5 Optics Flashcards
refractive index
sin i
n= ——
sin r
air into glass
-light bends towards the normal (goes into a more dense substance)
glass into the air
light bends away from the normal
no refraction when the incident ray is along the normal
refractive index 2
c
ns= —-
cs
ns = ratio of speeds of light c= speed in a vacuum cs= speed of substnace
critical angle
i < critical angle
(an arrow inside and outside the block)
ic= critical angle
(an arrow along the boundary)
i > critical angle
(reflected)
what substance has a high refractive index?
diamond
optical fibres
more dense —> less dense
the incident substance has a larger refractive index than the other substance
partial internal reflection always occurs at a boundary when the angle of incidence is less than or equal to the critical angle
optical fibres
inference
the core has to narrow to prevent dispersion
violet light travels slower than red light
medical endoscope
enclosed space
two bundles of fibres (one for light and another for viewing the image)
core has to be very optically clear t reduce absorption and thin
cladding = lower refractive index
coherence
- same frequency and constant phase difference
- fringe separation depends on the colour of the light
- vapour or discharge tubes = dominant colour
- filament lamp = all colours of the spectrum
single slit
diffraction of a single slit
central fringe is twice as wide
intensity reduces with distance
diffraction grating
plate with many parallel slits
N = 1
—-
d
number of spacing
maximum number of orders is given by the value of distance divided by wavelength rounded down to the nearest whole number
light sources
laser
- coherent (in phase)
- monochromatic
non laser light = polarised first
white light fringes
- blue fringes are closer together than red
- central fringe white (every colour)
Refractive index of air
1
Snell’s law
N2sin2 = n1sin1
Total internal reflection
Sin θ = n2 / n1
dispersion
dispersion. Visible light is actually made up of different colors. Each color bends by a different amount when refracted by glass. That’s why visible light is split, or dispersed, into different colors when it passes through a lens or prism.
optical fibres
Cladding is one or more layers of materials of lower refractive index, in intimate contact with a core material of higher refractive index. It is used in optical fibers. The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the two.
material dispersion
Material dispersion is a phenomenon in which different optical wavelengths propagate at different velocities, depending on the refractive index of the material used in the fibre core.
modal dispersion
the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode
pulse broadening definition
Pulse broadening is defined as the spreading of the light pulses as they travel down the fiber.
pulse broadening and absorption
light rays travelling straight down the axis of the fibre core travel the shortest distance and so reach the far end of the fibre in the shortest possible time. light rays entering the core at a small angle to the axis will be reflected internally a few times before emerging. These rays have a longer path because the core has a non zero diameter. because they’ve travlled a longer path they take a greater time to travel the length of the fibre.
rays that enter the fibre at the same time will emerge from the other end at different times - short duration pulses of light used for digital signalling will emerge as wider duration pulses - and it’s a limit on the rate you can send digital information through a long fibre because in that situation you would like one pulses, one pulse following another in as short a time as possible.
