6 Waves Topic overview Transverse and longitudinal waves

6 Waves Topic overview Transverse and longitudinal waves • Waves are either transverse or longitudinal • In Transverse waves, the vibrations are at right angles to the direction of energy transfer. • In longitudinal waves, the vibrations are parallel to the direction of energy transfer. • Longitudinal waves have areas of compression (C) and rarefaction (R). • When a wave moves through a medium, the medium does not move permanently. © Hodder & Stoughton 2016

6 Waves Topic overview Properties of waves © Hodder & Stoughton 2016

6 Waves Topic overview Wave equation © Hodder & Stoughton 2016

6 Waves Topic overview Reflection © Hodder & Stoughton 2016

6 Waves Topic overview Transmission and absorption of waves • A mirror is an example of a near perfect reflector. o A hard surface, e. g. walls or rocks will reflect sound. • Other surfaces will either transmit or absorb waves. • Some of the waves may pass through the surface and change speed. o If the wave passes through the surface at an angle, the change of speed will cause a change in direction of travel. • Other surfaces may absorb the wave, e. g. o black paint absorbs light o special surfaces can be used to absorb sound in a concert hall or recording studio o modern buildings are often designed to absorb sound. © Hodder & Stoughton 2016

6 Waves Topic overview Sound waves • Sound o is a longitudinal wave o is produced by vibrating objects o travels through air by a series of compressions and rarefactions o can travel through solids by causing vibrations in the solid. • The compressions and rarefactions in sound waves reaching the eardrum cause the eardrum to vibrate. o This vibration is transferred to the inner ear and causes nerve impulses to be sent to the brain. • The range of human hearing is between 20 Hz and 20 k. Hz although this depends on age and other factors. © Hodder & Stoughton 2016

6 Waves Topic overview Waves for detection and exploration • Ultrasound has frequencies above 20 k. Hz, the upper limit of human hearing. • Ultrasound is used for: o medical imaging, e. g. foetal scanning o sonar: depth measurements and ocean exploration o fishing: to find shoals of fish o industrial imaging, e. g. finding cracks in railway lines. © Hodder & Stoughton 2016

6 Waves Topic overview Waves for detection and exploration • Ultrasound is used for medical imaging, e. g. foetal scanning © Hodder & Stoughton 2016

6 Waves Topic overview Seismic waves • Seismic waves are either: o P-waves which are longitudinal o S-waves which are transverse. • P-waves travel through both solid and liquid rock. They travel at different speeds in each. • S-waves cannot travel through a liquid. • The different waves and their different speeds can be used to investigate the structure of the Earth’s core. © Hodder & Stoughton 2016

6 Waves Topic overview Electromagnetic waves • Electromagnetic waves: o are transverse waves o transfer energy from one place to another o travel through a vacuum o have a wide range of wavelengths from about 10 -12 m to over 103 m o all travel at the same speed in a vacuum; 300 000 m/s (3. 0 x 108 m/s) • The range of wavelengths is called the electromagnetic spectrum. © Hodder & Stoughton 2016

6 Waves Topic overview Properties of electromagnetic waves 1 • Different wavelengths are reflected, refracted, transmitted and absorbed differently by different substances. • Refraction occurs when a wave travels from one medium to another at an angle because the wave has different speeds in the different mediums. © Hodder & Stoughton 2016

6 Waves Topic overview Properties of electromagnetic waves 2 • Radio waves and microwaves are produced by oscillating charges in an electrical circuit. • When received, the radio waves causes an alternating current in a circuit. Electromagnetic waves from infra-red to X-rays: oare emitted when changes in atoms occur ocause changes in atoms when they are absorbed by the atom. • γ-rays are emitted when changes occur in the nucleus of an atom. o. When this happens, the nucleus loses energy. © Hodder & Stoughton 2016

6 Waves Topic overview Hazards of electromagnetic waves • Ultraviolet, X-rays and γ-rays can cause mutations in the cells of the body. o This damages or kills the cells. • Higher frequencies cause more damage. o Ultra-violet radiation from the Sun in excess can cause skin cancer. o X-rays and γ-rays are used medically and the dose received is carefully controlled so that the benefit outweighs the risk. • Radiation dose is measured in sievert (Sv) which is a measure of the damage to living tissue. © Hodder & Stoughton 2016

