Purpose of core in a step-index optical fibre
Core is transmission medium for electromagnetic waves to progress by total internal reflection (wit low absorption)
Define wavelength
Distance between two adjacent peaks on a wave
Define amplitude
The maximum displacement of the wave from its equilibrium position
What are areas of high and low pressure called in a longitudinal wave?
High -> compressions
Low -> rarefactions
What does a polarising filter do?
Only allows oscillations in one plane
How is polarisation used as evidence of the nature of transverse waves?
Polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (as they are in transverse waves)
Describe the appearance of the first harmonic
Two nodes at either end and an antinode in the middle
What is meant by coherence?
- Constant phase difference
- and the same frequency and wavelength
When does most diffraction occur in single/two source intereference?
- When slit is close to wavelength in size
- so lots of diffraction occurs
What is phase?
The position of a certain point on a wave cycle
What is a progressive wave?
A wave that transfers energy without transferring material
4 safety measures when using lasers
- Never shine the laser towards a person
- Wear high opacity safety goggles
- Have a warning sign on display
- Turn the laser off when it is not needed
State the principle of superposition
When two or more waves cross, the resultant displacement of a particle is the sum of the displacements caused by the individual waves
Explain how double slit diffraction occurs
- Intense light from a laser hits a pair of slits
- Light diffracts from the two slits, spreading out in circles
- The light from the two slits is coherent as it is derived from the same source
- The light from both slits strikes a screen
- When two waves cross, they superpose
- The path difference from each slit to a point on the screen causes a phase difference
- Phase difference of 0 = whole number of half wavelength causes constructive interference–> max amplitude and bright spot
- and vice versa
What happens as slit width approaches the size of the wavelength?
Diffraction becomes more observable
Why does single slit diffraction occur?
- Waves from different edges of the slit travel difference distances to the screen
- So there is a path difference
- A path difference means a phase difference
- –> Constructive and destructive interference.
Describe the single slit pattern
- Bright central fringe with fringes either side of decreasing intensity
- The central fringe has twice the width of those either side
- The central fringe has a much larger intensity than those either side
2 applications of diffraction gratings
- Measuring the wavelength of light from stars (using absorption spectra) to determine the constituents in the star’s atmosphere
- X-ray crystallography for finding the spacing between atoms - a crystal sheet acts as the diffraction grating for X-rays to pass through
Which colour makes the smallest angle of refraction? (think Pink Floyd)
Red
Why can chromatic dispersion be observed easily using a glass prism?
The light refracts twice, which makes the change in angle of refraction more obvious
Why does modal dispersion occur?
Different rays enter the core at different angles, and so travel different paths (some may travel middle while others reflect repeatedly), so the time of arrival at the end of the fibre varies
Why does material/modal dispersion mean less data being sent down the fibre? (3*)
- Material/modal dispersion causes pulse broadening, which causes received signals to be wider than the original signal
- Light pulses that are too close will become merged at the end of the fibre
- So signals must be sent at longer time intervals
Why does material dispersion occur?
- Different wavelengths travel at different speeds in a fibre
- Because refractive index depends of wavelength
How do you fix material dispersion?
Using monochromatic light
