4.4 Waves Flashcards
Define monochromatic.
All light waves have the same wavelength.
What conditions does there need to be to measure wavelength of light?
- Light must be monochromatic
- There must be an accurate method of producing very small path differences, and of measuring these.
What did Thomas Young do to establish the wave theory of light?
He was the first person to measure wavelength successfully and doing so established the wave theory of light- until them people thought of light as a stream of tiny particles called corpuscles. To demonstrate this, he used his double-split experiment.
What was the Young double-slit experiment?
Young used a monochromatic red light source, which he placed behind a single slit in apiece of black card. Light passes through the slit and spreads out by diffraction, until it reaches another obstacle, in which there are two narrow parallel slits. Waves from both slits that are in phase will interfere constructively. Alternate bright and dark vertical bands are seen on the other screen.
What do waves transfer?
Energy without any net transfer of matter.
What are the two types of waves?
- Transverse
- Longitudinal
What are some examples of longitudinal waves?
- Sound waves
- Ultrasound
- Types of seismic waves (P waves)
What are vibrations in waves called?
Oscillations
What causes longitudinal waves?
Vibrations/ oscillations parallel to the direction of the energy transfer.
What are some examples of transverse waves?
Anything on the electromagnetic spectrum. e.g. UV light, visible light, microwaves, X-rays, gamma rays.
What produces transverse waves?
Vibrations/ oscillations at right angles to the direction of energy transfer.
What speed do electromagnetic waves travel at?
The speed of light: 3 x 108
What are progressive waves?
Waves that move away fro the source.
What way do particles oscillate in in terms of waves?
All particles oscillate vertically- they do not move forward or backwards although the waves move forwards.
What are the two graphs we can draw waves on? And on both, what does the space between the waves represent?
Displacement (y axis) against distance travelled (x axis)- the distance between two waves us the wavelength.
Displacement (y axis) against time (x axis)- the distance between two waves is a period.
What is a wavelength?
The distance between two successive identical points that have the same pattern of oscillation/ the distance the wave travels before the wave repeats itself.
What is a period?
The time it takes for one complete pattern of oscillation to take place.
What is the frequency of a wave?
The number of waves per unit time at any point and is related to the time period T by using the equation f=1/T.
What is displacement in terms of waves?
The distance of any part of the wave has moves from its mean position- it can be positive or negative.
What is amplitude?
The maximum displacement- the distance from a peak or trough to the mean (rest) position.
What is phase difference?
Phase difference concerns the relationship between the pattern of vibrations between two points.
What is meant if two waves are in phase?
They have exactly the same pattern of oscillations.- there is zero phase difference between them.
What is an oscilloscope?
A device for viewing oscillations by a display on the screen of a cathode ray tube.
what does an oscilloscope display?
It displays a voltage-time signal and can be used as a voltmeter to display and measure the output from a microphone or signal generator. The time-varying voltage trace represents displacement against time for longitudinal waves.
Define the principle of superposition.
When two or more waves of the same type meet, the resultant waves can be found be adding the displacements of the individual weaves.
What is constructive interference?
If two waves, with the same amplitude exist at the same point and are travelling in phase, the amplitude of the resultant wave will be twice the individual weaves.
What is destructive interference.
When two waves with the same amplitude exist at the same point and are travelling in antiphase, they will cancel each other out, and the resultant wave will have an amplitude of zero.
What does it mean if two waves are coherent?
The two waves have a constant phase difference.
What is path difference?
The distance between the distances travelled by two waves arriving at the same point.
What is phase difference?
The difference in the phases of two waves of the same frequency.
Phase difference is measured in degrees, if its 180° difference, it’s completely out of phase (destructive interference), while if its 360° its completely in phase (constructive interference).
What does the path difference for constructive interference look like?
For constructive interference, the path difference = nλ. The peak must arrive with another peak, this inly happens when the wave length is a whole number.
What doers the path difference look like for destructive interference?
For destructive interference, the path difference= (n+½)λ. The peak must meet a trough- this only happens when the path difference is an odd number of wavelengths. e.g. ½, 3/2, 5/2 ect.
What is the difference between longitudinal and transverse waves?
Longitudinal waves have vibrations that are parallel to their direction of energy transfer, whereas transverse waves have vibrations that are at right angles to their direction of energy transfer.
Longitudinal waves require a medium to travel through, whereas some transverse waves (electromagnetic waves) do not.
How is an oscillation different from a wave? Give an example to explain this.
An oscillation describes the periodic motion of a particle about its mean position or equilibrium position, whereas a wave refers to the motion of the energy outwards from the initial disturbance.
When a stone is dropped into a pond, the individual water particles will oscillate up and down about their equilibrium positions in the pond, but the water wave will spread out across the surface of the pond, transferring energy as a transverse wave across the pond’s surface, and at right angles to the oscillating motion of the water particles.
How can you demonstrate interference using sound?
Using two load speakers connected to the same generator, walk along in front of the speakers, you you go along, you should hear a loud sound where the waves reinforce each other and a quiet sound where the waves partially cancel each other out.
This variation is clearer if you cover up one ear.
How does frequency affect the distance between maxima and minima waves?
The distance between the loud and quiet regions is longer at low frequencies.
What equation relates frequency, wave speed and wave length?
wave speed= frequency x wavelength
v= fλ
How can we work out frequency from time?
f= 1/T
where T is equal to one time period.
What is the spectrum of visible light?
20Hz - 20,000Hz
What is the equation the relates area, intensity and power?
Intensity= power/ area
=(energy/time)/area
=Power/ 4πr2
How is the energy of a wave related to it amplitude?
Energy of a wave is proportional to the square of it’s amplitude.
How is the energy of a wave related to it amplitude?
Energy of a wave is proportional to the square of it’s amplitude.
What happens to the amplitude of a wave the further you go away from a wave?
It decreases as the wave spreads away from it’s source.
What would happen to the intensity of a wave if it’s amplitude decreases by a factor of 2?
It’s intensity will decrease by a factor of 4 because it’s intensity is also proportional to the square of its amplitude.
What can polarization be used as evidence of?
The wave nature of light- all transverse waves can be polarized.
What sort of wave cannot be polarized?
Longitudinal waves
What is ultrasound?
Ultrasound is a sound wave with a very high frequency- so we cannot hear it- above 20,000Hz.
How do u ultrasounds produce an image?
A transmitter produces an ultrasound pulse. The ultrasound reflects at each boundary it crosses. Time for reflected pulse to return to the detector is measured and used to calculate distance. A computer changes this information into a picture.
How do radio waves transmit information?
Radio waves can be used to transfer information as an alternating current is put through a transmitter, which causes electrons to oscillate up and down a wire in the transmitter. This causes the electromagnetic fields around the transmitter to oscillate and form electromagnetic waves. These wave travel from the transmitter to a receiver, causing the electrons in the receiver to oscillate. Data is transmitted as the frequency of the waves is the same from the transmitter then those in the receiver- the data is encrypted in the frequency.
What is a boundary for a wave?
A boundary is between two mediums where the wave travels at different speeds.
At a boundary, what will the wave do?
A mixture of reflection, absorption and transmission.
How many radians is 360 °?
2π
How many radians is 180degrees
π
What do waves transfer?
Energy
What is the power of a wave?
The energy transferred per second from a wave.
What are the units for intensity of a wave.
Wm2