and absorbed, particularly in relation to the behavior of matter at a molecular level. It provides a mathematical formula that quantifies the intensity of radiation as a function of wavelength and temperature, bridging concepts of thermodynamics and quantum mechanics.
Radio waves: Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light, typically ranging from about 1 millimeter to 100 kilometers. They are used for various communication technologies, including radio, television, and wireless networks, showcasing their fundamental role in modern communication systems and the transmission of information.
Reflection: Reflection is the process by which light or electromagnetic waves bounce off a surface, changing direction without being absorbed. This phenomenon is crucial in understanding how we perceive objects and images, as it allows light to travel from a source, interact with surfaces, and return to our eyes. Reflection is a fundamental principle that governs various optical systems, affecting how light behaves in different media and environments.
Refraction: Refraction is the bending of light as it passes from one medium to another due to a change in its speed. This phenomenon occurs because light travels at different speeds in different materials, causing it to change direction when entering a new medium. Refraction plays a critical role in various optical phenomena and is fundamental to understanding how light interacts with biological tissues and other materials.
Snell's Law: Snell's Law describes how light refracts when it passes from one medium to another with a different refractive index. The law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, which helps explain how light behaves in various materials and contributes to our understanding of optics and electromagnetic radiation.
Spectroscopy: Spectroscopy is a technique used to measure and analyze the interaction between light and matter, helping to identify substances based on their light absorption, emission, or scattering characteristics. It plays a crucial role in understanding various biological and chemical processes, making it essential in areas like medical diagnostics, material science, and environmental monitoring.
Total Internal Reflection: Total internal reflection is a phenomenon that occurs when a light wave traveling through a medium hits the boundary of a less dense medium at an angle greater than the critical angle, causing the light to be completely reflected back into the denser medium. This principle is crucial in understanding how light is transmitted in fiber optics, making it essential for applications in communication and medical imaging, as well as in comprehending the behavior of light as an electromagnetic wave.
Ultraviolet radiation: Ultraviolet radiation is a type of electromagnetic radiation that falls between visible light and X-rays on the electromagnetic spectrum, with wavelengths ranging from about 10 nm to 400 nm. This form of radiation is significant for its effects on biological systems, including its ability to cause skin damage and its applications in sterilization and phototherapy.
Visible spectrum: The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye, ranging from approximately 380 nm to 750 nm in wavelength. This range includes all the colors that can be seen, from violet at the shorter wavelengths to red at the longer wavelengths. The visible spectrum plays a critical role in understanding how light interacts with biological tissues and the fundamental principles of light and electromagnetic radiation.
Wavelength: Wavelength is the distance between consecutive peaks (or troughs) of a wave, commonly measured in meters, and is a fundamental property of light and electromagnetic radiation. It plays a critical role in determining how light interacts with matter, influencing absorption, scattering, and transmission processes in various applications such as photodynamic therapy, laser-tissue interactions, and low-level light therapies.