Electromagnetic Radiation, Wave Properties, and Astrophysics Knowledge Organiser

Fundamentals and Properties of Waves

General Wave Concepts:
- A wave is a mechanism for transferring information or energy from one location to another.
- Waves can cause matter to oscillate without causing a permanent displacement of the medium itself.
- Waves are capable of being reflected or refracted at boundaries.
Classifications of Waves:
- Waves are categorized into two primary forms: transverse waves and longitudinal waves.
- Transverse Waves:
- In transverse waves, energy moves perpendicular to the direction of oscillation of the matter (e.g., water waves and light waves).
- The highest displacement point on a transverse wave is called the peak.
- The lowest displacement point on a transverse wave is called the trough.
- Longitudinal Waves:
- In longitudinal waves, energy moves parallel to the direction of oscillation of the matter (e.g., sound waves).
- Longitudinal waves propagate through alternating regions of high pressure, known as compressions, and regions of low pressure, known as rarefactions.
Nature of Sound Waves:
- Sound travels exclusively in waves.
- Sound is generated when an object causes surrounding air particles to vibrate.
- The frequency of a sound wave determines its pitch: increasing the frequency produces a higher pitch sound.
- The amplitude of a sound wave determines its loudness: increasing the amplitude increases the volume.
- Human Hearing and Ultrasound:
- The standard human hearing frequency range spans from $20\,\text{Hz}$ to $20000\,\text{Hz}$ .
- Sound waves with frequencies exceeding $20000\,\text{Hz}$ are classified as ultrasound.

Wave Measurements and Calculations

Key Wave Parameters:
- Frequency ( $f$ ): Defined as the number of waves that pass a given point each second. It is measured in hertz ( $\text{Hz}$ ).
- Period ( $T$ ): Defined as the length of time it takes one complete wave cycle (one second to pass a given point).
- Wavelength ( $\lambda$ ): The physical distance from a point on one wave to the identical point on the subsequent wave (e.g., peak to peak or trough to trough), measured in meters ( $\text{m}$ ).
- Amplitude: The maximum distance of a point on the wave from its central rest position.
- Velocity ( $v$ or $S$ ): The speed of a wave in the specific direction in which it is travelling.
Mathematical Wave Equations:
- The fundamental displacement velocity formula:

$\text{Velocity of the wave} = \frac{\text{displacement}}{\text{time}}$

$S = \frac{d}{t}$

The universal wave equation relating velocity, frequency, and wavelength:

$\text{Velocity of the wave}\,(\text{m/s}) = \text{frequency}\,(\text{Hz}) \times \text{wavelength}\,(\text{m})$

$v = f \times \lambda$

Practical Methods for Observing and Measuring Wave Speeds:
- The speed of water waves can be experimentally determined using a ripple tank setup.
- Components of the ripple tank apparatus include a power supply, lamp, motor, wooden rod (dipper), transparent water tank, white card placed beneath the tank to project shadows of wavefronts, and a metre ruler to measure wave displacement and wavelength.
- The speed of a wave travelling along a vibrating string can also be directly measured.

Wave types, ripple tank apparatus, and electromagnetic spectrum wavelength comparison

The Electromagnetic Spectrum Overview

Core Characteristics of Electromagnetic Waves:
- Electromagnetic (EM) waves are transverse waves that transfer energy from the source of the waves to an absorber.
- Electromagnetic waves form an unbroken, continuous spectrum.
- All types of electromagnetic waves travel at the identical velocity through a vacuum (outer space) or through air: $3 \times 10^6\,\text{m/s}$ .
Arrangement of the Spectrum:
- Electromagnetic waves are categorized based on their wavelength ( $\lambda$ ) and frequency ( $f$ ).
- Arranged in order from longest wavelength / lowest frequency to shortest wavelength / highest frequency, the groups are:
1. Radio waves
2. Microwaves
3. Infrared radiation
4. Visible light (ordered red to violet)
5. Ultraviolet waves
6. X-rays
7. Gamma rays
Approximate Scales of Electromagnetic Wavelengths:
- Radio waves: comparable in scale to buildings ( $10^3\,\text{m}$ ) and humans ( $10^0\,\text{m}$ .
- Microwaves: comparable to butterflies ( $10^{-2}\,\text{m}$ ) and needle points ( $10^{-3}\,\text{m}$ ).
- Infrared: comparable to protozoans ( $10^{-5}\,\text{m}$ ).
- Visible light: approximately $0.5 \times 10^{-6}\,\text{m}$ .
- Ultraviolet: comparable to molecules ( $10^{-8}\,\text{m}$ ).
- X-rays: comparable to atoms ( $10^{-10}\,\text{m}$ ).
- Gamma rays: comparable to atomic nuclei ( $10^{-14}\,\text{m}$ ).
Higher Tier (HT Only) Interaction Principles:
- Different substances may absorb, transmit, refract, or reflect electromagnetic waves in ways that differ depending on the wavelength of the incident radiation.
- Wave effects such as refraction arise directly due to differences in the propagation velocity of the waves across differing substances.
- Wave front diagrams are used to illustrate refraction by modeling the change in wave speed that occurs across the interface between two dissimilar media.

