P1.5 Waves

Wave basics

• The amplitude is the displacement (the shortest distance) from the rest position to the crest
• The wavelength is the length of a full cycle of the wave eg crest to crest or trough to trough
• Frequency is the number of complete waves passing a certain point per second OR the number of complete waves produced per second
• Frequency is measured in Hertz (Hz) 
• 1 Hz is 1 wave per second

Wave basics

Slinky spring demo        

• Transverse waves (a) have sideways vibrations - the vibrations are at 90 degrees to the direction of energy transfer of the wave
• Light is an example of a transverse wave

• Longitudinal waves (b) have vibrations along the same line - the vibrations are parallel to the direction of energy transfer of the wave
• Sound, ultrasound and shock waves are examples of longitudinal waves

Wave formula

Example: A radio wave has a frequency of 92.2 X 10   Hz. Find its wavelength. The speed of all radio waves is 3 X 10   m/s

Solution: wavelength = velocity / frequency

                              = 3 X 10   / 92.2 X 10

                              = 3.25m

Reflection

Angle of incidence = Angle of reflection

The image formed in a plane mirror

The image formed in a plane mirror is:

• the same size as the object
• as far behind the mirror as the object is in front
• virtual (cannot be projected on to a screen)
• upright
• laterally inverted (right side of object appears to be the left side of the image and vice versa

Diffraction of waves

• The amount of diffraction depends on the size of the gap relative to the wavelength of the wave
• The narrower the gap or the longer the wavelength, the more the wave spreads out

Refraction

• As the light passes from air into glass it bends (refracts) towards the normal
• As the light passes from glass to air it bends (refracts) away from the normal
• Waves are only refracted if they meet at an angle
• If they are travelling along the normal (the angle of incidence is zero) they are not refracted

Electromagnetic Spectrum

• EM waves with different wavelengths (or frequencies) have different properties
• All the different types of EM wave travel at the same speed in a vacuum
• EM waves with higher frequencies have shorter wavelengths
• Different EM waves have different properties and therefore different uses
• EM waves have different uses for communication because of their different wavelenths

Radio waves are used mainly for communication

• Long wavelength radio waves diffract around the curved surface of the Earth. They can also get around hills and into tunnels
• Short wave radio signals can be received at long distances from the transmitter. This is because they are reflected by the Earths electrically charged upper atmosphere, the ionosphere (diagram)
• To receive TV and FM signals, there must be nothing between the aerial and the transmitter - the signal will not bend around hills or travel far through buildings

Microwaves are used for satellite communications and mobile phones

Infrared waves are used for remote controls

Remote controls work by emitting different patterns of infrared waves to send different instrutions to an appliance

Visible light is used for photography

• Cameras use a lens to focus visible light onto a light-sensitive film or electronic sensor
• The lens aperture controls how much light enters the camera
• The shutter speed allows you to control how long the film or sensor is open to the light
• The longer the film or sensor is open to the light, the more light that will enter the camera and react with the film

Sound travels as a wave

• Sound waves are caused by vibrating objects
• The vibrations are passed through the surrounding medium eg air as a series of compressions
• Sound waves are a type of longitudinal wave
• Sound cannot travel in a vacuum (no particles)

The high the frequency, the higher the pitch