3.2 Refraction, 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 (same wavelength/colour) light so diffraction and interference 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 coheret light source The light interferes constructively and destructively to create an interference pattern
Describe the interference pattern created using white light
A bright white 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 light diffracts as it passes through the slit Where the waves are in phase constructive interference occurs making bright fringes Where the waves are 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 would become narrower and more intense
Is the following a double slit pattern, single slit pattern or a diffraction grating pattern?
Single slit
What is the approximate refractive index of air?
1
When light enters a more optically dense medium does it bend towards or away from the normal?
Towards the normal
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 is the purpose of the cladding in a step index optical fibre?
Protects core from scratches which would allow light to escape and degrade the signalAllows TIR as it has a lower refractive index than the core
How does signal degradation by absorption in an optical fibre affect the recieved 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 memory 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 mode fibre (very narrow) 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 more information as light has a high frequencyNo energy lost as heatNo electrical interferenceCheaperVery fast
What path does a light ray take when the angle of incidence is equal to the critical angle?
It goes along the boundry
What formula can be used to find the critical angle for 2 materials whose refractive indicies 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 boundry if water has a refractive index of 1.33?
sinC = n2/n1 n2 = air and n1 = water C = sin-1(1/1.33) C = 48.8
Using snells law of refraction, find the angle of refraction in a material with RI = 1.53 when the angle of incidence is 32 from a material with a RI = 1.23
n1sin(i) = n2sin(r) 1.23sin32 = 1.53sinr sinr = 1.23sin32/1.53 sinr = 0.426 r = 25.2
Glass has a refractive index of 1.5, water has a refractive index of 1.33, which is more optically dense?
Glass
What formula is used to determine the refractive index of a material?
n = c/(c_s) n = refractive index c = speed of light in a vaccum (c_s) = 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 starX-ray crystallography, a crystal sheet acts as the diffraction grating the X-rays pass through, used to find the spacing between atoms
Derive the formula (d \sin \theta = n \lambda)
For the first order maximum, the path difference between two adjacent rays of light is (\lambda), the angle between the normal to the grating and the light ray is (\theta)A right angled triangle is formed, with side lengths (d) and (\lambda). The upper angle is (\theta).For the first maximum (\sin \theta = \frac{\lambda}{d}), rearrange to (d \sin \theta = \lambda)Other maxima occur when the path difference between the two rays of light is (n \lambda), where (n) is an integer, replace (\lambda) with (n \lambda) to get: (d \sin \theta = n \lambda)
When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?
The wavelength of the 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
How did Young’s double-slit experiment provide evidence for the wave nature of light?
Diffraction and intergerence are wave properties, hence the interference pattern of light shows light has wave properties
What are 4 safety precautions that must be followed when using a laser?
Wear laser safety gogglesDont shine the laser at reflective surfacesDisplay a warning signNever shine the laser at a person
What formula is associated with Young’s double-slit experiment?
(w = \frac{\lambda 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
How could you investigate stationary sound waves?
Place a speaker at one end of a closed glass tubeLay powder across the bottom of the tubePowder will be shaken from the antinodes and will settle at the nodesThe distance between each node is half a wavelength
What is ‘phase’?
The position of a certain point on a wave cycle Units are radians, degrees or fractions of a cycle
True or False: Only light can produce interference patterns
False: interference patterns can be produced by sound waves and all EM waves too
