3.2 refraction , diffraction and interference Flashcards
Define coherence
Coherent waves have fixed phase difference and the same frequency and wavelengths
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 coherent light source , the light then interferes constructively and destructively to cause an interference pattern
Describe the interface pattern using white light
A white central maximum at the zero order with alternating spectral fringes on either side of decreasing intensity with violet closest to the zero order and red the furthest
Why does an interference pattern occur when light is passed through a single slit ?
Where light diffracts as it passes through the slit where the waves are in phase constructive interference takes place making bright fringes and where the waves are completely out of phase destructive interference occurs making dark fringes
What happens when the slit is widened?
Less diffraction so the zero order maximum is narrower and more intense
What is the approximate refractive index of air?
1
When light enters a more optically dense medium does it bend away or towards the normal?
Towards
When does total internal reflection occur?
When light is on the boundary of a less optically dense medium and the angle of incidence is greater than the critical angle
What is the purpose of cladding in a step index optical fibre?
1) protects core from scratches which would allow light to escape degrading the signal
2) Allows total internal reflection as it has a lower refractive index than the core
How does signal degradation by absorption in the optical fibre affect the received signal?
Part of the signals energy is absorbed by the fibre so the amplitude is lower
What is pulse broadening?
When the received signal is wider than the original, this can cause overlap in signals meaning information lost
How does modal dispersion cause pulse broadening ?
Light rays enter the fibre at different angles causing them to take different paths, meaning some take longer than others causing pulse broadening
What is material dispersion?
When light with different wavelengths is used some wavelengths slow down more than others causing them to reach the end at different times causing pulse broadening
How can modal dispersion be reduced?
By using a single mode fibre reducing the number of different possible paths making pulse broadening smaller
How can material dispersion be reduced?
By using a light with only 1 wavelength / 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 using optical fibres over traditional copper fibres
1) signal can carry more information as light has a high frequency
2) No energy lost as heat
3) No electrical interference
4) Cheaper
5) 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 so the angle of refraction is 90 degrees
What formula can be used for the critical angle of two materials whose refractive indices are known?
sinC = n2/n1 n2>n1
C = critical angle n1= refractive index of material 1 n2 = refractive index of material 2
What is the critical angle of water to air if the refractive index of water is 1.33 ?
sinC = 1/1.33 sin^-1(1/1.33) C = 48.8
What is snell’s law of refraction?
n1sini=n2sinr i= angle of incidence r = angle of refraction
What formula is used to determine the refractive index of a material?
n= c/v c= speed of light in a vacuum v = speed of light in material
State 2 applications of diffraction gratings?
1) Splitting up light from stars to make line absorption spectra used to identify elements in the star
2) X-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 dsinϑ=λ
1) for the first order maximum the difference between 2 adjacent rays of light is 1λ , the angle between the normal to the grating and the light ray is ϑ
2) A right angled triangle is formed with sides d and λ, the upper angle is ϑ
3) for the first maximum sinϑ= λ/d . rearrange to dsin= λ
When light passing through a diffraction grating go from blue to red do the orders get closer ?
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 as they pass through or around a gap
How did Young’s double slit experiment provide evidence of the wave nature of light?
Diffraction and interference are wave properties so the interference pattern of light shows light has wave properties
What are 4 safety precautions that must be followed when using a laser ?
1) Wear laser safety goggles
2) Dont shine the laser at reflective surfaces
3) Display a warning sign
4) Never shine the laser at person
What formulas is linked with Young’s double slit experiment?
w = λD/s w-fringe spacing λ-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 waves?
Place a speaker at the end of a long glass tube , place powder along the bottom of the glass tube , the powder will be shaken at the antinodes and settle at the nodes. The distance between 2 nodes is half a wavelength
What is phase?
The position of a certain point on a wave cycle ( units are in degrees radians or fractions of a cycle)