Applications and Properties of Electromagnetic Waves

Suitability and Properties:
- Each electromagnetic wave type is uniquely suitable for specific practical applications due to its particular wavelength and frequency band.
Practical Applications Across Groups:
- Radio Waves:
- Employed in television broadcasting and radio communications.
- (HT Only) Radio waves can be artificially produced by oscillations in electrical circuits (e.g., alternating current in an antenna transmitter).
- (HT Only) When radio waves are absorbed by an aerial receiver, they can induce an alternating current with the exact same frequency as the incoming radio wave itself, thereby regenerating electrical oscillations within the circuit.
- Microwaves:
- Employed in satellite communications and cooking food.
- (HT Only) High-frequency microwaves are readily absorbed by water and fat molecules in food, increasing their internal thermal energy.
- (HT Only) Microwaves easily pass through Earth's atmosphere without being absorbed or reflected, making them suited for signal transmissions between Earth base stations and satellites in orbit.
- Infrared Radiation:
- Employed in electrical heaters, thermal cooking of food, and infrared thermal imaging cameras.
- Visible Light:
- The human eye is biologically sensitive only to visible light, detecting a narrow band of the total electromagnetic spectrum.
- The visible light spectrum consists of seven distinct colors in increasing frequency: red, orange, yellow, green, blue, indigo, and violet.
- Red light has the longest wavelength and lowest frequency; violet light has the shortest wavelength and highest frequency.
- Employed in high-speed fibre optic data communications.
- Ultraviolet Waves:
- Employed in energy-efficient fluorescent lamps and artificial sun tanning equipment.
- X-Rays and Gamma Rays:
- Employed extensively in medical imaging (diagnostic radiography) and cancer radiotherapy treatments.

Applications and typical sources across the electromagnetic spectrum

Hazards and Risks of Electromagnetic Radiation

Radiation Origin and Atomic Transitions:
- Changes within atoms and the electron arrangements or nuclei of atoms can cause electromagnetic waves to be generated or absorbed across a broad frequency band.
- Gamma rays specifically originate from structural changes within the nucleus of an unstable atom.
Biological Tissue Hazards:
- High-frequency electromagnetic radiation—specifically ultraviolet waves, X-rays, and gamma rays—can have hazardous effects on human body tissues.
- The severity of biological harm depends directly on the specific type of radiation and the total absorbed dose.
- Radiation Dose: Measured in units of sieverts ( $\text{Sv}$ ), serving as a direct quantitative measure of the risk of harm resulting from the exposure of human body tissue to ionizing or penetrating radiation.
Specific Health Consequences:
- Ultraviolet Waves: Cause premature aging of the skin and significantly increase the risk of skin cancer.
- X-Rays and Gamma Rays: Classed as ionising radiation, meaning they carry sufficient energy to strip electrons from atoms; this causes chemical changes, dangerous gene mutations, and carcinogenic tumor development.

Reflection and Refraction (Physics Only)

Types of Reflection:
- Specular Reflection: Occurs when parallel light waves reflect off a completely smooth, flat surface in a single uniform direction.
- Diffuse Reflection: Occurs when light reflects off a rough or uneven surface, scattering the reflected rays in multiple directions.
Principles of Refraction:
- Refraction is the change of direction observed in a wave when it passes across an interface from one medium to another, caused by a change in its propagation speed.
- When a wave passes from a less dense medium into a more dense medium, it slows down, which causes the wave path to bend towards the normal.
- When an incident light ray encounters an interface between two different media, a portion of the energy is typically reflected while the remainder is transmitted and refracted.
Ray Terminology and Conventions:
- Incident Ray: The incoming ray travelling towards the boundary surface.
- Normal Line: An imaginary reference line drawn at right angles ( $90^\circ$ ) to the boundary surface at the exact point of incidence.
- Angle of Incidence: The measured angle between the incident ray and the normal line.
- Refracted Ray: The ray travelling through the second medium whose path has bent relative to the normal.

