Describe the air pressure at compressions and rarefactions
regions where the particles are pushed closer together (compressions) are at a higher pressure than atmospheric
regions where they are further apart (rarefactions) are at a lower pressure than atmospheric
Equation for wave velocity
V=f λ
What are mechanical waves?
consist of vibrating particles, so they can only move through a material medium. They cannot travel through a vacuum as there are no particles to vibrate
What are EM waves?
do not need a material medium. Oscillations of perpendicular electric and magnetic fields. All EM waves travel at the speed of light
Examples of mechanical waves
sound
ultrasound (f greater than 20kHz)
seismic
water
waves on a string
Explain the nature of EM waves
charged particles such as electrons set up an electric field in the space around them. When they are made to vibrate a magnetic field if also produce. The pattern of electric field and magnetic field vibrations travels outwards as an electromagnetic wave.
What is amplitude?
the maximum displacement of a particle in the wave from its equilibrium position
What is a period?
the time taken to complete one full oscillation
What is frequency?
the number of oscillations per unit time at a point in the wave
1Hz = 1 oscillation per second
Equation for wave speed
wave speed=distance/time
Law of reflection
angle of incidence=angle of reflection
both angles are taken from the normal
Condition for reflection
if the surface is smooth all the normals are parallel to one another so all waves are reflected in an orderly way
What happens when a light ray enters a more optically dense material?
(material with a higher refractive index)
it slows down and bends towards the normal
its wavelength decreases
What happens when a light ray enters a less optically dense material?
(material with a lower refractive index)
it bends away from the normal and speeds up
wavelength increases
What happens when the light rays are travelling along the normal when they enter a new material?
they continue in the same direction but theeirsp changes
Explain partial reflection
it is unusual for all of the incident wave to be reflected, some is likely to be absorbed by the surface material and some will refract into it if the material is transparent.
ENERGY IS CONSERVED AT THE BOUNDARY
incident energy=transmitted energy + reflected energy + absorbed energy
Explain the doppler effect
the apparent change in the frequency and wavelength of waves due to relative motion between the source and observer
Explain reflection from a curved surface
the normal must be perpendicular to the surface so a tangent is drawn perpendicular to the surface where the ray hits it
What is the relationship between the incident and emerging ray when refracted through a rectangular prism
the emerging ray is parallel to the incident ray because the change in speed at both boundaries has the same ratio and the boundaries are parallel
Why is a ray of white light entering a prism split into different colours?
different wavelengths of visible light travel at different speeds in the glass so are refracted by different amounts. Shorter wavelengths slow down more than longer so blue light refracts more than red
The larger the change in speed of the waves as they enter the new medium, the larger the change in…
the direction of the waves
How are sound waves produced?
a vibrating source causes the surrounding medium to vibrate, and this pattern of vibration travels away from the source as sound waves. For example in a loudspeaker the cone vibrates back and forth, moving the air in front of it. The pattern of vibrations travels through the air as sound waves
Explain the relationship between vibrations of the source and the sound produced
the sound waves have the same frequency as the vibrations of the source
the amplitude of the sound waves depends on the amplitude of the vibrations of the source
the speed of the sound waves is determined by the medium through which they travel and not the source
Can sound waves travel through solids, liquids and gases?
yes as it relies on compressions and rarefactions of the medium
speed of the sound depends on the nature of the medium
