OCR (B) Physics GCSE Chapter 1: Radiation and Waves Study Notes

Electromagnetic Radiation and the EM Spectrum

• The Electromagnetic (EM) Spectrum: Consists of seven distinct groups. You are required to recall the order of these waves from the longest wavelength to the shortest wavelength, though specific numerical values for wavelengths are not required.
• Order of the Spectrum (Long to Short Wavelength):
  1. Radio waves
  2. Microwaves
  3. Infrared
  4. Visible light
  5. Ultraviolet
  6. X-rays
  7. Gamma rays
• Differentiating Characteristics:
  • Wavelength ($\lambda$): Radio waves have the longest wavelength; gamma rays have the shortest.
  • Frequency ($f$): Radio waves have the lowest frequency; gamma rays have the highest. The human eye is only capable of detecting a limited range of frequencies known as the visible light spectrum.
  • Energy ($E$): Radio waves have the lowest energy; gamma rays have the highest energy.
• Common Characteristics of EM Radiation:
  • They are all transverse waves.
  • They consist of radiation and do not require particles or a medium to move.
  • They travel at a speed of $3 \times 10\,ms^{-1}$ through a vacuum.
  • Interaction with objects: Depending on the wavelength, objects may absorb, transmit, or reflect the radiation.
• Energy Transfer: EM radiation transfers energy from a source to an absorber.
  • Example 1: Microwaves transfer energy from the source to food.
  • Example 2: The Sun emits energy that is transferred to the Earth.

Production and Absorption of EM Radiation

Changes within molecules, atoms, and nuclei generate and absorb radiation across a vast range of frequencies.

• Gamma Rays: These are emitted specifically from the nuclei of atoms.
• X-rays, Ultraviolet, and Visible Light: These are generated when electrons within atoms lose energy.
• Ultraviolet (UV) and Atmospheric Protection: UV radiation is absorbed by oxygen to produce ozone. Ozone itself also absorbs ultraviolet radiation, which serves to protect life on Earth.
• Infrared (IR): This radiation is both emitted and absorbed by molecules.

Ionisation and Biological Risks

Gamma rays, X-rays, and high-energy ultraviolet rays possess sufficient energy to cause ionisation when they are absorbed by certain atoms.

• Properties of Ionising Radiation: These waves have short wavelengths, high frequencies, and high energy.
• Mechanism of Ionisation: Electrons in an atom are arranged at specific distances from the nucleus. These arrangements can change when EM radiation is absorbed or emitted. Atoms become ions through the loss of outer electrons.
• Biological Impact: Ionisation can cause cellular mutation, which potentially leads to cancer.
  • Radiotherapists: Because they work constantly with gamma sources, they maintain minimal exposure by measures such as leaving the room during procedures.
  • Pilots: Flying at high altitudes exposes pilots to increased levels of UV radiation, resulting in a higher risk of cancer.

Uses of Electromagnetic Radiation

• Radio Waves:
  • Use: Communications.
  • Explanation: They have long wavelengths, allowing them to travel great distances without a loss in quality.
  • Production: Radio waves can be produced by oscillations in electrical circuits.
  • Detection: When absorbed, they create an alternating current (AC) in the conductor at the same frequency as the radio waves.
• Microwaves:
  • Use: Cooking and Satellite communications.
  • Explanation: In cooking, they heat the water or fat within foodstuffs. For communication, they can penetrate the atmosphere to reach orbiting satellites.
• Infrared (IR):
  • Use: Cooking food, infrared cameras, short-range communication, and remote controls.
  • Explanation: Transfers thermal energy.
• Visible Light:
  • Use: Illumination and Fibre optics.
  • Explanation: Visible light provides the best reflection and scattering within glass (other wavelengths are either too short or too long).
• Ultraviolet (UV):
  • Use: Sun tanning, energy-efficient lamps, and sterilisation.
  • Explanation: Radicates the least heat but relatively more energy; it is effective at killing bacteria.
• X-rays:
  • Use: Medical imaging and medical treatment.
  • Explanation: Very high energy allows them to penetrate materials easily.
• Gamma Rays:
  • Use: Medical treatment (Radiotherapy).
  • Explanation: Used to kill cancer cells.