$Ray diagrams of light refraction at a boundary and light convergence through a convex lens$

Lenses, Image Formation, and Magnification (Physics Only)

Optical Transmission Properties of Matter:
- Transparent: Objects that allow most incident light to transmit cleanly through them.
- Translucent: Objects that allow only some incident light to transmit through, scattering the rest.
- Opaque: Objects that do not transmit light at all; light is absorbed or reflected, causing distinct shadows to form behind them.
Lens Types and Characteristics:
- A lens forms images by systematically refracting light rays passing through it.
- Convex Lens (Converging):
- Geometrically thicker in the middle than at the outer edges.
- Causes parallel rays of incoming light to converge and meet at a single focal point called the principal focus.
- The distance from the center of the lens to the principal focus is termed the focal length.
- Capable of forming either real or virtual images depending on object distance.
- Practical applications include photographic cameras, human eyes, optical projectors, and handheld magnifying glasses.
- Represented in formal ray diagrams by a vertical line with two outward-facing arrows at its tips.
- Concave Lens (Diverging):
- Geometrically thinner in the middle than at the outer edges.
- Causes parallel rays of incoming light to diverge outward away from a common point.
- The resulting image is always virtual.
- Practical applications include peepholes in doors.
- Represented in formal ray diagrams by a vertical line with two inward-facing arrows at its tips.
Image Descriptors:
- An image formed by any lens system is classified along three pairs of attributes:
- Upright versus inverted (orientation relative to the original object).
- Magnified versus diminished (relative size compared to the original object).
- Real (formed where light rays physically converge and can be projected onto a screen) versus virtual (appears to originate from behind the lens where diverging light rays appear to meet).
Magnification Calculations:
- Magnification is calculated using the linear size relationship:

$\text{magnification} = \frac{\text{image height}}{\text{object height}}$

Units: Magnification is a pure dimensionless ratio and therefore carries no units.
Both image height and object height must be measured and entered using identical units, typically either millimeters ( $\text{mm}$ ) or centimeters ( $\text{cm}$ ).

Colour and Absorption of Light (Physics Only)

Spectral Color Bands:
- Each individual color within the visible light spectrum corresponds to its own distinct, narrow band of wavelength and frequency.
Energy Absorption at Surfaces:
- Waves can be absorbed at the boundary between two different materials.
- When electromagnetic waves are absorbed by a surface, the radiant energy of the wave is transferred directly to the particles comprising the surface, elevating their internal kinetic and thermal energy.
Mechanism of Colour Filters:
- Colour filters operate through selective transmission and absorption: they absorb certain specific wavelengths (colours) while transmitting other designated wavelengths (colours).
Colour Perception of Opaque Objects:
- The perceived color of an opaque object is determined strictly by which wavelengths of visible light are most strongly reflected from its surface.
- Wavelengths of light that are not reflected are absorbed by the material.
- If all wavelengths of the visible spectrum are reflected equally, the object appears white.
- If all wavelengths of the visible spectrum are completely absorbed, the object appears black.

Infrared Radiation, Surfaces, and Leslie Cube Practical

Thermal Emission and Absorption by Objects:
- Every physical body, irrespective of its temperature, continually emits and absorbs infrared radiation.
- The hotter a body is, the greater the quantity of infrared radiation it radiates within a given period of time.
Surface Texture and Color Dependences:
- Dark, matte surfaces: Highly effective, efficient emitters and absorbers of infrared radiation.
- White, silver, shiny surfaces: Poor emitters and poor absorbers (good reflectors) of infrared radiation, as they reflect virtually all incident visible light and infrared wavelengths.
- Practical Radiator Design: Household heating radiators are frequently painted white so that they release their infrared radiation steadily over an extended period rather than discharging thermal energy too quickly.
Required Practical Activity (RPA) - Infrared Emission and Absorption:
- Aim: Investigate how the quantity of infrared radiation absorbed or radiated by a surface depends on the nature and finish of that surface.
- Apparatus:
- An infrared thermometer is used to accurately measure the emission of infrared radiation from surfaces without requiring direct physical contact.
- A Leslie cube is utilized to compare different surfaces directly. The Leslie cube is a hollow metal cube manufactured with four distinct vertical faces (e.g., matte black, gloss black, white, and shiny silver/metallic), which is filled with boiling or hot water to maintain all four surfaces at an identical internal temperature.