6 Waves Topic overview Uses and applications of electromagnetic waves • Electromagnetic waves have many uses and applications: o Radio waves: used for communications, radio and television. o Microwaves: used for satellite communication and mobile phones. Microwaves are used for cooking food as some wavelengths are absorbed by water molecules. o Infra-red: used for heating and cooking. Infra-red is also used for remote controls and for night-time photography. o Light: as well as using light to see, it is used in fibre-optic communications. Lasers are used for fibre optics and in medicine and industry. o Ultra-violet: some substances absorb ultra-violet and re-emit it as visible light – called fluorescence. Fluorescent lamps make use of this. Ultra-violet is also used for security marking. o X-rays and γ-rays are used for medical imaging and treatment. © Hodder & Stoughton 2016

6 Waves Topic overview Lenses • Optical instruments use lenses to form images. • Lenses rely on refraction to change the direction of light. • There are two types of lens: o Converging (convex) lens, shown in (a) o Diverging (concave) lens, shown in (b) © Hodder & Stoughton 2016

6 Waves Topic overview Converging lenses • Parallel rays of light entering a convex lens meet at the principal focus. • The distance between the centre of the lens and the principal focus is the focal length of the lens. • When a lens forms an image on a screen, the image is called a real image. © Hodder & Stoughton 2016

6 Waves Topic overview Ray diagrams • Ray diagrams can be used to find the position of an image. • Any two of three rays are drawn from the top of the object: 1. 2. 3. • • a ray parallel to the principal axis which then passes through the focus a ray that passes straight through the centre of the lens undeviated a ray that passes through the focus and emerges parallel to the principal axis. Where the rays meet is the top of the image. The image is inverted. © Hodder & Stoughton 2016

6 Waves Topic overview Magnification © Hodder & Stoughton 2016

6 Waves Topic overview Diverging lenses • Parallel rays of light entering a concave lens are spread out. • The rays appear to come from a point called the virtual focus. • Ray diagrams are drawn in the same way as for a converging lens. • A virtual image is formed where the rays appear to cross; it cannot be seen on a screen. • The image is always: o virtual o upright o diminished. © Hodder & Stoughton 2016

6 Waves Topic overview Visible light • Each colour in the visible spectrum has a different band of frequencies. • Light is reflected: o from a smooth surface in a single direction. This is called specular reflection o from a rough surface in a variety of directions. This is called diffuse reflection. © Hodder & Stoughton 2016

6 Waves Topic overview Visible light • Coloured filters absorb all wavelengths of light apart from a narrow range which they transmit. • Opaque objects have colour because certain wavelengths of light are strongly reflected while others are absorbed. • A black object absorbs all wavelengths and a white object reflects all wavelengths. • Objects that transmit light are transparent or translucent. © Hodder & Stoughton 2016

6 Waves Topic overview Black body radiation • Everything emits and absorbs radiation. o The hotter the body, the more radiation it emits. o As temperature increases more of the energy is emitted as visible light. o The higher the temperature, the shorter the wavelength of visible light emitted. • A perfect black body absorbs all the radiation it receives. • Good absorbers are good emitters so a black body is a good emitter. o A body at constant temperature absorbs radiation at the same rate as it emits radiation. o If a body: § receives more radiation than it emits, the body warms § emits more radiation than it receives, the body cools. © Hodder & Stoughton 2016

6 Waves Topic overview The earth and radiation • The temperature of the Earth depends on many factors including the rate of absorption and emission of radiation and the amount of radiation reflected. • It is hotter at the equator because more radiation is incident on the Earth at the equator than at the poles. © Hodder & Stoughton 2016

6 Waves Topic overview The earth and radiation • At night radiation is emitted into space but none absorbed so the Earth cools. • In the day, clouds reflect some radiation back into space so Earth is cooler. • At night, clouds reflect radiation back from the Earth so keeping Earth warmer. © Hodder & Stoughton 2016