Transcript Document 7804520

Waves part 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
• For a wave of amplitude A and frequency f,
the intensity I is proportional to A2f2
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
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.
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.
• 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
• The speed v, frequency f and wavelength
λ of a wave are related by v = fλ
• All wavelengths of electromagnetic
radiation have the same speed c=
3.00x108ms-1 in a vacuum
Questions
1. 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
2. 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.
Question cont..
b) A typical frequency for v.h.f.
television transmission is 250MHz.
Calculate the corresponding wavelength.
Answers
1. a) 200Hz
b) 5.0 x10-3s
2. a) 7.5 x10 14Hz to 4.3 x10 14Hz
b) 1.2 m
Polarisation of waves
• The condition for a wave to be planpolarised is for the vibrations to be in just
one direction normal to the direction in
which the wave is travelling
What type of wave is this?
• 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.
Why does polarisation have to be thought about with
aerials?