Physics unit 1 Flashcards
What are the two wave types?
Transverse and Longitudinal
What are transverse waves?
They oscillate at right angles to the direction of propagation (up and down or side to side)
Can be visible as travelling ripples in water or as standing waves
What are longitudinal waves?
Oscillate back and forth along the direction of propagation. They can cause compression and rarefaction which move with the wave pattern.
What happens with energy in waves?
Energy is transferred from one place to another with no net transfer of matter.
What are Displacement and oscillations?
Displacement - A vector it has a direction and magnitude, the distance moved from its original position
Oscillations - They occur around a rest value of the displacement
Explain displacement-time graphs
Focuses attention on the oscillations at one point in space. Shows how the displacement changes over time for a moving object.
Explain displacement-distance graphs
A snapshot in time showing the wave shape over the whole space at that instance. Shows the position of the particles in a wave relative to the reference point.
What is meant by out of phase?
Phase difference is a measured as an angle which is representative of the fraction of a cycle. Two waves of the SAME frequency could be in or out of phase. Its out of phase when for example a trough meets with the other waves peak.
What is the calculation involving wavelength and frequency?
(λ) Wavelength = (v) velocity/(f) frequency
What is the speed of light?
In free space 2.998x10^8
What is frequency?
Measured in s^-1 or Hz - its the number of waves per second produced or passing any fixed point
How can you tune stringed instruments without changing the length of the string?
Thicker strings -> More mass per unit length meaning that it has a lower speed therefore lower notes
Thinner strings -> More tension in the string meaning a faster speed and therefore a higher notes
What is tension?
A force which is measured in Newtons
How to change tune of air column instruments?
As speed of sound is fixed it can only tune brass or woodwind instruments by changing the length of the pipes being used.
What is the ratio which involves the speed of light (c)?
c/v = n (refractive index of a medium where v is the velocity speed at which light is traveling through the medium)
What is one complete cycle in degrees and radians?
360° or 2π Rad
Wave fronts, explain
Lines, plane if in 3D that join points in a wave where all oscillations are in phase. They are spaced one wavelength apart and move forward in a direction perpendicular to the wave front.
What is the principle of superposition?
Where waves from different sources cross and their amplitudes add together.
When do interference patterns occur?
When the wave sources are coherent i.e. they have the same frequency and a fixed relationship
What are gratings?
Flat arrays of regularly spaced lies which are designed to break a plane wave-front into separate wave sources.
How are dark and bright fringes created?
A path difference is created by dividing a light source so that different rays of light will travel different paths. That difference causes a phase difference, the path difference increases as the angle through the grating increases. When the path difference is a whole number nλ the light ray is in phase - creating a bright fringe (Constructive interference).
In between where the path difference is λ/2, 3λ/2 etc. dark fringes occur - destructive interference.
What is a spectra?
Lines produced from different wavelengths in light. As the conditions for constructive interference happens at different angles for each wavelength it means a set of coloured lines are produced rather than bright fringes.
White light for example
What is emission spectra?
Light emitted by electrons when excited by heating or an electric discharge, it is characteristic of the electronic structure of each element.
How do we identify elements?
Flame tests and emission spectra
What is the relation between emission spectra, photons and electrons?
Emission spectra is produced by electrons which have been excited to the next energy level through an energy transfer for example absorbing a photon to reach a higher energy level or if they emit a photon to lower an energy level.
What is a stationary wave?
Can be considered as two moving waves which move in opposite waves moving in opposite directions, superimposed on one another. This results in a fixed pattern of nodes and anti-nodes.
What happens with energy in a stationary wave?
Energy is stored within a resonator
What is a node and an anti-node?
Node - Have minimum (ideally zero) oscillation, zero amplitude so zero displacement
Anti-node - Have maximum amplitude so maximum displacement
What is a resonator?
They store wave energy through reflecting the wave back upon itself to form a stationary wave pattern, have a fundamental (a lowest) natural mode of oscillation and higher harmonics.
Only efficiently receive energy if its from a source which has a frequency to its own natural frequency - Resonance
What is resonance?
when energy is only efficiently received if its from a external source which has a frequency to its own natural frequency
What are some uses of resonance?
Reflection of TV signals in order to create a node at the reflector, the dipole detector will be placed at an anti-node
In microwaves, a resonant cavity is used to build up power. Reflections inside when cooking result in spacing nodes and anti-nodes so food is rotated to avoid hot & cold spots
How do string instruments link to waves?
