waves - Refraction, diffraction and interference Flashcards
What is path difference?
The difference in the distance travelled by two waves
What is a coherent light source?
The same frequency and wavelength and a fixed phase difference
What does young’s double slit experiment demonstrate?
Interference of light from two sources
How can you form an interference in young’s double slits experiment ?
Use two coherent light sources or use single source with a double split.
What should you do if you don’t have a coherent source of light in young’s double slit experiment?
Place a single split before the double slit to make the light have a fixed phase difference and a filter to make the light monochromatic
When are light fringes formed in young’s double slit experiment?
When the path difference of the waves are n lambda - difference between the waves is a whole number of wave lengths
When are dark fringes formed in young’s double slit experiment?
When the phase difference of the waves are completely out of phase (antiphase) and the path difference will be whole number and a half wavelengths n1/2 lambda
What happens if white light is used instead of a monochromatic laser?
-Wider maxima
-less intense diffraction pattern
-Central white fringe
-Alternating bright fringes which are spectra , violet closest to central maxima red the furthest
What hazards do lasers provide and what can be done to minimise the risk of these hazards.
Hazard - lases can permanently damage eyesight
steps taken to minimise risk:
-wear laser safety googles
-Don’t shine laser at reflective surfaces
-Display a warning sign
-Never shine the laser at another person
How can the type of interference seen in young’s slits be displayed using sound waves?
- Use two speakers connected to the same single generator
- measure the intensity of the wave using a microphone to find maxima and minima
What did young’s double slit experiment provide evidence for light/EM waves and why?
That light has a wave nature as diffraction and interference are wave properties so EM waves must act as waves most of the time
What is diffraction?
The spreading out of waves when they pass through or around a gap
When does the greatest diffraction occur?
When the gap is the same size as the wavelength
What happens when monochromatic light is diffracted through a single split onto a screen?
-Pattern has a bright central fringe, which is double the width of all other fringes
-Alternating dark and bright fringes on either side
-bright fringes caused by constructive interference
-dark fringes caused by destructive interference
-intensity of bright fringe decreases from central fringe
What happens when white light is diffracted through a single split onto a screen?
-Different wavelengths in white light are diffracted by different amounts
-Get a spectrum of colour
-Has a central white maximum
-Alternating fringes which are spectra, red furthest away and violet closest to central maxima
What happens if you increase the widths of the slits in a diffraction grating
-decreases amount of diffraction
-central maximum becomes narrower
-light intensity increases
What happens if you increase the wavelength of light going through a diffraction grating
-increases amount of diffraction
-central maximum becomes wider
-light intensity decreases
What is a diffraction grating?
A slide containing many equally spaced slits very close together
Why is monochromatic light’s interference pattern much sharper and brighter through a diffraction grating than a double slit?
Many more rays of light reinforcing the pattern
What are some application for diffraction gratings?
1.split up light from stars using a diffraction grating to get line absorption spectra which can be used to find elements in the star
2.X-ray crystallography which is where x-rays are directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern, this is because the wavelength of x-rays
is similar in size to the gaps between the atoms. This diffraction pattern can be used to measure the atomic spacing in certain materials
What is refractive index?
A property of a material which measures how much it slows down light passing through it.
What is refraction?
when a wave changes direction either towards or away from the normal depending on the refractive index
What happens if n1 > n2 in snells law?
n2 is less optically dense than than n1 so ray of light will bend away from the normal
What happens if n2 > n1 in snells law?
n1 is less optically dense than than n2 so ray of light will bend toward the normal
What is the critical angle?
The angle of incidence when the angle of refraction is 90 degrees
When does total internal reflection occur?
When angle of incidence is greater than the critical angle and incident refractive index n1 is greater than the refractive index of the material at the boundary n2.
How does TIR occur in fibre optic cables?
They have an optically dense core surrounded by cladding with a lower optical density
What does the cladding do in a fibre optic cable apart from allow TIR?
-protects the core from damage
-prevents signal degradation through light escaping the core
What cause signal degradation?
absorption and dispersion
What is absorption?
Where signals energy is absorbed by the fibre reducing the amplitude of the signal - can lead to loss of infomation
What does dispersion cause?
Pulse broadening
What is pulse broadening?
Where the received signal is broader than the original transmitted signal. Broadening can cause signals to overlap causing information loss
What is modal dispersion?
-where light enters the optic fibre at different angles
-so light takes different paths along the wire
-rays take different times to travel down the wire causing pulse broadening
How can modal dispersion be reduced?
Make the core narrow making possible difference in path lengths smaller
What is material dispersion?
-where light has different wavelengths
-light rays will travel at different speeds along the fibre
-leads to pulse broadening
How can material dispersion be reduced?
Use a monochromatic light.
How can both modal and material dispersion be reduced?
Use a optical fibre repeater - regenerates the signal on the way to its destination.
