Rarefaction, diffraction and interference Flashcards
Define coherence
Coherent waves have a fixed phase difference and the same frequency and wavelength
Why is a laser useful in showing interference and diffraction?
It produces monochromatic wavelength so diffraction and interreference patterns are more defined
What was Young’s double-slit experiment?
A single light source is directed towards two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interreference pattern
Describe the interreference pattern created using white light
A bright light central maximum flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest
Why does an interference pattern form when light is passed through a single slit?
The diffracts as it passes through the slit, where the waves are in phase constructive interference occurs making bright fringes and where the waves are completely out of phase destructive interference occurs making a dark fringe
Increasing the slit width increases the width of the central diffraction maximum. True or false?
False, the slit is not so close to the wavelength in size so less diffraction occurs - the central maximum becomes narrower and more intense
Refractive index of air
1
When light enters a more optically dense medium does it bend towards or away from the normal?
Towards
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
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 signal degradation by absorption in an optical fibre affect the received signal?
Part of the signals energy is absorbed by the fibre so its amplitude is reduced
What is pulse broadening?
When the received signal is wider than the original, his can cause overlap of signals leading to information loss
How does modal dispersion cause pulse broadening?
Light rays enter the fibre at different angles so they take different paths along it, some may travel down the middle while others are reflected repeatedly, so the rays take different times to travel along the fibre, causing pulse broadening
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
How can modal dispersion be reduced?
Use a single model fibre so the possible difference in path lengths is smaller
How can material dispersion be reduced?
Use monochromatic light
How can both absorption and dispersion be reduced?
Use an optical fibre repeater to regenerate the signal now and then
State the advantages of optical fibres over traditional copper wires
*Signal can carry information as light has a high frequency
*No energy lost as heat
*No electrical interference
*Cheaper
*Very fast
What path does a light ray take when the angle of incidence is equal to the critical angle?
It goes along the boundary
What formula can be used to find the critical angle for 2 materials whose refractive indices are known?
*sinC = n2/n1 where n1>n2
*C = critical angle
*n1 = refractive index of material 1
*n2 = refractive index of material 2
What is the critical angle of a water to air boundary if water has a refractive index of 1.33?
*sinC = n2/n1
n2 = air =1 n1 = water = 1.33
C = sin^-1 (1/1.33)
C = 48.8
Snells law of refraction
n1sini = n2sinr
Formula used to determine the refractive index of a material
n = c/v
n = refractive index
c = speed of light in vacuum
v = speed of light in material
State 2 applications of diffraction gratings
*Splitting up light from stars to make line absorption spectra - used to identify elements present in the star
*X-ray crystallography, a crystal sheet acts as the diffraction grating the X-rays pass through, used to find the spacing between atoms
When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?
The wavelength of light has increased so it will diffract more, the orders will become further apart
What is diffraction?
The spreading out of waves when they pass through or around a gap
Double slit experiment providing evidence for the wave nature of light
Diffraction and interference are wave properties hence the interference pattern of light shows light has wave properties
4 safety precautions that must be followed when using a laser
*Wear laser safety goggles
*Dont shine the laser at reflective surfaces
*Display a warning sign
*Never shine the laser at a person
Youngs double slit experiment formula
w = lambda x D / s
w = fringe spacing
lambda = wavelength of light used
D = distance from screen to slits
s = slit separation
What is path difference
The difference in distance travelled by 2 waves
What is a phase?
The position of a certain point on a wave cycle
