Waves Waves are energy carriers. They carry energy from the source to the point where they are absorbed. Waves can be produced by a disturbance in the medium such as throwing a stone in water. When a stone is thrown in water, the kinetic energy of the stone is converted to kinetic energy of the molecules of water which move up and down perpendicularly passing the energy to the neighbouring molecules. This consecutive movement of water molecules appears as a wave motion. It is like a Mexican wave. You must have seen a soccer game where the spectators raise their hands one by one and to us it seems like a wave moving around the stadium. Let’s assume that the line represents still water. A,B,C,D and E are molecules of water. Rest position (still water) A
B
C
D
E
When a stone is thrown in the water, A gains kinetic energy and moves upwards.
A B
C
D
E
After reaching its maximum displacement , A comes down passing the kinetic energy to B which now moves upwards.
B A
C
D
E
B also reaches its maximum displacement and comes down to its rest position passing on the energy to C. This continues and to us it seems as if a wave is moving forward. It is actually energy which is being carried forward and the consecutive movements of the molecules of water produces a wavelike motion. Types of waves 1. Transverse waves 2. Longitudinal waves Transverse Waves In this type of wave, the particles of the medium move at right-angles(perpendicular) to the direction of wave motion. Up
wave motion
Down
Example:
water waves, radio waves, micro waves, etc.
See Wave on a String, Physics Interactive on my website: http://www.physics.com.pk/ .
You can produce transverse waves in slinky springs as well.
Labelling a transverse wave
Rest position
A = amplitude(m):
it is the maximum displacement of the particles of the medium from the rest position.
λ = wavelength(m): it is the distance between two consecutive crests/troughs. Crest: it is the highest point on a wave. Trough: it is the lowest point on a wave. T = Time period(s): it is the time taken to produce one wave. f = Frequency(Hz): it is the number of waves in one second. The wave equation: This equation can be used to calculate the speed of a wave.
𝒗=𝒇𝝀 V = wave speed (m/s) F = frequency (Hz) λ = wavelength(m) Note: you can use the wave equation to find the speed of a wave or the simple distance time equation. Both work.
𝒗=
𝒔 𝒕
Reflection of Transverse waves Reflection means to bounce back. When waves hit a barrier they reflect just like light. Examples: Incident wave
incident waves
Barrier Reflected wave
reflected waves Figure 1
Figure 2
It is difficult to draw waves like this so we draw them as shown in Figure 2.
Wavefronts
A wavefront is an imaginary line that joins points which are in phase. Phase: all those points on a wave that travel in the same direction with same speed such as crests or troughs. wavelength
wavefronts
Remember: crests move downwards while troughs move upwards as a wave motion passes by.
Properties of reflected waves: 1. 2. 3. 4.
There is no change in their speed There is no change in their wavelength There is no change in their frequency There is only change in their direction.
Refraction of Transverse waves Refraction: it is bending of waves due to change in their speed when they pass from shallow to deep water or from shallow to deep water.
Normal
Waves bend towards the normal when they pass from deep to shallow water and vice versa. Properties of refracted waves going from deep to shallow water. 1. 2. 3. 4.
Their speed decreases. Their wavelength decreases. Their frequency remains unchanged. They bend towards the normal.
Properties of refracted waves going from shallow to deep water. 1. 2. 3. 4.
Their speed increases. Their wavelength increases. Their frequency remains unchanged. They bend away from the normal.
Ripple Tank
A ripple tank can be used to demonstrate reflection and refraction of waves. Water waves are produced in the tank by the rotating motor on the paddle. The paddle moves up and down creating transverse waves in the water. The reflection of these waves is seen on the viewing screen. To show reflection we can place a barrier in the tank and when waves strike it they reflect.
To show refraction we can place a glass block in the tank. Water above the block will be shallow and waves passing above it will be refracted.
By Shafaq Hafeez shafaq@physics.com.pk