Black Body Radiation and Thermal Equilibrium (Physics Only)

Definition of a Black Body:
- No known physical substance in nature is perfect at absorbing or emitting all incident radiation, but objects that closely approach this behavior are classified as black bodies.
- A perfect black body is a theoretical object that absorbs $100\%$ of the radiation incident upon it, reflecting or transmitting none.
- Because an ideal absorber is also an ideal emitter, a perfect black body represents the theoretical best possible emitter of radiation.
Astronomical Approximations:
- Astronomical stars are treated as black bodies because they are extremely efficient emitters and absorbers across almost all wavelengths in the electromagnetic spectrum.
- Planets and black holes also represent near-perfect black bodies in astrophysical systems.
Thermal Equilibrium and Radiation Balance:
- A body held at a constant temperature absorbs electromagnetic radiation at the exact same rate at which it emits radiation.
- If the rate of radiation absorption exceeds the rate of radiation emission, the body's internal energy and overall temperature increase.
- If the rate of emission exceeds the rate of absorption, the body cools, and its temperature decreases.

Earth's Radiation Balance and the Greenhouse Effect (Physics Only)

Factors Determining Earth's Global Temperature:
- The global temperature of planet Earth is governed by a dynamic thermodynamic balance determined by:
- The rate at which solar and cosmic radiation is absorbed by the surface and atmosphere.
- The rate at which terrestrial radiation is emitted back out into space.
- The proportion of incoming radiation that is reflected back into space (albedo).
- The atmospheric concentration of greenhouse gases.
Atmospheric Energy Transfer Mechanisms:
- Earth absorbs incoming short-wavelength infrared radiation and visible light from the Sun, increasing its internal energy.
- The heated Earth transfers thermal energy back into the lower atmosphere via physical conduction and atmospheric convection.
- The surface of the Earth continually emits longer-wavelength infrared radiation outward toward space.
- Naturally occurring greenhouse gases in the atmosphere absorb portions of this emitted terrestrial infrared radiation.
- Once absorbed, these greenhouse gas molecules re-radiate infrared radiation in all directions, including back down towards the Earth's surface where it is reabsorbed.
Natural versus Enhanced Greenhouse Effect:
- Natural Greenhouse Effect: A vital natural phenomenon caused by baseline atmospheric greenhouse gases that stabilizes the surface temperature of the Earth at a level capable of sustaining biological life.
- Enhanced Greenhouse Effect: Human industrial and land-use activities—predominantly the large-scale burning of fossil fuels and extensive deforestation—are substantially increasing the atmospheric concentration of greenhouse gases.
- As greenhouse gas concentrations rise, a greater proportion of outbound infrared radiation is absorbed and re-emitted toward the surface, leading to an enhanced greenhouse effect and increased planetary temperatures.

Solar radiation and the enhanced greenhouse effect in Earth's atmosphere

Space Physics: Red-Shift, Dark Energy, and Dark Matter (Physics Only)

Stellar Emission and Absorption Spectra:
- Light emitted from a star does not contain an uninterrupted, continuous spectrum of all electromagnetic wavelengths.
- Chemical elements present in the star's cooler outer layers absorb specific characteristic wavelengths of light.
- When the star's spectrum is passed through a spectroscope, these absorbed wavelengths manifest as distinctive dark absorption lines across the spectrum.
- Because different elements possess unique electronic configurations, each element produces its own distinct absorption and emission spectra, acting as a chemical fingerprint.
Astronomical Red-Shift:
- Astronomers observe an increase in the measured wavelength of light coming from distant galaxies, shifting spectral absorption lines towards the red end of the spectrum (known as red-shift).
- The further away a galaxy is located from Earth, the faster it is moving away, and the greater the observed increase in wavelength.
- This observed red-shift provides evidence that space itself (the universe) is expanding outward in all directions.
- The systematic relationship between a galaxy's recessional speed and its distance provides observational evidence supporting the Big Bang theory.
The Big Bang Theory:
- States that the entire universe originated from an initial, extremely hot, and infinitely dense point-like region of space that expanded outward.
- Scientific theories such as the Big Bang model are constructed and refined directly through observational evidence and empirical measurements.
Modern Cosmological Discoveries:
- Accelerated Expansion: Since 1998 onwards, precision observations of distant type Ia supernovae revealed that distant galaxies are receding away from each other at an accelerating rate.
- Dark Energy: Scientists do not yet fully understand the underlying mechanism driving the accelerating expansion of the universe, but have proposed the existence of dark energy, which is theorized to constitute over $\frac{2}{3}$ of the total energy-mass content of the Universe.
- Dark Matter: Astronomers have demonstrated that galaxies rotate at speeds far too high for the gravitational pull exerted solely by the observable mass of their stars. This led to the deduction that there exists undetected, invisible mass that interacts via gravity, termed dark matter.
- Current Composition of the Universe: Only approximately $5\%$ of the entire universe is comprised of the standard baryonic matter that modern science currently understands.