Climate Change and EM Emission

• Emission Principles: All objects emit electromagnetic radiation. The principal frequency of this emission increases as temperature increases. The distribution of intensity and wavelength depends on the body's temperature.
• Effects of Increasing Temperature:
  • A greater amount of radiation is released per second (increased power and intensity).
  • A greater proportion of shorter-wavelength radiation (higher energy waves like X-rays) is released.
• Temperature and Radiation Balance:
  • Constant Temperature: A body is at constant temperature when it absorbs radiation at the same rate it emits it (though it is still actively receiving and radiating energy).
  • Increasing Temperature: Occurs when a body absorbs more energy than it emits.
  • Cooling Down: Occurs when a body releases energy at a greater rate than it absorbs it.
• Temperature of the Earth: Determined by the amount of energy from the Sun that is absorbed by the atmosphere, the amount reflected, and the amount re-radiated.

Wave Motion and Equations

• Wavelength ($\lambda$): The shortest distance between the same points on two consecutive waves.
• Amplitude: The distance from the equilibrium line to the maximum displacement (the crest or the trough).
• Frequency ($f$): The number of complete waves that pass a single point every second.
• Period ($T$): The time taken for one whole wave to completely pass a single point.
• Wave Velocity Equation:
  • $V = f \times \lambda$
  • If frequency increases, velocity increases.
  • If wavelength increases, velocity increases.
• Frequency and Period Equation:
  • $F = \frac{1}{T}$
  • Period is inversely proportional to frequency. A smaller period results in a higher frequency and greater velocity.

Types of Waves and Particle Motion

• Transverse Waves:
  • Examples: Light, all electromagnetic waves, waves on a rope.
  • Structure: Consist of peaks and troughs.
  • Vibration: Particles vibrate at right angles ($90^\circ$) to the direction of wave travel.
• Longitudinal Waves:
  • Examples: Sound waves in air, oscillations on a spring.
  • Structure: Consist of compressions and rarefactions.
  • Vibration: Particles vibrate in the same direction as the direction of wave travel.
• Important Concept: In both wave types, the wave (energy) moves, but the medium (the matter it passes through) does not.
  • Sound: Air regions move right then left due to pressure changes, causing adjacent regions to move; the wave travels, but the air as a whole does not travel with it.
  • Ripple Tank: A ping pong ball in a ripple tank moves up and down but is not carried in the direction of the wave.

Investigating Waves: Sound and Ripple Tanks

• Measuring Sound Speed in Air:
  • Method 1: Make a noise $\sim 50\,m$ from a solid wall; record the time for the echo to return. Use $\text{speed} = \frac{\text{distance}}{\text{time}}$.
  • Method 2: Use two microphones at a large distance apart connected to a datalogger. Record the time difference as sound passes from one to the other.
• Ripple Tank Mechanics: Shallow glass tanks use an oscillating needle or paddle to produce water waves at a set frequency.
  • Visualizing Crests and Troughs: Light shone through the tank creates dark and light patches. Crests contain the most water, scattering the most light, thus appearing as dark patches.
  • Calculating Frequency: Count how many dark maxima pass a point in $60\,s$ and divide by $60$.
  • Calculating Wavelength: Use a stroboscope set to the wave frequency to make the pattern appear fixed; measure the distance between two maxima.
  • Modeling Reflection: Water waves hitting a wall in the tank.
  • Modeling Refraction: Placing a thick glass sheet in the tank makes the water shallower. Because wave speed depends on depth, the ripples slow down, mimicking waves entering a denser medium.

Reflection and Refraction

• Reflection:
  • Waves reflect off flat surfaces.
  • Specular Reflection: Smooth surfaces provide a single, strong reflection.
  • Diffuse Reflection: Rough surfaces scatter light in all directions, appearing matt.
  • Law of Reflection: Angle of incidence = Angle of reflection.
  • Material Interaction: Reflection occurs if the object is opaque and the light is not absorbed. Electrons absorb the light energy and re-emit it as a reflected wave.
• Refraction:
  • Denser Material: Light slows down and bends towards the normal. Shorter wavelengths (e.g., Blue) slow down more than longer wavelengths (e.g., Red).
  • Less Dense Material: Light speeds up and bends away from the normal.
• Refraction through a Prism: Dispersion occurs because different wavelengths refract by different amounts, spreading white light into a rainbow.
• Wave Properties during Refraction (Air to Glass):
  • Wave speed decreases (entering a denser medium).
  • Wavelength ($\lambda$) decreases because $v = f \times \lambda$ must hold true.
  • Frequency remains constant because energy must be conserved; since frequency is unchanged, the color of the light does not change.

