Waves Flashcards
Describe a Transverse Wave Diagram
Describe a Longitudinal Wave Diagram
What is a Transverse Wave
When particles of the wave vibrate at right angles/ perpendicular to the direction that energy travels in
What is a Longitudinal Wave
When particles of the wave vibrate parallel to the direction that energy travels in
What is a Wavelength (λ)
The distance from any point on the wave to the next identical point
What is Frequency (f)
The number of vibrations/cycles of the wave per second
What is Time Period (T)
The time for one complete cycle. This is equal to 1/f
What is Amplitude (A)
The maximum displacement from the equilibrium position
What is Phase (e)
The position of a particle within the cycle relative to the start of the cycle
What is the Wave Speed Equation
c = f x λ
Wave Speed ( m s-1) = Frequency (Hz) x Wavelength (m)
Describe the Direction of Particles in Transverse Waves (When the Wave Direction is Towards the Right)
- P - Down
- Q - Up
- R - Up
- S - Down
- T - Down
What are Progressive Waves
A wave that travels through a substance or space, transferring energy
What is Phase Difference
The fraction of a complete cycle between two point of a wave
Describe different Phase Differences
- In Phase - particles in parts of a wave that are moving at the same speed in the same direction
- Out of Phase - particles in different parts of a wave at a particular time
- Antiphase - particles in parts of a wave moving at the same speed in opposite directions
Describe a Phase Difference Diagram
- Phase Difference -
- A - 0° or 0 radians
- B - 90° or ℼ/2 radians
- C - 180° or ℼ radians
- D - 270° or 3ℼ/2 radians
- E - 360 or 2ℼ radians
What is a Mechanical Wave
Waves that cannot transmit energy in a vacuum, either transverse or longitudinal
Give Examples of Mechanical Waves
- Sound waves
- P and S waves
- Water waves
What are Electromagnetic Waves
Waves that can transmit energy through a vacuum, always transverse waves
Describe the Properties of All Electromagnetic Waves
- All travel at the speed of light in a vacuum
- Energy carried by oscillating electric current and magnetic field
- All are in phase and vibrating at 90° to each other
Describe the Electromagnetic Spectrum (Smallest Wavelength First)
- Gamma Rays
- X-Rays
- Ultra
- Visible Light
- Infrared
- Microwaves
- Radio Waves
What is the wavelength of visible light, red light and violet light
Visible Light = 400nm - 700nm
Red = Approximately 700nm
Violet = Approximately 400nm
What is Polarisation
When the oscillations of a wave are confined to one plane (e.g. vertically or horizontally polarised) and only happens to transverse waves
Describe a Polarisation Diagram
Why can only transverse waves be polarised
- Transverse waves’ displacement is at a right angle to the direction of energy transfer
- Longitudinal waves’ displacement is parallel to the direction of energy transfer
- Longitudinal is not vibrating in the direction of travel it is vibrating parallel and therefore will not be restricted to one plane
Describe Polarisation Applications
- Polaroid material (sunglasses)
- The alignment of aerials for transmission and reception
How is polarisation used in aerials/ antennas
- Ariels/ Antennas recieve signals that are polarised
- Therefore the orientation of the rods on the transmitting aerials and recieving aerial must be algined in the same plane
- In order to receive the signal at full strength
Describe Polarisation on a Light Intensity Graph during Rotation
What is Refraction
The change of speed and direction at a boundary when a wave travels from one medium to another
What happens when a light wave travels from air into a medium such as glass or water
The wave slows down, the wavelength gets shorter and frequency stays constant
When do waves change direction
- Waves arriving along the normal - do not change direction because all the waves reach the boundary simultaneously
- Waves arriving at a different angle - change direction as some parts arrive earlier and slow down before the rest
What does TAGAGA stand for
- Towards
- Air
- Glass
- Away
- Glass
- Air
What is the Refractive Index Equation
n = C / Cs
Refractive Index = Speed of Light / Speed of Light in the material
What is Snell’s Law of Refraction
n1 sin(𝛳) = n2 sin(𝛳)
Refractive Index x sin( angle of incidence) = Refractive Index x sin(angle of refraction)
What is a Critical Angle
The smallest angle that traps light into a denser medium
What is Total Internal Reflection
- The complete reflection of a wave at a boundary
- Within a material that has a higher refractive index than its surroundings
What are the two conditions for total internal reflection
- The inside material must have a larger refractive index than the surrounding material
- The angle of incidence must be greater than the critical angle
What is the Critical Angle Equation
Sin (𝛳) = n2 / n1 n1 > n2
Sin ( critical angle) = Refractive Index / Refractive Index
Why does n1 need to be greater than n2 in the critcal angle equation
Because you would be doing the inverse sin() of a number larger than 1 which does not exist
What is Optical Fibre
A thin glass or plastic fibre that transmits light
What are the advantages of optical fibre over traditonal copper wire
- Signal can carry more information as light has higher frequency
- No energy lost as heat
- Cheaper
- Very fast
What is Step Index Optical Fibre
An optical fibre with a uniform refractive index in the core and a uniform refractive index for the cladding
How does an Optical Fibres use Reflection
- A light ray will undergo a series of total internal reflections each time it reaches the boundary
- The the ray is reflected back and forth inside the fibre
- Eventually reaching the end of the fibre
What are the Benefits of Cladding the Optical Fibres
- With no cladding there are multiple reflections at a fairly small angle,
- Cladding the fibres provides a lower refractive index and decreases the critical angle so that total internal reflection takes place
- Therefore the distance travelling is smaller or less energy is lost and the time of transmission is shorter
- Also the inner fibre is protected from damage
Why does the core need to be protected from scratches in a optical fibre
- Water can get into the scratches, this will increase the refractive index and could prevent total internal reflection from occuring
- The scratch may alter the angle at which the signal reflects with the cladding so the angle may be lowered below the critical angle which will prevent total internal reflection from occuring
What are Types of Dispersion in an Optical Fibre
- Modal
- Material
What is Modal ( or Multipath) Dispersion
Lightrays enter the fibre at different angles so they take different paths along it, so the rays take different times to travel along the fibre
What is Material ( or Spectral) Dispersion
When different colours of light travel through the fibre at different speeds (red travels fastest) causing a pulse broadening
How do you produce a good image using optical fibre
The less dispersion, the more coherent the bundle will be at the other end ( this means all the rays will arrive at the same time and in the same relative position giving a good image)
Describe two of the Problems with Optical Fibre
- Pulse Broadening - when the duration of a pulse increases due to dispersion, this is a problem because it causes an overlap in signals leading to information loss
- Absorption - when the energy from the signal is absorbed by the optical fibre, this is a problem because it means the signal strength fails
Describe Ways to Reduce Problems with Optical Fibre
- Pulse Broadening - use a monochromatic source so the speed is constant or use of monomode fibre (very narrow fibre) to reduce multipath dispersion
- Absorption - make optical fibre out of material with low absorption or amplify the signal
