waves Flashcards
waves
all types of waves are caused by a vibrating source, and transfer energy and information WITHOUT transferring matter. all waves can be reflected, refracted (and diffracted) they also undergo the doppler effect
transverse waves
the oscillations in a transverse wave are perpendicular to the direction of energy transfer
longitudinal waves
the oscillations in a longitudinal wave are parallel to the direction of energy transfer
properties of a wave
- amplitude
- wavelength
- frequency
- time-period
- wave speed
wavefronts
a line or surface that goes through the crest of the wave
time-period
the time it takes for one complete oscillation
amplitude
the maximum displacement of particles from their equilibrium position
wave speed
how fast the wave is moving
wavelength
the distance of one entire oscillation
frequency
number of complete oscillations per second
examples of transverse waves
- light waves
- ripples on the surface of water
- electromagnetic waves
examples of longitudinal waves
- sound waves
- seismic p waves
- ultrasound
frequency formula
F=1/T
wave speed formula
wave speed=frequency x wavelength
doppler effect
the apparent change in the frequency of a wave caused by relative motion between the source of the wave and its observer
electromagnetic spectrum
radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays
they all travel at the same speed in free space
- radio waves have the longest wavelength and lowest frequency
- gamma rays have the highest frequency and the shortest wavelengths
uses of radio waves
- communication and broadcasting : radio waves are emitted by a transmitter. as they arrive at an aerial they are detected and the information they carry can be received. televisions and FM radios use radio waves with the shorter wavelengths to carry their signals
use of microwaves
- cooking: water molecules in the food absorb the microwaves, they penetrate a few centimeters into the food before being absorbed. the energy is then conducted or convected to other parts of the food
- satellite communication: the signal from a transmitter is transmitted into space, where its picked up by the satellite dish orbiting thousands of kilometres above the Earth.
uses of infrared radiation
- night-vision equipment: the infrared radiation given out by objects can be detected in the dark of night by night-vision equipment. the equipment turns it into an electrical signal, which is displayed on a screen as a picture, allowing things which would otherwise be hidden in the dark to be seen
- heating
uses of visible light
- optical fibres: an optical fibre is a very thin piece of fibre that is made of two different types of glass. the centre is made of a type of glass with a high refractive index, surrounded by a different type of glass with a lower refractive index. since the fibre is very narrow, light entering the inner core always strikes the boundary of the two glasses at an angle greater than the critical angle. no light escapes across this boundary, the light is totally internally reflected
- photography: cameras use a lens to focus visible light onto a light-sensitive film. the lens aperture controls how much light enters the camera. the shutter speed controls how long the film is exposed to the light
uses of ultraviolet light
- fluorescent lamps: ultraviolet radiation is absorbed and then visible light is emitted. the UV radiation is absorbed by a phosphor coating on the inside of the glass which emits visible light
uses of x-rays
- internal structure of objects: x-rays are directed through the object onto a detector plate. the brighter bits are where fewer x-rays get through. they can pass through soft tissues in the body but not bone
uses of gamma rays
- sterilising medical equipment and food: gamma rays kill the microbes
detrimental effects of EM waves
- microwaves: internal heating of body tissue
- infrared: skin burns
- visible light: retina damage
- ultraviolet: blindness, skin cancer (damage to surface cells)
- gamma rays: cell DNA mutation, cancer
precautions for EM waves
- microwaves: don’t stand directly against the microwave ovens while operating
- infrared: protective clothing and eyewear
- ultraviolet: protective goggles and skin creams
- gamma rays: store sources in lead-lined boxes
law of reflection
the angle of incidence=the angle of reflection
speed of light in air + vacuum
300,000,000 m/s
reflection
when a light bounces off an even surface, this is called reflection
refraction
when a wave slows down or speeds up as it crosses the boundary between two medias
snells law formula
n=sin i/sin r
light going from an optically denser to less dense medium…
light going from an optically denser to a less dense medium (higher refractive index to lower) speeds up and bends away from the normal
wavelengths and wavefronts in refraction
when a wave is refracted the wavelengths and wavefronts of the refracted ray are compressed. since the frequency stays the same and v=fxλ, the wave-speed decreases
refraction in glass or water
when a ray of light travels from air to glass or water, it slows down when it crosses the border between the two medias. this change in speed may cause a change in direction. this change in direction of a ray is known as refraction
the critical angle
the critical angle is the smallest possible angle of incidence at which light rays are totally internally reflected
total internal reflection
light going from an optically denser to less dense medium (higher refractive index to lower) speeds up and bends away from the normal. if you keep increasing the angle of incidence then eventually the angle of refraction (r) will reach 90 degrees. eventually i reaches a critical angle (c) for which r=90, the light is refracted along the boundary. above this critical angle, so when i is greater than c, you get total internal reflection meaning that no light leaves the medium
the prismatic periscope
total internal reflection allows us to see objects that aren’t in our direct line of sight using prisms, this is how a periscope works. the ray of light travels into one prism where it is totally internally reflected by 90 degrees. it then travels to another prism lower down where it is totally internally reflected by another 90 degrees. the ray is now travelling parallel to its initial path but at a different height
optical fibres
an optical fibre is a very thin piece of fibre that is made of two different types of glass. the centre is made of a type of glass with a high refractive index, surrounded by a different type of glass with a lower refractive index. since the fibre is very narrow, light entering the inner core always strikes the boundary of the two glasses at an angle greater than the critical angle. no light escapes across this boundary, the light is totally internally reflected
critical angle formula
sin c=1/n
frequency range for human hearing
20-20,000Hz
