3.1 General Waves Properties
Wave front: Peak of a transverse wave of compression of a longitudinal wave
Speed: How fast the wave is travelling (m/s)
Frequency: number of complete waves in one second (1/period)
Period: number of time for one complete wave
Amplitude: Maximum distance from peak to rest position
Wavelength: Distance from a certain point to the same point of another wave
Speed= Frequency x wavelength
when the water waves pass through shallower water they slow down the waves. When waves slow down they change direction.
–waves slow down when they pass from a less to a more dense material and vice versa
-when a wave is slowed down, it is refracted towards the normal (i > r)
-when a wave is sped up, it is refracted away from the normal (i < r)
-deep water is denser than shallow water
waves bend round the sides of an obstacle, or spread out as they pass through a gap. Wider gaps produce less diffraction.
-Light reflects from mirrors; sound reflects from hard surfaces.
-Light bends when it passes from air into glass or water-Sound bends around when obstacles such as walls and buildings
-Light spreads when it passes through tiny holes and slits.
-Some radio signals can bend round very large obstacles such as hills
-They are caused by vibrations, they are longitudinal, need material to travel through Speed of sound in air- 330m/s liquid- 1400m/s solid-5000m/s
-Speed of sound isn’t affected by pressure, but will increase in higher temperature
-human hearing range= 20Hz- 20000Hz, beyond the hearing range= ultrasound
– To increase sound level, increase amplitude; to increase pitch, increase frequency
speed of sound= 2 x distance to wall/ echo time
Uses of echoes- Echo-sounder ( Measure depth of water), Radar( Measuring the ‘echo times’ of microwave pulses reflected from them), Electronic tape measure.
Reflection of light (Plane Mirror)
Plane (flat) mirrors produce a reflection. Rays from an object reflect off the mirror into our eyes, but we see them behind the mirror.
The image has these properties:
-the image is the same size as the object
-the image is the same distance from the mirror as the object
-a line joining equivalent points of the image and object meet the mirror at a right angle
-the image is virtual: no rays actually pass through the image and the image cannot be formed on a screen
Laws of reflection: Angle of incidence = angle of reflection.
sin i (Air)/ Sin r (Medium)
Speed of light in vacuum/speed of light in medium
sin critical angle= (1/refractive index)
Critical Angle and Total Internal Reflection
Optical fibres: light put in at one end is totally internally reflected until it comes out the other end. This is used in communications where signals are coded and sent along the fibre as pulses of laser light, and in medicine: an endoscope, an instrument used by surgeons to look inside the body. It contains a long bundle of optic fibres.
•Principal focus: the point where rays parallel to the principal axis converge with a converging lens.
•Focal length: distance from the principle focus and the optical centre.
•Principal axis: the line the goes through the optical centre, and the 2 foci.
•Optical centre: the centre of the lens
Dispersion of light
-If a beam of white light is passed through a prism, it splits into a range of colours called a spectrum. Red is deviated the least and Purple is deviated the most.
-travel at the speed of light: 3 × 108m/s
–do not need a medium to travel through i.e. can travel through a vacuum
–are producedby particles oscillating or losing energy in some way
–are transverse waves
Types of electromagnetic waves
Radio waves –radio and television communications
Microwaves –satellite television and telephones (causes internal heating of body tissues)
Infrared – electrical appliances (radiant heaters and grills), remote controllers for televisions and intruder alarms
X-rays – medicine (x-ray photography and killing cancer cells) and security (cause cancer)
Monochromatic light= light with only one colour and one wavelength