Waves 2025 (1) PDF

Electromagnetic waves CHAPTER 1: FUNDAMENTALS AND PROPERTIES OF WAVES REFLECTION AND REFRACTION OPTICAL PHENOMENA Key knowledge From the study design: 1A Introduction to Waves A wave is a transfer of energy without the net transfer of matter. Mechanical waves require a medium through whi...

Electromagnetic waves CHAPTER 1: FUNDAMENTALS AND PROPERTIES OF WAVES REFLECTION AND REFRACTION OPTICAL PHENOMENA Key knowledge From the study design: 1A Introduction to Waves A wave is a transfer of energy without the net transfer of matter. Mechanical waves require a medium through which to travel. Electromagnetic waves do NOT require a medium. In transverse waves (eg. water waves), oscillations are perpendicular to the direction of the wave (direction of energy transfer). In longitudinal waves (eg. sound waves), oscillations are parallel to the direction of the wave (direction of energy transfer). Electromagnetic waves Electromagnetic waves are transverse oscillations of electric and magnetic fields They travel at the speed of light in a vacuum (c = 3.0 × 108 m s-1) They do not require a medium and so can travel through outer space Examples: light, microwaves, gamma rays, radio waves VCAA 2009 VCAA 2016 VCAA 2013 1B Wave fundamentals Waves are described using a number of (related) quantities: Amplitude (units dependent on the wave type): The maximum displacement of particles from their median position. Wavelength (m): The distance between two successive ‘in phase’ points (eg. two peaks). Frequency (Hz): The number of complete cycles per second. Period (s): The time between two successive ‘in phase’ points. Frequency and period are related by: Where: o frequency (Hz or s−1) o period (s) VCAA 2011 Wave fundamentals Displacement-distance graphs of a wave plot the displacement of every point along the wave at one instant in time. The horizontal axis represents the distance of each point from the wave source. The horizontal distance between two in-phase points is the wavelength. Displacement-time graphs of a wave plot the displacement of one specific point over time as the wave passes through it. The horizontal distance between two in-phase points is the period. In both cases, the units on the vertical axis are the units of the wave’s amplitude. Wave fundamentals – Example 1 Wave speed Wave speed is the speed at which a wave transfers energy through a medium. The speed of a wave depends on the medium Wave speed Applying yields the wave equation for all mechanical waves: or and Where: o wave speed (ms−1) o wavelength (m) o period (s) o frequency (Hz or s−1) In a given medium, this implies: Wave frequency The frequency of a wave is determined by the source A vibrating speaker cone creates sound waves at the same frequency (left) A vibrating electron creates electromagnetic waves at the same frequency (right) When a wave moves from one medium to another, its speed will change and so will its wavelength. Frequency is NOT changed Wave speed – Example 1 VCAA 2008 VCAA 2019 1C The electromagnetic spectrum The electromagnetic spectrum comprises all wavelengths of electromagnetic radiation, (ambiguously) divided into ‘regions’. You need to know (in order of priority): I. Higher frequency = shorter wavelength = higher energy (more on the maths of this later in the topic). II. The names of each region and their order in terms of wavelength. III. Roughly the frequencies/wavelengths around the visible spectrum: ~750 nm (red) to ~380 nm (violet). IV. Applications/uses of the different regions (these are summarised on the following slides, but don’t labour over learning them – they should be obvious if you know points I. and II.) Electromagnetic waves Radio waves: Long wavelength enables them to diffract around buildings etc. and travel long distances. Mainly used to transmit radio and television signals. Microwaves: Used to transmit mobile phone and Wi-Fi signals. Match the resonant frequency of water molecules, causing them to vibrate (and therefore increase in temperature) in microwave ovens. Infrared: Used in some forms of signal transmission (such as television remotes). At ‘life’ temperatures (~37°C), objects emit mainly infrared, which can be picked up (and converted to visible light) by thermal imaging cameras. Can also be used to increase the temperature of objects (it’s what we detect as ‘heat’). Electromagnetic waves Visible light: Used for all standard forms of imaging. Ultraviolet: High energy means it can be used in sterilisation processes. Used in ‘black lights’ as it causes