Waves part 2

1
Waves part 2
2
Intensity of a wave

• The intensity is proportional to the square of
  the amplitude of a wave. Thus doubling the
  amplitude of a wave increases the intensity of
  the wave by a factor of 4.
• The intensity also depends on the frequency
  intensity is proportional to the square of the
  frequency

3

• For a wave of amplitude A and frequency f, the
  intensity I is proportional to A2f2

4
Example

• The amplitude of a wave in a rope is 15mm. If the
  amplitude were changed to 20mm, keeping the
  frequency the same, by what factor would the
  power carried by the rope change?
• 20/15
• So the power increases by a factor of (20/15)2
  1.8

5
Your turn

• A beam of red light has twice the intensity of
  another beam of the same colour. Calculate the
  ratio of the amplitudes of the waves
• Two waves travel at the same speed and have the
  same amplitude, but the first has twice the
  wavelength of the second. Calculate the ratio of
  the intensities transmitted by the waves.

6
(No Transcript)
7

• E.m. waves show all the properties common to wave
  motions they can be r__________, r__________,
  and d____________.
• Because they are transverse waves they can be
  polarised. In a vacuum all electromagnetic waves
  travel at the same speed 3.00 x 108 ms-1
• The complete e.m. spectrum is divided into a
  series of regions based on the properties of
  electromagnetic waves in these regions.

8

• A progressive wave travels outwards from the
  source, carrying energy but without transferring
  matter.
• In a transverse wave the oscillations are at
  right angles to the direction in which the wave
  carries energy
• In a longitudinal wave, the oscillations are
  along the direction in which the wave carries
  energy.
• The intensity of a wave is the energy passing
  through unit area per unit time. Intensity is
  proportional to the square of the amplitude

9

• The speed v, frequency f and wavelength ? of a
  wave are related by v f?
• All wavelengths of electromagnetic radiation have
  the same speed c 3.00x108ms-1 in a vacuum

10
Questions

• A certain sound wave in air has a speed 340ms-1
  and a wavelength 1.7m. For this wave calculate
  a) the frequency, b) the period
• The speed of electromagnetic waves in vacuum (or
  air) is 3.0x108ms-1.
• a) the visible spectrum extends from a
  wavelength of 400nm (blue light) to 700nm (red
  light). Calculate the range of frequencies of
  visible light.

11
Question cont..

• b) A typical frequency for v.h.f. television
  transmission is 250MHz. Calculate the
  corresponding wavelength.

12
Answers

• a) 200Hz
• b) 5.0 x10-3s
• a) 7.5 x10 14Hz to 4.3 x10 14Hz
• b) 1.2 m

13
Polarisation of waves

• The condition for a wave to be plan-polarised is
  for the vibrations to be in just one direction
  normal to the direction in which the wave is
  travelling

14
What type of wave is this?
15

• In a plane-polarised wave, the vibrations of the
  wave are in one direction only, which is normal
  to the direction of travel of the wave.
• Transverse waves can be polarised longitudinal
  waves cannot.
• Plane-polarised light can be produced from
  unpolarised light by using a polariser, such as a
  sheet of polariser, such as a sheet of Polaroid.

16
Why does polarisation have to be thought about
with aerials?