General Properties of Waves

Progressive waves

A progressive wave or travelling wave is a disturbance which carries energy from one place to another without transferring matter.

Wavefront

An imaginary surface representing points of a wave that are at the same point in their cycle. a wavefront is a line on which the disturbance has the same phase at all points. A vibrating source produces a succession of wavefronts, all of the same shape. In a ripple tank, straight wavefronts are produced by a vibrating bar (a line source) and circular wavefronts are produced by a vibrating ball (a point source).

Investigating Diffraction

Diffraction can be shown in a ripple tank by placing small barriers and obstacles in the tank As the water waves encounter two obstacles with a gap between them, the waves can be seen to spread out as follows. As the water waves encounter the edge of an obstacle, the waves can be seen to spread out as follows. The amount of diffraction depends on the size of the gap compared to the wavelength of the water wave.

Diffraction at an edge

For a single edge, diffraction will occur at the edge. The spreading and curvature of the wavefront around the edge will be more noticeable for longer wavelengths. Diffraction of radio waves around an obstacle

Investigating Reflection

Reflection can be shown by the waves hitting a plane (straight) surface, such as a wall or mirror.

Investigating Refraction

Refraction can be shown by placing a glass block in the tank The glass block should sit below the surface of the water and cover only some of the tank floor The depth of water becomes shallower here the glass block is placed Since speed depends on depth, the ripples slow down when travelling over the block This is a good model of refraction showing how waves slow down when entering a denser medium

Transverse waves examples

Seismic S-waves Water waves electromagnetic radiations

Mechanical waves

Several kinds of wave motion occur in physics. Mechanical waves are produced by a disturbance, such as a vibrating object, in a material medium and are transmitted by the particles of the medium vibrating about a fixed position. Such waves can be seen or felt

Reflection

The bouncing back of a wave at a boundary.

Refraction

The changing of speed, and consequently the direction, of a wave as it changes medium. The wavelength of the wave will also change but the frequency remains constant.

ray

The direction of the path in which light is travelling is called a ray and is represented in diagrams by a straight line with an arrow on it. A line drawn at right angles to a wavefront, which shows its direction of travel.

wavelength and frequency relationship

The faster the end of a rope is vibrated, the shorter the wavelength of the wave produced. That is, the higher the frequency of a wave, the smaller its wavelength.

The frequency f

The frequency f is the number of complete waves generated per second. The frequency of a wave is also the number of crests passing a chosen point per second. The frequency of the wave is two vibrations per second or 2 hertz which is the same as the frequency of the movement of the end of the rope.

Frequency of motor and wavelength

The higher the frequency of the motor, the shorter the wavelength The lower the frequency of the motor, the longer the wavelength

The amplitude a

The maximum displacement of a wave from its undisturbed (equilibrium) position.

Normal

The perpendicular to the strip at the point where the incident ray strikes is called the normal.

The wave speed v

The wave speed v of the wave is the distance moved in the direction of travel of the wave by a crest or any point on the wave in 1 second.

Compressions C

They are regions of high pressure due to particles being close together.

Rarefactions R

They are regions of low pressure due to particles being spread further apart.

Diffraction

Waves spread out when they go around sides an obstacle or through gap. It is the bending of waves around gaps or corners. It occurs when the size of the aperture or obstacle is of the same order of magnitude as the wavelength of the incident wave.

Longitudinal waves

Waves with oscillations that are parallel to the direction of travel/energy transfer. A longitudinal wave can be sent along a spring, stretched out on the bench and fixed at one end, if the free end is repeatedly pushed and pulled sharply.

Transverse waves

Waves with oscillations that are perpendicular to the direction of travel/energy transfer. A transverse wave can be sent along a rope (or a spring) by fixing one end and moving the other rapidly up and down.

How can waves can be explained?

displacement-distance graph It shows, at a certain instant of time, the distance moved (sideways from their undisturbed positions) by the parts of the medium vibrating at different distances from the cause of the wave.

Longitudinal waves examples

sound waves seismic P-waves

Speed of wave equation

speed of wave = frequency × wavelength or v = fλ

Law of reflection

the angle of incidence and angle of reflection are always equal.

Wavelength

the distance between successive crests (peaks) The distance from a point on one wave to the same point on the adjacent wave

Effect of wavelength and gap size on diffraction

the gap width is about the same as the wavelength of the waves (1cm). the wavefronts that pass through become circular and spread out in all directions. the gap is wide (10 cm) compared with the wavelength and the waves continue straight on some spreading occurs but it is less obvious.

Behavior of waves in water

the shallow region are found to have a shorter wavelength than those in the deeper parts, i.e. the wavefronts are closer together. Both sets of waves have the frequency of the vibrating bar and, since v = fλ, if λ has decreased so has v, since f is fixed. Hence waves travel more slowly in shallow water.

Examples of progressive waves

transverse and longitudinal waves.

Examples of mechanical waves

waves on a rope or spring water waves sound waves in air or in other materials.