Interactions with Materials (Physics Only)

• Transmission: Waves pass through a transparent material. The more transparent the material, the more light passes through. This process allows the wave to emerge on the other side, even if it refracts internally. Opaque materials do not allow transmission.
• Absorption: If the frequency of light matches the energy levels of the electrons in a material, the light is absorbed and not re-emitted as light. Instead, it is re-emitted over time as heat. This removes that specific frequency from the light.
• Opaque Colors: The color of an opaque object is determined by which wavelengths it reflects.
  • A green object reflects only green light and absorbs all other visible frequencies.
  • White objects reflect all wavelengths.
  • Black objects absorb all wavelengths.
• Translucent Materials: These scatter most light and only allow some to pass through.
• Color Filters: These absorb every wavelength except the desired one, which is transmitted through.

Lenses and Vision (Physics Only)

• General Rules: Light passing through the center of a lens does not change direction. Lenses are represented by dashed vertical lines. Focal points are located on either side of the lens where light can converge.
• Concave Lenses:
  • Shape: Thinner at the center than at the edges ("caves" inward).
  • Function: Spreads light outwards (diverging). Light appears to originate from the focal point.
  • Images: Only produces virtual images.
  • Medical Use: Corrects short-sightedness by spreading light out so it focuses correctly on the retina rather than in front of it.
• Convex Lenses:
  • Shape: Wider at the center.
  • Function: Focuses light inwards (converging).
  • Images: Can produce real images (opposite side of the object) or virtual images (same side as the object).
  • Medical Use: Corrects long-sightedness by focusing rays closer. Also used in magnifying glasses and binoculars.
• Magnification Formula:$\text{Magnification} = \frac{\text{Image Height}}{\text{Object Height}}$

How the Ear Works (Physics Only)

1. Collection: The outer ear collects sound and channels it down the ear canal as a pressure air wave.
2. Eardrum Vibration: The wave hits the eardrum (a stretched membrane). Compression forces it inward; rarefaction forces it outward. It vibrates at the same frequency as the sound wave.
3. Amplification: Small bones (stirrup bones) vibrate at the same frequency and act as an amplifier.
4. Fluid Transmission: Vibrations are transmitted to the fluid in the cochlea (inner ear).
5. Hair Cells: As the fluid moves, small hairs lining the cochlea move. Each hair is attuned to specific frequencies.
6. Signal Transmission: When a hair moves, it triggers a nerve cell to release an electrical impulse to the brain for interpretation.

Limitations of Human Hearing (Physics Only)

• Range: Humans can hear between $20\,Hz$ and $20\,kHz$.
• Evolutionary Advantage: This range provides a survival advantage. Humans do not hear ultrasound because they rely on accurate vision rather than sonar for hunting.
• Cochlear Damage: Hairs attuned to high frequencies can be damaged by:
  • Constant loud noise over years.
  • Aging changes in the inner ear.
  • Smoking, diabetes, or chemotherapy.

Ultrasound and Infrasound (Physics Only)

• Ultrasound:
  • Waves are partially reflected when they reach a boundary between two media.
  • Distance calculation: $\text{distance} = \frac{\text{speed} \times \text{time}}{2}$ (divide by two because the wave travels to the object and back).
  • Uses: Imaging under surfaces (finding cracks in metal), fetal scans (non-invasive), and Sonar (calculating seabed depth or locating shoals of fish).
• Infrasound (Seismic Waves): Frequencies lower than $20\,Hz$.
  • P waves: Longitudinal; can pass through both solids and liquids.
  • S waves: Transverse; slower and can only pass through solids.
  • Earth's Core: The lack of S waves on the opposite side of the Earth from an earthquake suggests the Earth's core is liquid.