Waves : a comprehensive review of the waves physics topic, Study notes of Physics

Download Waves : a comprehensive review of the waves physics topic and more Study notes Physics in PDF only on Docsity! Waves- A review 1 Waves- A review How to use this review: This review of the Waves topic was created by Anshul Jadhav. These notes are NOT a substitute for actual learning and being taught in school, and as such the reader is warned to use these notes at their own risk. While I have tried to make these notes as comprehensive and accurate as possible, there may be minor inaccuracies as a result of simplifications when writing. Please let me know if any mistakes or inaccuracies are found. Please be patient and wait for images and gifs to load. Also, these notes are best used in light mode, as some diagrams are hard to see in dark mode. These notes are set out to inform loosely based on the outcome sheets provided at the start of the unit but do go into more depth in many sections, with extension notes for more advanced readers. If you just using these notes to study the test briefly, summaries of key points are in callouts like this: 💡 Example of a summary callout A wave is essentially a disturbance that travels through space and matter from point to point transferring energy. Some must have a medium to travel through, made of the movements of particles and as a result, are slower (mechanical waves). Others can travel through a vacuum, made of oscillating fields, and as Waves- A review 2 such travel at incredibly high speeds (electromagnetic waves). Waves are all around us, from ocean waves to seismic waves to even light and sound waves. Waves are grouped into two fairly distinct categories: transverse and longitudinal (compression). 💡 Waves are disturbances that transfer energy through space and matter. They can travel through a medium or a vacuum and are categorized as transverse or longitudinal. Waves include ocean waves, seismic waves, light waves, and sound waves. Transverse Waves Transverse waves are one of the two main types of waves, notably having many distinguishable features as shown in the diagram below. Transverse waves have several features including wavelength, crest, trough, amplitude, distance and the equilibrium position. Wavelength: The wavelength is the distance between any two consecutive crests, troughs or other matching parts of a wave, measured in meters (m). Crest and Trough: The crest of a wave is the peak or the highest point in a wave before it begins to go downwards. The trough is the opposite of the crest. Amplitude: The amplitude of a wave is the maximum distance between a moving particleʼs resting position and displaced position, or the distance between the equilibrium line and the waveʼs crest or trough. The amplitude of a wave really means the strength or energy of the whole wave. In the case of EM Waves- A review 5 As shown in the diagram above, longitudinal waves have some of the same features as the transverse wave, namely the wavelength. Longitudinal waves must travel through matter and really are just the movement (vibration) of particles and so are mechanical waves. Sound, for example, is a longitudinal wave, and is basically the movement of the particles in the medium it travels through. In sound waves, the particles of the medium move parallel to the direction of energy transfer. As energy transfers into particles, they move in “clumpsˮ that form the compressions shown. As they move along, the spaces between them expand, forming rarefactions. As shown below, the amplitude of a longitudinal wave is also detectable, shown as the maximum distance a particle is displaced from its equilibrium points (see how the red reference particles move back and forth) It is often difficult to show longitudinal waves and their characteristics, so they are commonly shown in the form of a transverse wave for simplicity. In the form of a transverse wave, they are much easier to interpret. The amplitude of the wave is now easier to measure as well. As for sound waves, features like amplitude directly correspond to real-world traits. Waves with higher amplitude (transverse or longitudinal) are stronger, so for sound waves, this means we find them louder. As mentioned before, another feature of the wave is the frequency of a wave. The frequency of a wave is the number of This diagram shows the movement of particles in a longitudinal wave. The particles move parallel to energy transfer. Waves- A review 6 complete oscillations a particle in a wave makes per second, or equally how many waves pass a given point in space per second. Frequency is measured in Hertz (Hz) and is why we hear that sound is of a certain pitch, in hertz. or Sound waves, which means a higher frequency results in a higher pitch for the wave, and vice versa. Humans can only hear sounds with a frequency between roughly 2020,000 Hz. Sounds with frequencies lower than this are called infrasound and are used by some animals like whales and elephants to communicate long distances. Ultrasound, on the other hand, is for frequencies above 20,000 Hz and has high energy. They have medical uses in generating images of the body, as high energy waves are bounced off the body into a sensor. 