Waves Flashcards
What is the definition of a mechanical wave?
Vibrations which passes through a substance.
Give 3 examples of a mechanical wave
Sound waves, seismic waves, and waves on strings
What are electromagnetic waves?
Oscillating electric and magnetic fields which progress through space without need of a substance.
Give 3 examples of E.M waves
Radio waves, microwaves, IR radiation
What are longitudinal waves?
Waves in which the direction of vibration is parallel to the direction in which the waves travel.
Give 3 example of longitudinal waves
Sound waves, primary seismic waves and compression waves.
What are transverse waves?
Waves in which the direction of vibration is perpendicular to the direction in which the waves travel
Give three examples of transverse waves?
E.M waves, secondary seismic waves and waves on strings.
What is the meaning of plane polarised?
When vibrations stay in one plane only
What is the meaning of unpolarised?
The vibrations change from one plane to another.
What happens when light from a filament lamp is passed through a polaroid filter?
Light from filament lamp is unpolarised, when it passes through the filter, the transmitted light is polarised, as the filter only allows through light which vibrates in a certain direction.
What does it mean when two polaroid filters are “crossed”
The two polaroid filters are at 90 degrees to each other, and this means that the transmitted light intensity is at a minimum, so no light should pass through the second slit.
What is the main use of polarisation? How do these ____ work?
In polaroid sunglasses. They reduce glare of light reflected by water or glass. Reflected light is polarised and the intensity is reduced when it passes through polaroid sunglasses.
Why does the aerial of a radio receiver need to be aligned correctly?
Radio waves from a transmitter are polarised, so the aerial needs to be aligned in the same plane as the radio waves to obtain the best reception.
What is the displacement of a wave?
Distance and direction of vibrating particle from its equilibrium position
What is the amplitude of a wave?
The maximum displacement of a vibrating particle. The height of a crest or the depth of a trough from equilibrium position.
What is the wavelength of a wave?
The least distance between 2 particles in phase. (Distance between adjacent crests)
What is a cylce of a wave?
One complete cycle of a wave is from max displacement to the next max displacement.
What is the period of a wave?
The time for one compete wave cycle to pass a fixed point.
What is the frequency of a wave?
Number of wave cycles occurring each second
What is the formula for the period of a wave?
1/frequency
What is the main wave formula?
c=fλ
What is phase?
The fraction of a cycle the particle has completed since the start of the cycle.
What is phase difference and what is the formula for phase difference?
It is the fraction of the cycle between vibrations of two particles .2πd/λ ( in radians or degrees ). One complete cycle of a wave is 2π.
What is the principle of superposition?
When two waves meet they superpose, and the total displacement at a point is equal to the sum of the individual displacements at that point.
What happens when a crest meets a crest, or a trough meets a trough?
The two waves superpose and form a supercrest or a supertrough.
What happens when a crest meets a trough of the same amplitude?
The two waves superimpose and the resultant displacement is zero, the two waves cancel.
How are stationary waves formed?
They’re formed when two progressive waves travelling in opposite directions of similar amplitude, opposite velocities and same frequency pass through each other and superpose. The two waves interfere, and at fixed points along the wave form nodes of minimum displacement and antinode of maximum displacement. At each node, the two sets of waves are always 180 degrees out of phase. No energy is transferred between them. Destructive interference at nodes, constructive interference at the antinodes.
In a ripple tank, why do we see gaps in the wavefronts?
These are points of cancellation, where a crest from one dipper meets a trough from another.
What is interference?
When waves continuously pass through each other at constant frequency and constant phase difference , cancellation and reinforcement occurs at fixed positions.
How are stationary waves formed on a guitar string?
The string is fixed at both ends, the middle part vibrates and progressive waves travel towards each end, reflect at the end and then pass through each other, which causes them to cancel.
What is the wavelength of the first harmonic on a string
2L ( where L is the length of the string)
What do stationary waves not transfer?
Energy to their surroundings when vibrating freely.
How does the amplitude of a vibrating particle vary in a stationary wave?
Amplitude of vibrating particle varies between zero at a node and max amplitude at an antinode.
When is phase difference zero in a stationary wave?
When 2 particles are between adjacent nodes or separated by an even number of nodes.
When is phase difference 180 degrees in a stationary wave?
When two particles are separated by an odd number of nodes.
What is the structure of the first harmonic? What is the wavelength of the first harmonic?
N,A,N. 2L
What is structure of the second harmonic? What is the wavelength of the second harmonic?
N,A,N,A,N. L.
What is structure of the third harmonic? What is the wavelength of the third harmonic?
N,A,N,A,N,A,N. 2/3L.
How can the pitch of a note be altered?
By changing the tension of the string or by altering its length. Raising the tension or shortening length increases the pitch. Lowering tension or increasing the length lowers the pitch.
Which equation links frequency to tension?
f = 1/2L * √t/μ. where μ is the mass per unit length.
