3.2 Refraction, diffraction and interference Flashcards
Define coherence.
Coherent waves have a fixed phase difference and the same frequency and wavelength.
Define path difference.
A measure of how far ahead a wave is compared to another wave.
Usually expressed in terms of the wavelength
Why are lasers used for showing interference and diffraction?
They are monochromatic sources of light, hence interference and diffraction patterns are more defined.
Describe the Young’s double slit experiment.
Single light source directed at two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interference pattern.
Use wave theory to explain how a fringe pattern is formed.
- Slits act as coherent sources
- Waves diffract at slits
- Waves superpose
- Bright patches: constructive
- Dark patches: destructive
Describe the interference pattern created using white light.
- Bright white central maximum
- Dark fringes
- Bright fringes beginging with violet and ending with red
- Repeating 2 and 3
What is the equation for fringe spacing?
What do the letters represent in this equation?
What are 4 safety precautions that must be followed when using a laser?
● Wear laser safety goggles
● Don’t shine the laser at reflective surfaces
● Display a warning sign
● Never shine the laser at a person
How does a diffraction pattern change if the slit width became narrower?
- Increased fringe separation
- Lower maxima intensity
What is diffraction?
The spreading out of waves when they pass through or around a gap.
What is the equation for a diffraction grating?
What do the letters represent in this equation?
State 2 aplications of diffraction gratings
- Absorption spectra to identify elements in a star.
- X-ray crystallography
Which direction does light bend when entering a more optically dense medium?
Towards the normal.
What is the equation for refractive index of a substance?
What do the letters represent in this equation?
What is Snell’s law?
(Law of refraction)
What do the letters represent in this equation?
What is the equation for the critical angle?
What do the letters represent in this equation?
When does Total Internal Reflection occur?
When light is at a boundary to a less optically dense medium and the angle of incidence is greater than the critical angle.
What are the two components of an optical fibre?
- Cladding
- Core
What are the purpose of cladding?
● Protects core from scratches which would allow light to escape and degrade the signal.
● Allows TIR as it has a lower refractive index than the core.
How does absorption in an optical fibre affect the signal?
Part of the signal’s energy is absorbed by the fibre so its amplitude is reduced.
What is pulse broadening?
When the received signal is wider than the original, this can cause overlap of signals leading to information loss.
What is material dispersion?
When light with different wavelengths is used some wavelengths slow down more than others in the fibre so they arrive at different times causing pulse broadening.
Causes pulse broadening
How can material dispersion be reduced?
Use monochromatic light.
What is modal dispersion?
When rays enter the fibre at different angles, causing each ray to travel a different distance. Resulting in the rays arriving at different times.
Causes pulse broadening
How can modal dispersion be reduced?
Use a very narrow fibre so the possible difference in path lengths is smaller.
How can both absorption and dispersion be reduced?
Use an optical fibre repeater to regenerate the signal.
State the advantages of optical fibres over traditional copper wires.
● Signal can carry more information as light has a high frequency
● No energy lost as heat
● No electrical interference
● Cheaper
● Very fast
