Waves Flashcards
What do waves transfer from one point to another?
Energy
In which direction do longitudinal waves travel?
The direction of wave travel.
In which direction do transverse waves move?
Perpendicular to the direction of energy travel
What does the amount of energy transferred in a wave depend on?
The amplitude of the wave
-The greater the amplitude, the greater energy transferred
What is the equation for the displacement of a wave travelling from left to right?
y = Asin2π(f t −x/λ)
What is the equation for the displacement of a wave travelling from right to left?
y = Asin2π(f t + x/λ)
What is meant by the phase difference of a wave?
A measure of how much one wave lags behind another, or one point lags behind another on the same wave.
What is a phase difference of one wavelength equal to?
2π radians
What is a phase difference of a half wavelength equal to?
π radians
How are stationary waves formed?
The constructive interference of two waves, of the same frequency and amplitude, travelling in opposite directions.
What are formed by the interference of waves?
Nodes with zero amplitude, and antinodes in between with max amplitude.
What is an example of where stationary waves are used?
Musical instruments - to produce different notes
Sketch the first harmonic mode of oscillation (Waves)
Remember:
Half a wavelength
Sketch the second harmonic mode of oscillation (Waves)
Remember: One wavelength
Sketch the third harmonic mode of oscillation (Waves)
Remember: One and a half wavelength
Sketch the fourth harmonic mode of oscillation (Waves)
Remember: Two waves
When does interference occur? (Waves)
When coherent waves overlap.
When are two waves determined as coherent?
If they have the same frequency, wavelength and speed
Or
If they have a constant phase relationship.
When are two waves determined as ‘in phase’?
When the crest of one matches up with the crest of the other
or trough of one matches up with trough of other
When are two waves said to be ‘out of phase’?
When the crest of one matches up with the trough of the other.
When are waves also said to be coherent
When they have a constant phase difference.
What is constructive interference? (Waves)
When two waves of equal amplitude meet in phase, and combine to form a wave of twice the amplitude.
What is destructive interference? (Waves)
When two waves of equal amplitude meet out of phase, and combine to form a wave of zero amplitude.
What are the two ways to produce an interference pattern for light. (Waves)
- Division of Amplitude
- Division of Wavefront
What are examples of division of amplitude? (Waves)
- Interference produced by thin films
- Coated Camera Lenses
- Thin wedge interference
What is an example of division of wavefront?
Young’s double slit experiment
What is produced when path difference = mλ (Waves)
A maximum (constructive interference)
What is produced when path difference = (m+1/2)λ (Waves)
A minimum (destructive interference)
What happens when a wave passes through a medium such as glass?
Its wavelength decreases due to refraction.
What is meant by optical path difference (opd)? (Waves)
The difference between two optical path lengths.
What phase change does a wave undergo when travelling in a less dense medium and reflects from an interface with a more dense medium?
π radians
What phase change does a wave undergo when travelling in a more dense medium and reflecting from an interface with a less dense medium?
No phase change.
When will a coherent source of waves produce constructive interference?
When opd is equivalent to whole number of wavelengths
When will a coherent source of waves produce destructive interference?
When the waves are completely out of phase.
What will happen to liquids that are less dense than water? (Waves)
They will form a thin film on the water surface.
What is the equation for destructive interference within a thin film? (Waves)
2nfilmd = (m+1/2)λ
What does the colour of the reflected light from a thin film depend on? (Waves)
Which wavelengths produce interference, which in turn depends on the thickness of the film, d.
What is meant by the blooming of lenses? (Waves)
Special coating placed on expensive camera/binocular lenses to ensure maximum light is transmitted through them.
Which type of interference occurs during the blooming of lenses? (Waves)
Destructive interference
What does complete cancellation happen for during the blooming of lenses? (Waves)
One particular wavelength only
-Partial cancellation occurs for other wavelengths
Where does the incident light reflect from during thin wedge interference? (Waves)
The inside surface of each glass sheet.
Why will the ray reflected from the upper glass sheet not undergo a phase change in thin wedge interference? (Waves)
It is travelling from a medium with a high refractive index (glass) into one with a lower refractive index (air).
Why will the ray reflected from the lower glass sheet undergo a phase change during thin wedge interference? (Waves)
It is travelling from a medium with a low refractive index (air) into one with a higher refractive index (glass).
What happens when a thin wedge is observed through a travelling microscope with a monochromatic light source? (Waves)
A series of interference fringes are produced.
What are interference fringes? (Waves)
Successive areas of constructive and destructive interference as the optical path difference increases.
What can the fringes produced inside the air wedge be used to determine? (Waves)
The diameter of the support at the end.
What is polarisation? (Waves)
An effect most commonly observed when using polarising lens in sunglasses which reduces the glare from reflective surfaces such as water or glass.
In which direction will electric field oscillations travel for light which is not polarised? (Waves)
Every plane perpendicular to its travel
(The vectors vibrate in all directions).
Where will electric field oscillations occur when light is polarised? (Waves)
Only one plane
What is the equation for initial and final energies and amplitudes of a wave?
E1 / A1^2 = E2 / A2^2
What is the equation for optical path difference? (Waves)
opd = n x gpd
What is the use of a polarising filter (polariser) (Waves)
To allow light to pass through in one plane only.
What happens if a second polariser (analyser) is placed in front of the the polariser? (Waves)
It will block the light totally if rotated through 90° relative to the polarised light.
Which waves can polarisation not be produced with? (Waves)
Longitudinal waves
When can polarisation also be produced? (Waves)
When light is reflected from an electrical insulator such as glass or water.
What is Brewster’s angle? (Waves)
The angle of incidence which causes the reflected light to be fully plane-polarised.
OR
It is the angle of incidence at which the angle between the reflected and refracted rays is 90 degrees.
Derive an expression for Refractive Index (Waves)
n = sin(theta1) / sin(theta2)
n = sin (ip) / sin r
n = sin (ip) / sin (90 - ip)
n = sin (ip) / cos (ip)
n = tan (ip)
What is the equation for constructive interference within a thin film? (Waves)
2nfilmd = m λ