💡 Longitudinal waves are mechanical waves that travel through matter and involve the movement of particles parallel to the direction of energy transfer. Sound waves are an example of longitudinal waves. They have compressions and rarefactions, and their amplitude and frequency determine their strength and pitch. Longitudinal waves are often represented as transverse waves for simplicity. The Wave Equation The wave equation is a formula that links the frequency, wavelength and wave speed for a given wave. As we know the frequency of a wave is equal to the number of oscillations of a particle in a wave in one second. A new term, however, is the period of a wave, which is equal to the time taken for one oscillation of a particle in a wave. Thinking about it, we find that frequency is essentially the number of waves per second. Period on the other hand is seconds per wave. As such, it is evident that the period is the inverse of the frequency. When calculating the speed of a wave, we understand that speed is distance over time, and in waves that can be found with the wavelength (distance between consecutive matching points) divided by period (time taken for a wave to travel a distance). Waves- A review 7 Since the period is not often easily found or given in an equation, we can use the frequency as the inverse of the period as a substitute( ) This equation is useful for many reasons, including finding one of the values once the equation is rearranged, for any type of wave. 💡 The Wave Equation: Speed (of a wave)  Wavelength x Frequency The waveʼs period is the time taken for a particle in a wave to oscillate Extension- Further uses of the Wave equation Not relevant) One other important reason is that once you consider EM waves in a vacuum, you can substitute V for the speed of light, c. This means there is a direct relationship between the speed of light and the wavelength and frequency of a light wave. Continuing from this it becomes evident that the wavelength of a wave and its frequency are inversely proportional to each other, meaning that as you increase the frequency of a wave, its wavelength decreases. Linking back to sound from earlier, infrasound has a low frequency and therefore a large wavelength, meaning the wave can travel a longer distance before running out of energy. The reverse can be said about ultrasound. Electromagnetic Radiation Electromagnetic EM radiation refers to a form of energy that is constituted by an electric field and a magnetic field. They travel in the form of a transverse wave and are self-propagating*. Since they are just the movement of fields they do not require a medium to travel through, and travel at incredibly high speeds. Speed = Time Distance = Period Wavelength f 1 Speed = Wavelength× Frequency V = fλ Waves- A review 10 At the lower end of the spectrum (with the lower frequencies and longer wavelengths), we have radio waves. Radio waves have wavelengths of between 1mm to 100 km and have relatively low energy. As such, they are the least penetrating of waves. Radio waves are used to broadcast signals, interpreted as audio, in everyday life. Radio waves have a frequency of between 10kHz and 100gHz. Microwaves have the next longest wavelengths, between 30 cm to 1 mm. They are slightly more penetrating and are used to broadcast signals and to cook food. Food is cooked with microwaves, as they excite water molecules in food it vibrates and heats up. They have frequencies between 1 and 1000 GHz. Infrared waves are EM waves with shorter wavelengths and higher energies. Their wavelength is between 780 nm and 1 mm and has frequencies of between 300 GHz to 400 THz. IR radiation is commonly known as heat radiation, as matter produces EM waves in the form of IR, when excited. IR is not visible to the human eye but can be felt as heat energy if it is intense enough. Visible light is the form of EM radiation we are most familiar with. With wavelengths between 380 to 740 nanometres, visible light is detectable by the human eye and makes up everything we can see. It has frequencies of 400 to 800 THz and makes up the middle of the electromagnetic spectrum. Visible light is broken down into 7 base colours; Red 700nm), Orange 600nm), Yellow 580nm), Green 540nm), Blue 470nm), Indigo 430nm) and Violet 380nm), listed from longest to shortest wavelength, from left to right in the diagram above. UV light has wavelengths of between 10 to 380 nanometres and cannot be detected by the human eye. and has a frequency of between 800 THz and 30,000 THz. It is quite high energy and is used for sterilisation as this light can kill some bacteria. UV light has extensive medical and industrial uses, and as it has a small wavelength, can penetrate many materials. X-rays make up the higher end of the spectrum, constituted of soft and hard X rays, the former having wavelengths of 100 picometers to 10 nanometres and frequencies of 30 petahertz to 30 exahertz. Hard X-rays have shorter wavelengths and have higher energy and penetrating power. X-rays can penetrate human flesh and are used in the medical industry to detect broken bones. The waves pass through flesh and bounce off bones back into sensors, that generate an image of the skeletal structure. Waves- A review 11 Gamma rays have the shortest wavelengths and as such the highest energy of waves on the EM spectrum. Gamma rays have wavelengths of less than 100 picometers and frequencies of more than 3  10^19 Hz. Such high energy and short wavelength mean that gamma rays can pass through nearly all materials, only stopped by lead or several metres of concrete. At such a small scale gamma rays pass through flesh and tear DNA strands, resulting in genetic mutations and cancers. On the other hand, it can be used medically, systematically eliminating any targeted bacteria or cancers in the body. 