Transcript Properties of Waves

Properties of Waves
Including light and sound
Wave Motion
A wave is a transfer of energy from one
point to another.
Longitudinal and Transverse Waves
The dark areas are
compressions
The light areas are
rarefactions
The highest points on the wave
are peaks
The lowest points on the wave
are troughs
Wave Diagrams and Terminology
Wavelength (λ) – one whole cycle of a wave
Amplitude – maximum displacement of wave from
its rest position
Frequency
Number of waves passing per second – unit hertz
[Hz]
The time it takes for one wave to pass is called
the period – unit second [s]
period = 1/f
A wavefront –
represents part of a
wave. The distance
between them is the
wavelength.
Wavefront direction the direction that the
waves are moving in
The Wave Equation
v = λf
v is the speed of the wave (m/s)
λ is the wavelength in meters (m)
f is the frequency in Hertz (cycle/s)
Reflection
Waves are reflected from a surface at the same angle they hit it.
The direction of the wave is changed
The speed of the wave is not changed
Reflection
Law of reflection: angle of incidence = angle of
reflection
•The incident ray is the incoming
ray of light.
•The reflected ray is the reflected
ray of light
•The normal line is at right angle to
the mirror
•The angles of incidence and
reflection are measured from the ray
to the normal
Normal
Refraction
When waves go from deep water
to shallow water they become
slower.
The frequency of the oscillations
remains the same but the
wavelength decreases due to the
depth change and so the speed
decreases.
If the barrier between the shallow
water and the deep water is at
an angle, the wave will change
direction
Diffraction
As a wave approaches
move through a gap,
they bend around the
corners. This is
diffraction
If the gap is the same size
as the wavelength, the
diffraction is greater.
Image in a plane mirror
Virtual image
Reflected ray
Object
Incident ray
Image is laterally inverted (back to front)
Image is virtual – it is formed at a point behind the mirror
The image is the same size as the object
The image is the same distance behind the mirror as the
object is in front
A line joining equivalent points on the object and image
passes through the mirror at right angles
Finding the position of an image in
a plane mirror
1. Put a mirror on a bit of paper, put a pin in
front of it and mark the position of the pin in
the mirror.
2. Line up one edge of a ruler with the image
of the pin. Draw a line along the edge to mark
its position then repeat with the ruler in a
different position
3. Take away the mirror, pin and ruler and
extend the two lines to find out where they
meet. This is the position of the image
4. Put the first pin back in and put another one
in where the lines meet. Put the mirror back
and the image and the second pin should line
up
Finding an image position by construction [1]
Object
mirror
Image
1. Draw an incident ray from
object to mirror
2. Draw in a normal
3. Draw a reflected ray
(remember angle of
reflection=angle of incidence).
4. Repeat steps 1-3 with a second
incident ray.
5. Extend the two rays (blue lines)
until they meet. This is the
position of the image.
Finding an image position by construction [2]
Object
mirror
Image
1. Draw a long line from the
object, through the mirror at a
right angle.
2. Measure the distance from the
object to the mirror.
3. At an equal distance behind the
mirror mark a pint on the
extended line – point I. This is
the image position
Refraction of light
When light travels from one medium, to a
medium of different density, light will be
refracted.
normal
Angle of
incidence
Incident
ray
air
glass
Angle of
refraction
The diagram shows a
ray of light travelling
through the air, hitting
a glass block and
being refracted.
Refracted
ray
Ray emerges parallel to
incident ray
Refraction calculations
Refractive index – the amount that light is bent
by a medium
speed of light in a vacuum
refractive index =
speed of light in medium
Or using Snell’s Law
sin i
refractive index =
sin r
Where i is the angle of incidence and r is the
angle of refraction
Dispersion of Light
When white light is refracted it splits into the
colours of the spectrum. This is called
dispersion.
As with a block that has parallel sides, the light
will come out parallel to the incident ray,
when light is refracted by a prism it is deviated
Total internal reflection
The inside surfaces of
transparent materials can
act like a mirror.
When light shines on these
surface some is reflected
and some refracted out of
the material.
Each material has a critical
angle (c): if the incident
angle is greater than this
critical angle the result will
be total internal reflection
refracted
ray
air
glass
i
reflected
ray
no
refraction
c
Optical fibres
Thin strands of glass that make use of total
internal reflection.
Digital signals in the form of pulses of light are
transmitted along optical fibres.
Uses include television and telephone signals to
your house and they can be used as camera’s
for medical examinations
Lenses
3.2 (c) Thin converging lens
• Describe the action of a thin converging lens on a beam of
light
• Use the term principal focus and focal length
• Draw ray diagrams to illustrate the formation of a real image
by a single lens
• Draw ray diagrams to illustrate the formation of a virtual
image by a single lens
• Use and describe the use of a single lens as a magnifying glass
3.2 (d) Dispersion of light
• Give a qualitative account of the dispersion of light
as shown by the action on light of a glass prism
Converging Lens
• Parallel light rays passing
though a converging lens
converge on the other side
• Diverging light rays will
emerge parallel to one
another from a converging
lens
• The point at which they meet
is called the focal point
• The distance between the
centre of the lens and the
focal point is the focal length
Formation of a real image by a single lens
The image is inverted
The image is real
If the object is moved further away (beyond 2F)
from the lens the image will be smaller
If the object is closer to the lens (between F and
2F) the image will be larger
Converging lens as a magnifying glass
• If an object is closer to a converging lens than
its principal focus, the rays never converge
• They form a virtual image behind the lens
The Electromagnetic Spectrum
Electromagnetic radiation
is a group of types of
radiation.
