Waves and Optics Flashcards
Properties of waves
Transfer energy without transferring matter
Define and describe longitudinal waves
? Vibration/oscillations are parallel to the direction of energy transfer. They consists of compressions and rarefractions.
Cannot travel in vacuum.
Eg: sound waves
Define and describe transverse waves
? Vibration/oscillations are perpendicular to the direction of energy transfer
Can travel in vacuum
Eg: EM Spectrum, waves on a string
Define displacement
? The distance and the direction from its equilibrium position
Metres (m)
Define amplitude
? Maximum displacement of a vibrating particle
Metres (m)
Define Wavelength
? Distance between 2 adjacent vibrating particles
Eg: distance between 2 adjacent crests
Metres (m)
Define cycle
? Complete cycle is from max displacement to the next maximum displacement
Define period (time period)
? Time for one complete oscillation. seconds (s)
f = 1/T
Define frequency
? Number of complete wave (cycles) passing a point per second
Hertz (Hz)
v=f𝜆
Plane polarised light
? Light that vibrates in a single plane. This plane includes the direction of propagation of waves. Vibrations are at 90o to the direction of travel.
Only transverse waves can be polarised.
Application of polarisation
Television/radio aerials work using polarisation
Radio waves transfer their energy into electrons within the aerials, causing a AC current
This is why aerials need to be aligned in a certain orientation
Define wave speed
? Distance travelled by a wave per second
Define phase difference
Measured in degrees or radians, the amount by which one wave lags behind the other wave.
Define progressive
? Waves whose oscillations travel and transfer energy
Define and describe superposition
? the point where 2 waves meet.
The total displacement is equal to the sum of the individual displacements.
Formula to calculate phase difference
phase difference (radians) = 2πd/𝛌
What is destructive interference ?
When they are in antiphase with each other (180 degrees out of phase)
A crest will meet a trough and cancel out each other to produce a minimum point.
What is constructive interference?
Where the waves are in phase with each other. The crest and troughs will constructively interfere with each other creating a ‘double’ creat/trough.
Define stationary waves
? When waves of the same frequency travel in opposite directions (reflected), the waves superpose/interfere and a stationary wave is formed. Stores energy instead of transferring.
Describe the process in creating a stationary wave
- A wave is generated at one end of the string and travels down it.
- At the other end, this wave is REFLECTED and travels back in the OPPOSITE direction
- The FREQUENCY of the wave generation and the LENGTH of the string are such that the next wave generated meets this reflected wave and undergoes superposition.
- At places where the two waves are in phase, they undergo constructive interference and form a MAXIMUM point known as an ANTINODE
- At places where the waves are in antiphase, they undergo destructive interference and form a MINIMUM point known as a NODE.
Equation to calculate FUNDAMENTAL frequency of a wave on a string.
f = 1/2L (T/u)^0.5
u(mu) - mass per unit length
T - tension of the string
L - length of the string
Can you describe stationary waves formed on a string and those produced with microwaves and sound waves?
Microwaves - Formed by reflecting a microwave beam at a metal plate to find the nodes and antinodes use a microwave probe
Sound waves - Formed by placing a speaker at one end of a closed glass tube. Lay powder across the bottom of the tube, it will be shaken at the anti nodes and settle at the nodes.
Can you describe stationary waves on strings in terms of harmonics?
? the lowest frequency at which a stationary wave forms is the first harmonic. ( 2 nodes and 1 antinodes.)
What is a coherent light source
Same frequency and wavelength and a fixed phase difference
What does young’s double-slit demonstrate?
The interference of light from two sources.
- Shine a coherent light source through 2 slits about the same size as the wavelength of the laser light so the light diffracts
- Each slit acts like a coherent light source.
Wave nature of light
When does light and dark fringes happen in young slits
Light - in phase and constructive interference
Dark - Completely out of phase and destructive interference
Formula for young slits experiment
w=λD/s
What happens if white light is used instead of monochromatic light. ( young slits)
wider maxima, less intense diffraction pattern with a central white fringe.
What are the safety precaution of using lasers
Wear laser safety goggles
Dont shine the laser at reflective surfaces
Display a warning sign
Don’t shine it at a person
What is diffraction and when does it happen.
Spreading out of waves when they pass through or around a gap. When a monochromatic light is diffracted through a single slit onto a screen.
When is the diffraction greatest?
When the gap size is the same as the wavelength.
What if white light is used instead of
monochromatic light ? (diffraction)
White light is made up of all colours. they have different wavelengths and would diffract by different amounts. You will get a spectrum of colours
What if white light is used instead of
monochromatic light ? (diffraction)
White light is made up of all colours. they have different wavelengths and would diffract by different amounts. You will get a spectrum of colours
What if white light is used instead of
monochromatic light ? (diffraction)
White light is made up of all colours. they have different wavelengths and would diffract by different amounts. You will get a spectrum of colours
What can you do to vary the width of the central maximum.
Increasing the slit width decreases the amount of diffraction, so the central maximum becomes narrower and the intensity increases
Increase the wavelength of the light increases the amount of diffraction, thus the maximum becomes wider and the intensity decreases
Formula assciated with diffraction grating
d sin(theta) = nλ
Application of diffraction grating
Split up light from stars to get line absorption spectra which is used to identify the elements present
X-ray crystallography, measure the atomic spacing in certain materials.
Define refractive index (n)
? property of a material whcih measures how much it slows down light passing through it.
formula associated with refractive index
n =c/cs
What is the refractive index of air
1
What is the equation for snell’s law
n1sin(theta1) = n2sin(theta2)
Define total internal reflection
Occrus when the angle of incidence is greater than the critical angle. Or incident refractive index is greater than the refractive index of the material
Equation to calculate n from IGCSE
sin (theta) in air / sin (theta) in medium
n = 1/sinc
real depth / apparent depth
Speed of light in air / speed of light in medium
Equation to calculate critical angle
sin (thetac) = n2/n1
n1>n2
Application of TIR
Optical fibres
Function of cladding
Lower optical density
Protects the core from damage
Prevents signal degration
Causes of signal degradation
Absorption, Dispersion
Describe absorption (Optical fibres)
The signal’s energy is absorbed by the fibre, reducing the amplitude of the signal
Describe modal dispersion
Caused by light rays entering the fibre at different angles, thus taking different paths. This leads to rays taking different amount of time along the fibre causing PULSE BROADENING
Material dispersion
Cause by light consisting of different wavelength, light travelling at different speeds, leading to pulse broadening
How to reduced signal degradation
Optical fibre repeater, which regenerates the signal during its travel
