Y9 Physics: waves 1.1 keywords:

Amplitude

The maximum displacement from the equilibrium position of the wave.

Transverse Wave

A wave where the oscillations of particles are perpendicular to the direction of energy transfer.

Longitudinal Wave

A wave where the oscillations of particles are parallel to the direction of energy transfer.

Frequency

The number of waves passing a point in a second. (Hz)

Time Period

The time it takes for one complete wave to pass a point. (s)

Peak

The top of a wave where there is the maximum displacement.

Trough

The bottom of a wave where there is the minimum displacement.

Reflection

When waves bounce off a surface with the reflected angle equal to the incident angle.

Refraction

When waves change speed and direction when they enter a different medium.

Wave Speed

The speed at which the wavefronts move through a medium. (m/s)

Wavefront

Lines marking peaks of waves in diagrams to show wavelengths. Wavefronts are perpendicular to the direction of wave travel.

Wavelength

The distance between two adjacent like points on a wave. (e.g. peak to peak or trough to trough)

Wave

A transfer of energy due to oscillations without transferring matter.

Compression

A region of high concentration of particles and higher pressure in a longitudinal wave.

Rarefaction

A region of low concentration of particles and lower pressure in a longitudinal wave.

Light ray

Narrow beam of light used to show the path and direction of light in a diagram. 

Umbra

Darkest part of a shadow where all light is blocked. 

Penumbra

Lighter part of a shadow where some light is blocked. 

Colour Filter

Something that only transmits its own colour but absorbs all others

Colour absorption

When a coloured object absorbs wavelengths of light rather than reflecting them. This makes the object seem black

Colour dispersion

When white light is refracted into a glass prism which disperses all the colours due to the different wavelengths and creates a rainbow

Luminous source

A source that creates its own light

Illuminated source

A source that is illuminated by a luminous source

Specular reflection

A type of reflection of a smooth surface

Law of reflection

The angle of incidence = The angle of reflection

Diffuse reflection

A type of reflection of a rough or unreflective surface where the light scatters in all directions forming a bad image. 

Refractive index

The ratio of wave speeds between a vacuum and a material.

Speed of light in air / speed of light in object

sin(angle of incidence) / sin(angle of refraction)

Refraction rule if a wave enters a denser medium.

Wave goes slower and wavelength decreases.

Bends towards the normal.

Refraction rule if a wave enters a less dense medium.

Wave goes faster and wavelength increases.

Bends away from the normal.

Refraction rule if a wave enters at the normal.

No refraction but change in speed and wavelength.

Why does refraction happen in terms of change in wave speed slowing down? (3)

different parts of the wavefront enter a denser boundary at different times

the part of the wavefront that enters first slows first as it hits first while the rest continues at the original speed

this causes the wavefront to change direction as the difference in speed bends the wavefront and wave

Why does refraction change wavelength?

Because of the wave equation, as the wave speed slows down, one of frequency and wavelength have to go down too. As frequency cannot change, the wavelength does and gets shorter.

Colour diffuse reflection

When white or coloured light hits a rough surface and the surface reflects some wavelengths in different directions, making the object appear a particular colour.