They use stationary transverse waves to make resonators out of stretched string. The fixed ends are ALWAYS nodes, the fundamental mode has a singular anti-node at L =λ/2.
Higher harmonics any number n of anti-nodes can fit on the the length so L =λ/2n
What determines the wave speed of stringed instruments?
Tension and weight
Also determine the resonate frequencies
Where is an anti-node on pipe instruments?
The open ends are anti-nodes, a pipe that is open at both ends has L= λ/2n a complete set of harmonics
What happens when you close one end of a two ended pipe instrument?
The closed end would become a node
What are fiber optics?
Optical fibers made of high-density glass which can carry signals for long distances without losing any light through their sides.
What is the refractive index equation?
n = c/v or n = sin i / sin r
Where n > 1 (more than 1 as n is 1 for a vacuum or air)
What is the critical angle?
The least angle of incidence at which total internal reflection TIR happens
- only happens when light is leaving an optically dense medium to a less dense medium at the boundary of the two
Relationship between angle of incidence and angle of refraction
As the angle of incidence increase the angle of refraction also increases and is always larger
What is the equation for critical angle?
Sin C = 1/n
Optical fibers vs Copper wires Benifits
Optical fibers are better than copper wires as signals:
- Have lower losses and can travel further before needing to be amplified
- Cannot be trapped into, better security
- Can carry higher frequencies, can provide a broader bandwidth
Optical fiber drawbacks
High costs and needs specialist installation
How do light paths work in optical fibers?
Light from a range of angles are refracted on entering the cut end of the fiber, hitting the inside at angles greater than critical angle so it is internally reflected.
Cladding has a lower n than the fibers core
What is broadband?
Term which describes the system of fast internet access with fiber optics, satellites and copper wires with a range of frequencies which create a long-distance network.
What is illumination in terms of optical fibers? (endoscopy)
Where a small fiber is used to pipe light from an external source into a remote end where it illuminates the area that is being investigated
How are images formed in illumination? (endoscopy)
Lenses are found at both ends
A tiny lens is used at the remote end of an endoscopy which focuses an image onto the cut ends in a second large bundle (main bundle). In the main bundle each fiber collects and carries light for one pixel. At the eyepiece, the operator end, light exiting is refocused to be viewed as an image by eye or camera
What are analogue images?
An image where the brightness and colour of each pixel vary with time in direct proportion to light collected by objective lens.
Other devices which use critical angles & internal reflection
Fingerprinting - where fingerprint lines touch the glass they change the critical angle hence the amount of light reflected back internally to a detector (device unlocking)
Rain detectors on windscreens - Water on the windscreen is optically dense so it alters the critical angle
What is sampling rate?
The number of times per second the quantity is measured
What is sampling sensitivity?
The smallest increment the quantity is measured and recorded at
How do digitised videos do to pixels?
Each pixel is converted to a set of numbers which represent its colour and brightness.
How to set up a ADC (analogue to digital converter)?
1 - connect the sensors to give analogue signal into ADC input
2 - Select he sampling rate & sensitivity
3 - Send digital output signal to display (or to a data storage device)
Why would you transmit in digital signals?
(Advantages)
More data can fit in the same bandwidth
Most interference can be eliminated producing a higher quality
Can be stored and processed by a computer
Why wouldn’t you transmit in digital signals?
(Drawbacks)
Time delay due to signal processing
Data accuracy is limited by sampling settings (rate & sensitivity)
What is the order of the electromagnetic spectrum, highest to lowest frequency ?
Gamma rays (10^21)
X-rays
Ultraviolet
Visible light - VIBGYOR
Infrared rays
Microwaves
UHF - TV
VHF - FM
Am radio (10^6)
What is the order of the electromagnetic spectrum,
smallest to largest wavelength ?
Gamma rays (10^-12)
X-rays
Ultraviolet
Visible light - VIBGYOR
Infrared rays
Microwaves
UHF - TV
VHF - FM
Am radio (10^3)
What is true for the whole EM spectrum?
All have the same speed
What differs for the EM spectrum?
Wavelength, frequencies and photon energies
How do dish antenas work?
They concentrate waves into one place, a directional beam with flat plane wave-fronts. (wave will lose intensity much slower)
