OCR A-Level Physics/Electrons, Waves and Photons NEW SPECIFICATION/Waves
A progressive wave is an oscillation that travels through space. All progressive waves transfer energy from one place to another.
Transverse and longitudinal wavesEdit
Transverse waves are waves where the oscillations of particles are perpendicular to the direction of travel of the wave. Longitudinal waves are waves where the oscillations of particles are parallel to the direction of travel.
Quantities to do with a waveEdit
The displacement of a particle along a wave is the distance of the particle from the line of equilibrium or the line of zero displacement. The amplitude of a wave is the maximum displacement attained by particles along the wave during a cycle. The wavelength of a wave is the distance between two adjacent points along a wave oscillating in phase. These quantities are measured in metres (m).
The period of a wave is the time taken for the wave to travel one wavelength. This is measured in seconds (s).
The frequency is the number of wavelengths passing a point per unit time. This is measured as the number per second (s-1), which is equivalent to the alternative unit, the Hertz (Hz)
The wave speed is the distance travelled by a wave per unit time. This is measured in metres per second (ms-1).
The wave speed equationEdit
Frequency is related to the time period of a wave by the equation;
Speed is related to distance and time by the equation;
The wave moves a distance of one wavelength in a time of one time period , therefore;
The above is known as the wave speed equation which relates wave speed to frequency and period.
The phase difference is the difference in phase between two oscillating particles. This is the difference between two particles in the fraction of an oscillation completed by the particles.
Where is the separation between the two particles.
Processes affecting wavesEdit
Reflection occurs when a wave changes direction at a boundary between two media but remains in the original medium. The angle made between the incident wave and the normal to the boundary at the point where the wave meets the boundary (angle of incidence ) is always equal to the angle made between the reflected wave and the normal (angle of reflection ).
Refraction occurs when a wave changes speed and direction moving from one medium to another. The frequency of the wave stays constant. This means that the initial and final wavelengths and speeds are related in the following manner;
Diffraction is the spreading of a wave when passing through a narrow gap or around an obstacle. In order for this to occur, the width of the gap must be roughly equal to the wavelength of the wave.
When the width is much greater than the wavelength, there is little diffraction.
Polarisation is the process of making a wave become plane-polarised. A plane-polarised waves has oscillations in one plane only.
Polarising filters block out light not aligned in the desired plane. If a wave has a component parallel to the desired plane, the wave resulting from a polarising filter will have a lower intensity. If a wave is perpendicular to the desired plane, the polarising filter will block out 100% of the radiation and the resultant intensity will be equal to zero.
Intensity of a waveEdit
The intensity of a wave is defined as the radiant power passing perpendicular to a surface per unit area , represented symbolically as;
The inverse square law states that the intensity of a wave is directly proportional to the square of the separation between the surface and the source;
Since intensity is a measure of power per unit area, it has the units Watts per square metre or Wm-2.
The EM SpectrumEdit
The Electromagnetic (EM) Spectrum records all the different types of waves, from radio waves (highest wavelength) to gamma waves (lowest wavelength).
|Type||Maximum wavelength / m|
Note that x-rays and gamma rays have an overlap between m and m.
All electromagnetic waves;
- can travel through a vacuum
- travel at the speed of light in a vacuum, , which is equal to ms-1
- are transverse
- consist of oscillating and magnetic fields perpendicular to one another
Since the velocity of an electromagnetic wave is equal to the speed of light in a vacuum, the wave speed equation can be written as;
Refraction, reflection and refractive indexEdit
Different media have different refractive indices. The refractive indices of the media either side of a boundary determine whether a wave will reflect or refract at the boundary. The more optically dense a material is, the lower the speed the wave travels through it. The refractive index of a material is the ratio between the speed of light in a vacuum and the speed of light in that material ;
Light travels at speed in a vacuum, so the refractive index of a vacuum is equal to one. Air is not much more optically dense than a vacuum, so the refractive index of air is roughly equal to one. The value of glass is roughly 1.5.
Snell's Law relates the angle made between an incident wave and the normal to a boundary (angle of incidence) to the angle made between a refracted wave and the normal to the boundary (angle of refraction) by the following equation;
Where is the refractive index of the original medium, is the refractive index of the resultant medium, is the angle of incidence and is the angle of refraction.