other substances to fluoresce (emit visible light). X-rays: High energy makes them highly penetrating. Easily pass through soft tissue but not bone, so can be used for medical imaging. Gamma rays: Ultra-high energy makes them highly penetrating and damaging (highly ionising). VCAA Sample exam VCAA 2018 NHT 1D Refraction and reflection The speed of light through a medium is dependent on its physical properties. This ‘optical density’ is quantified by a material’s refractive index: Where: o refractive index of medium (no units) o speed of light in a vacuum (3.0×108 ms−1) o speed of light in medium (ms−1) Since is the maximum possible speed of light, refractive indices must always be greater than 1 (a larger value of indicates more ‘slowing down’). Refraction and reflection For light moving between two mediums with refractive indices and : Where: o , refractive indices of medium 1 and 2 (no units) o , speed of light in mediums 1 and 2 (ms−1) When light moves into a medium with a higher refractive index: Velocity decreases. Frequency remains unchanged (since waves can’t ‘pile up’ at the boundary). Wavelength therefore decreases (as per ). https://commons.wikimedia.org/wiki/File:Refraction_animatio VCAA 2019 NHT VCAA 2019 Example 1 Refraction and reflection When light strikes the boundary between two mediums with different refractive indices at an angle (the angle of incidence): Some light is reflected back into medium 1 (at the same angle as the angle of incidence). Some light is transmitted, undergoes refraction (change in direction), and enters medium 2 at a different angle (the angle of refraction). The degree to which the light is refracted is given by Snell’s law: Where: o , refractive indices of medium 1 and 2 (no units) o angle of incidence to the normal (°) Example 2 Refraction and reflection Since : When light enters a medium with a higher refractive index, it refracts towards the normal (angle of refraction is smaller than the angle of incidence). When light enters a medium with a lower refractive index, it refracts away from the normal (angle of refraction is greater than the angle of incidence). When entering a medium with a lower refractive index: The angle of refraction will reach 90° before the angle of incidence does. At the critical angle of incidence, light is refracted at 90° along the boundary between the two mediums. Beyond this angle, total internal reflection occurs and no light is transmitted into medium 2. VCAA 2013 (Detailed study 5) Refraction and reflection Since , the critical angle at the boundary between two mediums is: Where: o critical angle of incidence to the normal (°) o , refractive indices of medium 1 and 2 (no units) Total internal reflection is utilised in telecommunications by optical fibres: A ‘core’ with a higher refractive index is surrounded by a ‘cladding’ with a lower refractive index. When light enters the fibre at an appropriate angle (within the ‘cone of acceptance’) total internal reflection occurs continually occurs along its length. VCAA 2017 1E Optical phenomena Chromatic dispersion Rainbows Mirages Optical fibres Dispersion Whilst all light travels at the same speed in a vacuum, the refractive index of a medium varies (slightly) inversely with wavelength: Longer wavelengths experience a lower refractive index (and therefore travel faster and refract less). Shorter wavelengths experience a higher refractive index (and therefore travel slower and refract more). White light is comprised of the entire visible spectrum. When it is passed through a prism: Red light refracts the least (exits ‘most parallel’ to the original path). Violet light refracts the most (exits ‘most bent’ from the original path). This ‘splitting’ of light into its constituent colours/wavelengths is called dispersion. VCAA 2021 VCAA 2019 VCAA 2019 Rainbows Rainbows are formed when: 1. The sun is behind the observer 2. There are water droplets in the air 3. Sunlight disperses through the droplets and reflects into the observer’s eye Mirages A mirage is formed because warm air near the ground has a lower refractive index than the cooler air above Light from the sky bends away from the ground, acting as a mirror (total internal reflection) Observers see an image of the sky near the ground Optical fibres Total internal reflection is used to send light beams through optical fibres Signals can be carried very fast and very efficiently VCAA 2008 (Exam 2 – Detailed study 2) VCAA 2008 (Exam 2 – Detailed study 2) VCAA 2008 (Exam 2 – Detailed study 2) VCAA 2018 VCAA 2018 VCAA 2018