It travels as a transverse
wave and in a vacuum
all wavelengths travel
at the same speed: the
speed of light 3x108m/s
Higher frequency
radiation has more
energy and is more
harmful
Summary of Electromagnetic Radiation
Frequency
Type of
electromagnetic
radiation
Typical use
Wavelength
highest
gamma radiation
killing cancer cells
shortest
X-rays
medical images of
bones
ultraviolet radiation
sunbeds
visible light
seeing
infrared radiation
optical fibre
communication
microwaves
cooking
radio waves
television signals
lowest
longest
Sound Waves
Core
• Describe the production of sound by vibrating sources
• Describe the longitudinal nature of sound waves
• State the approximate range of audible frequencies
• Show an understanding that a medium is needed to transmit sound
waves
• Describe an experiment to determine the speed of sound in air
• Relate the loudness and pitch of sound waves to amplitude and
frequency
• Describe how the reflection of sound may produce an echo
Supplement
• Describe compression and rarefaction
• State the order of magnitude of the speed of sound in air, liquids and
solids
Sound Waves
Sound waves are longitudinal waves.
Sound waves are produced when
objects vibrate.
The vibrations travel in the same
direction as the wave travels.
At some points the air molecules are
pushed together increasing
pressure (compression).
At other points the air molecules are
further apart decreasing
pressure (rarefaction).
As a result the sound wave travels
through the air, with the air particles
vibrating backwards and forwards.
c
c
c
c
c
wavelength
r
r
r
r
Sound in a Solid, Liquid and Gas
Sound waves need a medium to travel through - a solids,
liquids and gases.
The particles making up solids are very close together, so they
can transfer sound energy from particle to particle very
quickly.
In liquids the particles are relatively close together, so they can
also transfer sound waves from particle to particle fairly
quickly.
However, in gases the particles are widely spaced, therefore it
takes longer for the sound waves to pass from particle to
particle.
The more dense the medium, the
faster the sound waves travel
through it.
Medium
Speed (m/s)
gas (air)
340
liquid (water)
1500
solid
5000
Measuring the Speed of Sound
Use a data logger to measure the time it takes
for the sound to get from one microphone to
the other. Then use the speed equation.
Sound and Amplitude
When a wave travels through any material, it
causes the particles making up that material,
to move from their position of rest.
The maximum movement away from the rest
position is
known as the amplitude.
The greater the amplitude, the louder the
sound, and the more energy it has.
Sound and Frequency
The faster that an object vibrates, the higher the
pitch of the sound produced. The more
frequent the vibrations, the greater the
frequency.
The frequency range for normal human hearing
is between 20Hz and 20,000Hz.
Practice calculations
If a sound wave travels 1 m in 0.003s, what is its
speed?
If a sound wave has a speed of 340 ms-1, how far
does it travel in 2s?
For homework, find out what the speed of
sound is in a solid a liquid and a gas
Question
A man stands in front of a wall and shouts. It
takes 0.2s for the sound go to the wall and to
get back to him (the echo time), or echo.
What is his distance from the wall? (Using
speed of sound in air 330ms-1) Hint – work out
TOTAL distance first!
Echoes
When a sound wave reflects off a hard surface and we
hear it again – this is an echo.
The distance between a sound and an object can be
found by using the wave equation – but remember, it
will be double the distance between the sound and the
hard surface
s = 2d/t
Where s is the speed of sound in air (or whatever
medium it is travelling through), d is distance and t is
time.
This method is called echo-sounding and has many
applications
Mapping the ocean floor
You work aboard a scientific survey ship. Your job is to
map the profile of the sea floor in the locations that
you ship visits. The following is echo-sounding data
between two points of the journey.
Distance
along path
(km)
10
20
30
40
50
60
70
80
90
100
Echo time
0.1
0.3
0.1
0.1
0.2
0.3
0.4
0.5
0.4
0.5
Distance
Complete the table and calculate the distance from
the bottom of the ship the sea floor. Use graph
paper to plot a profile of this part of the sea floor.
What is this machine showing?
Can you recall the wave equation?
Making Waves
Adjust the equipment for each of the following.
Draw the waveform and describe how
frequency and amplitude have been changed.
1. Loud sound with a high pitch
2. Loud sound with a low pitch
3. Quiet sound with a high pitch
4. Quiet sound with a low pitch
Practice Calculations
1. Calculate the frequency of a wave with a
speed of 1200 ms-1 and a wavelength of 3m
2. Calculate the wavelength of a wave with a
speed of 200ms-1 and a frequency of 120Hz
3. Calculate the speed of a wave with a
frequency of 200Hz and a wavelength of 2m.
4. Calculate the speed of a wave with a time
period of 2s and a wavelength of 3m