Waves

Vocabulary ReviewProperties of Waves

The Electromagnetic Spectrum

Vocabulary Review• Progressive waves• Describing wave motion

– Displacement– Amplitude– Wavelength– Period– Frequency– Speed

Intensity• Energy passing through a unit area in unit

time.• Intensity ~ Square of the amplitude• I ~ A2

• Intensity ~ 1/d2

• Where d is distance from source

Example 1• A health inspector is measuring the

intensity of a sound. Near a loudspeaker his meter records an intensity I. This corresponds to an amplitude A of the sound wave. At another position the meter gives an intensity reading of 2 I. What is the corresponding sound wave amplitude?

Example 2• The intensity I of a sound at a point P is

inversely proportional to the square of the distance x of P from the source of the sound. Air molecules at P, a distance r from S, oscillate with amplitude 8.0 μm. Point Q is situated a distance 2r from S. What is the amplitude of oscillation of air molecules at Q?

Phase

• The corresponding angular displacement relative to a complete wave of a point on the wave

Phase

180⁰ or п

0⁰

Phase

90⁰ or п/2

270⁰ or 3п/2

Phase Difference

• The difference in angles between two waves at a specific point.

Phase Difference

Phase difference of п at all points

Phase Difference

Phase difference of п at all points

Coherence

When two waves have constant phase difference, then they are coherent.

Coherence occurs when two waves have the same frequency or period.

Non-Coherence

In Phase

Phase difference is a multiple of 2п

at all points.

Out of Phase

Phase difference is not a multiple

of 2п.

Out of Phase

Phase difference of п at all points

Wave Properties• For all waves

– Reflection• Bouncing of waves off a surface

– Refraction• Change of wave speed as it passes from one medium

to another (from one depth to another)• Bending of waves

– Diffraction• Bending of waves around a barrier

Wave Properties Question• Which wave properties change when

light passes from air into glass?

• A colour and speed• B frequency and wavelength• C speed and wavelength• D wavelength and colour

Polarisation• Oscillation that takes place in one

direction.• Only true for transverse waves.

Polarisation

The Electromagnetic Spectrum

• Electromagnetic waves are transverse waves

• They are non-mechanical waves– Do not require a medium to transfer energy

• Travel at the same speed in a vacuum– 3 x 10 8 ms-1

Orders of Magnitude (Wavelength)

• Gamma Rays• X-Rays• Ultraviolet Rays• Visible Light

– ROY G BIV

• 10 -14 m• 10-10 m• 10-7 m • 10-7m • (700 nm -400 nm)

Orders of Magnitude (Wavelength)

• Infrared• Microwaves• Radio Waves

• 10 -6 m• 10-2 m• 10 2 m