Waves

• waves transfer energy without transferring matter
• a transverse wave, the direction of vibration is at right angles to the direction of propagation
• electromagnetic radiation, water waves and seismic S-waves (secondary) can be modelled as transverse
• a longitudinal wave, the direction of vibration is parallel to the direction of propagation
• sound waves and seismic P-waves (primary) can be modelled as longitudinal
• for reflection, the angle of incidence is equal to the angle of reflection
• refractive index, n, is the ratio of the speeds of a wave in two different regions
• a virtual image is formed when diverging rays are extrapolated backwards and does not form a visible projection on a screen
• the use of a single lens as a magnifying glass
• the use of converging and diverging lenses to correct long-sightedness and shortsightedness
• the dispersion of light as illustrated by the refraction of white light by a glass prism
• visible light of a single frequency is described as monochromatic
• all electromagnetic waves travel at the same high speed in a vacuum
• use of radio waves; radio and television transmissions, astronomy, radio frequency identification (RFID)
• use of microwaves; satellite television, mobile phones (cell phones), microwave ovens
• use of infrared; electric grills, short range communications such as remote controllers for televisions, intruder alarms, thermal imaging, optical fibres
• use of visible light; vision, photography, illumination
• use of ultraviolet; security marking, detecting fake bank notes, sterilising water
• use of X-rays; medical scanning, security scanners
• use of gamma rays; sterilising food and medical equipment, detection of cancer and its treatment
• harmful effects of microwaves; internal heating of body cells
• harmful effects of infrared; skin burns
• harmful effects of ultraviolet; damage to surface cells and eyes, leading to skin cancer and eye conditions
• harmful effects of X-rays and gamma rays; mutation or damage to cells in the body
• communication with artificial satellites is mainly by microwaves
• some satellite phones use low orbit artificial satellites
• some satellite phones and direct broadcast satellite television use geostationary satellites
• mobile phones (cell phones) and wireless internet use microwaves because microwaves can penetrate some walls and only require a short aerial for transmission and reception
• Bluetooth uses low energy radio waves or microwaves because they can pass through walls but the signal is weakened on doing so
• optical fibres (visible light or infrared) are used for cable television and high-speed broadband because glass is transparent to visible light and some infrared; visible light and short wavelength infrared can carry high rates of data
• a sound can be transmitted as a digital or analogue signal
• the benefits of digital signaling including increased rate of transmission of data and increased range due to accurate signal regeneration
• sound travels faster in solids than in liquids and faster in liquids than in gases
• approximate range of frequencies audible to humans is 20 Hz to 20 000 Hz
• a medium is needed to transmit sound waves
• the speed of sound in air is approximately 330–350 m/s
• an echo is the reflection of sound waves
• ultrasound as sound with a frequency higher than 20 kHz
• uses of ultrasound in nondestructive testing of materials, medical scanning of soft tissue and sonar