💡 EM spectrum (from longest wavelengths downwards): Radio waves 1mm to 100 km), Microwaves 30cm-1mm), Infrared 780nm-1mm), Visible light (all colours we see; 380700 nm), UV light 10380nm), X rays 100pm-10nm), Gamma rays 100pm). Frequency is inverse and goes from lowest to highest. Optics- Manipulating light When light hits an object, one of three things can happen; It can be reflected, absorbed or refracted. Reflection Reflection is the transformation in which light “bounces backˮ from a material it impacts. This happens when the material cannot absorb the incoming lightʼs wavelengths, and so they are reflected back. Light is always reflected at the same angle that it came in at. This is called the law of reflection In essence, this means that as light hits an object, it will bounce back at the same angle, at a direction determined by a hypothetical line perpendicular to the surface called the normal. Waves- A review 12 Depending on the surface the light impacts, the reflection can be either diffuse or specular. Specular reflection occurs when the surface the light hits is smooth and even. Light that hits this surface bounces off in predictable even directions. As a result, the material looks shiny and as you look at it, you can see a reflection of yourself in it. Things like mirrors demonstrate this property. Diffuse reflection, on the other hand, occurs when the material is uneven or rough, and has small imperfections that cause the light to bounce off in random, hard-to-calculate directions. Most materials are like this and look dull. In specular reflection, for smooth surfaces like mirrors, light beams reflect predictably and in the same direction. When you look in a mirror, light from the sun or other light sources reflects off your face into the mirror and back into your eyes. Since the mirror is smooth, you see yourself in the mirror. In diffuse reflection, the light bounces off the object in all different directions. This means that while you cannot see yourself in it, most of the light from your face goes in directions away from your eyes. We can still see the object, though, as the light from the sun and other objects still reflects off it and into your eyes. The colour you see the object as is determined by which wavelengths of light the object reflects and which it absorbs. For example; a leaf we see as green, is in reality absorbing all colours of light except green. The green light reflected goes into your eyes and processed. A white object reflects all colours of light back to your eyes (since white light contains all colours of visible light). When there is no light reflecting off of an object and into your eyes, the object appears black. Waves- A review 15 This is why things in water seem to be closer than they really are; the light from the object refracts as it hits leaves the water and reaches your eye at a different angle than it really is. If light hits an object at 90° to its surface (on the normal), it will not refract, instead passing straight through (still slowing down). 💡 As light passes through a transparent object it bends (changes angle), then it goes back to its original angle once it leaves the object. Refraction can also occur for curved, transparent objects. These are called lenses. A lens curved outwards (on both sides) is called biconvex (or just convex), and one that curves inwards is called biconcave (or just concave). The red lines show the path of light through a medium Waves- A review 16 A convex lens refracts the light rays inwards, where they converge at a focal point. This focal point has the energy of all the light rays combined. Our eyes use convex lenses to focus the incoming light into one point that the brain can interpret. A concave lens refracts the light rays outwards, so they diverge. The focal point is in front of the lens in a concave lens. 💡 A concave lens makes light diverge, focal point in front of lens. A convex lens makes light converge, focal point behind lens. Radioactivity Recalling the periodic table unit from earlier, isotopes are versions of elements that have unusual mass numbers, they have different numbers of neutrons to a standard atom of that element. If an element atom has a different number of neutrons, its charge doesnʼt change, but its mass does. It is important to note, that the nucleus of an atom generally likes to have the same or just more number of neutrons than protons. When the nucleus of an atom has less or more neutrons than usual, it may become unstable. Sometimes, if the change is minimal, the isotope of the element may not be unstable and instead form a similar particle with slightly different properties. However for many atoms, in fact, all atoms after uranium in the periodic table, the isotope of the element is Waves- A review 17 unstable. This means the nucleus has too much energy and wants to release some of this energy and become stable. The release of this energy is called radiation, and the process of a nucleus releasing energy is called radioactive decay. There are three types of radiation that can be released; Alpha (α), Beta (β), and Gamma (γ). 💡 Isotopes are versions of an element with a different number of neutrons than usual. They can be radioactive, as since they have a different mass, they have different properties. If they are unstable (radioactive) they will release some energy to become stable (radiation). Alpha radiation is the emission of 2 protons and 2 neutrons (or a helium nucleus) from the nucleus of a radioactive atom. Alpha radiation is relatively low energy and quite large, so it can only travel a short distance in the air. It is stopped by air or paper and as it is just protons and neutrons, is positively charged and can be influenced by magnetic fields. When an atom releases an alpha particle, its atomic number decreases by 2 (as the number of protons decreases by 2, and its mass number decreases by 4 2 protons  2 neutrons). For example: In a decay equation, we use the symbol for Helium to show the alpha particle (for simplicity) Beta radiation is the emission of a high-speed electron from the nucleus. As the nucleus becomes unstable, a neutron in the nucleus turns into 1 proton and 1 neutron. The proton stays in the nucleus, increasing its atomic number by 1 (as the atomic number is the number of protons). The mass stays the same as a neutron is replaced by a proton and the electron is practically massless. Beta radiation (the one high-speed electron), can travel a bit further than alpha Uranium-235 releases an alpha particle- its mass decreases by 4, and its atomic number by 2.