Waves Flashcards
What is a wave?
A wave is a disturbance of particles that transfers energy through a medium.
Define Mechanical wave
A mechanical wave is a wave that propagates through a medium.
Compare transverse and longitudinal waves
Transverse waves:
- Oscillation of particles is perpendicular to the propagation of energy
- Crests and troughs
- Amplitude is measured from the equilibrium
- Wavelength is the distance between consecutive crests/troughs
- Example: water and light
Longitudinal waves:
- Oscillation of particles is parallel to the propagation of energy.
- Amplitude is measured from the equilibrium
- Wavelength is the distance between the middle of wave compressions
- Example: Sound
Compression
a region in the longitudinal wave where the particles are closest
Rarefaction
A region in a longitudinal wave in which the particles are furthest apart
Crest
the point on a transverse wave of maximum positive displacement
Trough
the point on a transverse wave of maximum negative displacement
Amplitude
The maximum distance from an equilibrium
Period
The amount of time taken for one cycle to complete
P = 1/F
Frequency
The number of cycles per second
F = 1/P
Wavelength
the minimum distance between two consecutive points that are in phase.
The velocity of a wave
V = d/t
Reflection of a wave
A process whereby an incident wave changes direction back into the same medium according to the law of reflection.
Refraction of a wave
A process whereby an incident wave changes direction and speed at a boundary when passing into another medium.
Diffraction of a wave
The process by which an incident wave bends around a barrier or the wavefront is broken into smaller wave sources.
Describe the superposition of a wave
The process whereby two waves overlap in space whose resultant wave is the algebraic sum of the individual waves.
Using the wave model of light, explain phenomena relating to reflection and refraction
Due to the fact that light is both a wave and a particle, it can be reflected at a boundary according to the law of reflection and can refract through different media by changing speed and direction. This explains why the speed of light varies when moving from different media.
Describe what happens when an incident wave of a certain medium comes in contact with a boundary of the same medium, but of different density?
What happens at fixed and free ends?
Lighter to heavier density
- The incident wave is reflected and inverted
- The transmitted wave is in phase
- If the density is negligible, the transmitted wave will be larger than the reflected wave. However, if the density is large, then the transmitted wave will be smaller
Heavier to a lighter density
- The incident wave is reflected and phase
- The transmitted wave is larger and in phase
Fixed ends
- The reflected wave is inverted
Free Ends
- The reflected wave is in phase
Explain Constructive and Destructive interference of two waves
Constructive interference occurs when two waves of the same frequency superimpose to create a super crest with a larger amplitude. Destructive interference occurs when two waves of the same frequency but 180 degrees out of phase overlap to create a node.
Explain the process by which standing waves are created.
A standing wave occurs when two waves of similar frequency and amplitude continuously constructively and destructively superimpose on one another, creating set nodes and antinodes of minimum and maximum displacement. They first meet and constructively interfere, then destructively interfere and create a node, then meet again to produce a super-crest.
Define the concept of resonance in a mechanical system. Is energy transferred efficiently in resonating systems?
Resonance is the process by which the energy of one vibrating system causes another to vibrate at its natural frequency.
Energy is transferred efficiently in resonating systems.
Define Natural Frequency
The frequency at which an object naturally oscillates in resonance.
Is light a mechanical or electromagnetic wave?
It is an electromagnetic transverse wave that can travel in a vacuum.
Explain Total internal reflection
An incident ray of light that encounters a boundary of media reflects according to the law of refraction and refracts away from the normal if the medium is denser and towards it if it is less dense. if a ray of light hits a boundary at the critical angle, the light reflects back into the medium but does not refract because the angle is such that the refracted ray becomes the boundary.
Why is the wave model of light useful?
It helps us explain the reason for why light can reflect, refract, interfere, diffract and disperse, which is not possible with the particle model.
Describe the process of Polarisation
Light is an electromagnetic wave, propagated by the changing magnetic and electric fields that exist in two planes. Polarisation is the process whereby only the direction of the electromagnetic field that is parallel to the polarization material is ‘let through’ a material. Therefore, a plane wave is created.
Snell’s Law
When light travels from one medium to another, the ratio of the sin of the angle of incidence to the sine of the angle of refraction is equal to the refractive index of the material
Recall the speed of light in a vacuum. Contrast the speed of light and the speed of mechanical waves
3 * 10^8 m/s. Light is much faster than any other mechanical wave becasue it is an electromagnetic wave. Unlike sound, which requires particles for its waves to propogate, light can travel without a medium, with less resistance, and therefore it can travel faster.
Define intensity
Intensity is the average rate of flow of energy per uni area.
I = P/A
