Do now!
Can you discuss with your partner all the things you can remember
about the WAVES topic
(topic 3)
Topic 3 Waves
Waves
Waves can transfer energy and information without a net motion of the medium through which they travel.
They involve vibrations (oscillations) of some sort.
Rays
Rays highlight the direction of energy transfer.
Transverse waves
The oscillations are perpendicular to the direction of energy transfer.
Direction of energy transfer
oscillation
Transverse waves
peak
trough
Transverse waves
• Water ripples
• Light
• On a rope/slinky
• Earthquake
Longitudinal waves
The oscillations are parallel to the direction of energy transfer.
Direction of energy transfer
oscillation
Longitudinal waves
compression
rarefraction
Longitudinal waves
• Sound
• Slinky
• Earthquake
Wave measurements
Amplitude - A
The maximum displacement from the mean position.
amplitude
Period - T
The time taken (in seconds) for one complete oscillation. It is also the time taken for a complete wave to pass a given point.
One complete wave
Frequency - f
The number of oscillations in one second. Measured in Hertz.
50 Hz = 50 vibrations/waves/oscillations in one second.
Period and frequency
Period and frequency are reciprocals of each other
f = 1/T T = 1/f
Wavelength - λ
The length of one complete wave.
wavelength
Wave speed - v
The speed at which the wave fronts pass a stationary observer.
330 m.s-1
The Wave Equation
The time taken for one complete oscillation is the period T. In this time, the wave will have moved one wavelength λ.
The speed of the wave therefore is distance/time
v = fλv
λf x
1) A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving?
2) A water wave travels through a pond with a speed of 1m/s and a frequency of 5Hz. What is the wavelength of the waves?
3) The speed of sound is 330m/s (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
4) Purple light has a wavelength of around 6x10-7m and a frequency of 5x1014Hz. What is the speed of purple light?
Some example wave equation questions
0.2m
0.5m
0.6m/s
3x108m/s
Electromagnetic spectrum
Long Wavelength Short
Wavelength
Low Frequency High Frequency
What do they all have in common?
• They can travel in a vacuum• They travel at 3 x 108m.s-1 in a vacuum
(the speed of light)• They are transverse• They are electromagnetic waves (electric
and magnetic fields at right angles to each oscillating perpendicularly to the direction of energy transfer)
Light travels faster than sound
Speed of light = 300 000 000 m/s
Speed of sound (in air) = 330 m/s
Can you copy this please?
Law of Reflectionnormal
Angle of incidence
Angle of reflection
mirror
angle of incidence = angle of reflection
Can you copy please?
Image in a mirror
• Upright
• Same size as object
• Distance from mirror to object = “distance” from image to mirror
• Laterally inverted
Why?
Refraction
Glass block
Beam of lightLight going from air to glass bends towards the normal
Light going from glass to air bends away the normal
Can you copy this please?
Refraction
When a wave changes speed (normally when entering another medium) it may refract (change direction)
Snell’s law
i
r
Ray, NOT wavefronts
speed in substance 1 = sin(i)speed in substance 2 sin(r)
Snell’s law
i
r
Ray, NOT wavefronts
In the case of light only, this quantity is called the refractive index (η)
η = sin(i)
sin(r)
Critical Angle
Critical angle (when angle of refraction is 90º)
Copy!
90º
Total Internal
Reflection
Critical angle
sin c = 1/η
sin(critical angle) = 1/(refractive index)
Examples of Total Internal Reflection
Examples of Total Internal Reflection
Diffraction
Waves spread as they pass an obstacle or through an opening
Diffraction
Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave
Diffraction
Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave
Analogue and digital
Let’s read pages 164 and 165.
104 103 102 101 100 10-1 10-2 10-3
1 5
25 24 23 22 21 20 2-1 2-2 2-3
1 1 1 1
01111001101111001101111011101111
7 0 1 1 1
9 1 0 0 1
11 1 0 1 1
12 1 1 0 0
13 1 1 0 1
14 1 1 1 0
14 1 1 1 0
15 1 1 1 1
15 1 1 1 1
15 1 1 1 1
Advantages of digital signals
• Less affected by interference
• Interference is not increased when the signal is amplified
• Uses weaker signals than analogue.
Sound Longitudinal waves
The oscillations are parallel to the direction of energy transfer.
Direction of energy transfer
oscillation
The Cathode Ray Oscilloscope
Can you stick the sheet in?
Amplitude = volume
Can you copy this
CAREFULLY please?
Pitch = frequencyCan you copy
this CAREFULLY too please?
Range of hearing
Humans can hear up to a frequency of around 20 000 Hz (20 kHz)
Can you copy this too please?
Your own mind-map