Waves & Quantum

Longitudinal waves

Oscillations of the particles are parallel to the direction of the wave traveled

Transverse waves

Oscillations of the particles are perpendicular to the direction of the wave traveled

Polarisation

Restricting the oscillations of a transverse wave to one plane

How is a stationary wave produced

• A wave reaches the end of the string and reflects back creating a same wave travelling in the opposite direction

• The two waves superpose which creates a standing wave

• When they interfere constructively, positions of maximum amplitude are produced which are called antinodes

refractive index of light

• Red light has a lower refractive index due to its larger wavelength

• Blue light has a higher refractive index due to its smaller wavelength

Total Internal Reflection

When the incident ray hits the boundary at an angle greater than the critical angle all the light is reflected inside the material

Optical fibres

• Three main components : an optically dense core, a lower optical density cladding and an outer sheath

• Used in communication such as telephone and internet transmission, and medical imaging such as endoscopes

Purpose of outer sheath of optical fibres

• Prevents physical damage to the fibre

• Strengthens the fibre

• Protects the fibre from scratches

Purpose of cladding in optical fibres

• Protects core from damage

• Prevents signal degradation through light escaping the core

• Keeps core away from adjacent fibre cores to prevent crossover of information to other fibres

• Provides the fibre with strength to prevent breakage as the core is very thin

Material dispersion

• Occurs when white light is used because different wavelengths of light travel at different speeds

• Red light travels faster than blue light due to a lower refractive index

• Causes pulse broadening

• Monochromatic light is used to prevent this

Modal dispersion

• Occurs when the light rays in the core spread out due to different angles of incidence in the original pulse

• Causes pulse broadening

• The core needs to be very narrow to prevent this

Advantages of a narrow core in a optical fibre

• Less light is lost by refraction out of the core

• There is a smaller change in angle between each reflection

• Less overlapping pulses hence reduction of modal dispersion

• The signal will be transferred quicker leading to improved data and information transfer

• The quality of the signal will be better and less distorted

Consequence of pulse braodening

Different pulses could merge resulting in a completely distorted final pulse

Absorption

• Occurs when part of the signal’s energy is absorbed by the fibre

• The signal is attenuated by the core

• Reduces the amplitude of the signal which can lead to loss of information

• Prevented by using optical fibres repeaters so the pulse is regenerated before significant absorption has taken place

Interference

• Occurs when waves overlap and the resultant displacement is the sum of the displacement of each wave

• Constructive interference has a path differences of nλ

Coherent waves

• Have the same frequency

• Have a constant phase difference

Path difference

The difference in distance travelled by two waves from their sources to the point where they meet

Double slit interference

• sources of the observed wave must be monochromatic and coherent

• Produce a bright central maximum and dimmer side maxima with constructive interference

Coloured interference pattern (white light source)

• White central fringe as all wavelengths are in phase

• All maxima will appear wider in a spread colour spectrum

• Blue maximum will appear closest to the centre as blue light has the shortest wavelength

• Red light with the longest wavelength will appear furthest from the centre

Diffraction

The spreading out of waves when they pass on an narrow slit

Single slit diffraction pattern

• A central maximum with a high intensity

• Subsidiary maxima equally spaced, successively smaller in intensity and half the width of the central maximum

• Blue monochromatic light will produce narrower fringes than red light as it gets diffracted less

• When the slit gets narrower, intensity decreases and finge spacing increases

Fluorescent tube’s mechanism

• Electric current passed through the mercury vapour and its electrons are excited into a higher energy level

• Electrons de-excites back to its original state and releases energy in the form of UV photons

• The UV light excites the electrons in the phosphor coating

• Visible light photons are releases when the electrons de-excites back to original energy level

Work function

The minimum energy required to release a photoelectron from the surface of a metal

Photoelectric Effect

• Electron excites to a higher energy level

• De-excites back to its original state and releases energy in the form of photon

• A single photon interacts with a single electron (so no. of photoelectrons released depends on the light intensity)

Electron Diffraction

• Bright rings occur when waves interfere in phase

• The pattern in produced as electrons diffract

• Particles would only produce a small spot of light

Stopping potential

• Photoelectrons are released with a range of KE

• KEmax = eVs

• The potential difference that stops the fastest photoelectrons from crossing the gap, effectively stopping the current

Fluorescent Tubes

• electric current pass through the vapour and the electrons in mercury are excited and move to a higher energy level

• The electrons de-excites back to a lower energy level which release UV photons

• UV light excites the electrons in the phosphor coating

• Visible light photons are released when the electrons de-excites