ᴅɪғғʀᴀᴄᴛɪᴏɴ ɢʀᴀᴛɪɴɢs

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interference patterns can be created using more than
2 slits

diffraction grating use large number (1000+ per cm) 
of equally spaced, parallel slits

spread light into spectrum of colours due to interference

compared to double slit, more spread out, dark fringes
broader because greater number of slits = more opportunities
for destructive interference

ex: etching find lines on glass using diamond point,
space between scratches act as slits

CD - contain equally spaced grooves, 1 micrometer or
1 millionth of a meter apart

separate different wavelengths of light towards eye

because surface is reflective, can be viewed by looking

diffraction effects in nature: striated muscle tissue, crystals,
bacterial layers, useful to scientists in determining 
dimensions and structure

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     Thin Film Interference

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Sir Issac Newton observed interference effects

spherical lens on top of flat piece of glass, small air gap

when illuminated by light, concentric circular fringes 

appear when viewed from above known as Newton's Rings

caused by incident light moving through different thicknesses

of glass, interfere with light reflected from top of lens

test curvature of precisely ground lenses

interference effects also responsible for iridescent colours

thickness of transparent layer close to wavelength of

light illuminating it

some light reflected from top surface, others transmitted

through film, reflect back up from bottom layer

when illuminated by white light, thickness determine which

wavelength undergoes destructive interference

light emerging appear as complementary colour of light

that was canceled

red- cyan, magenta-green, yellow-blue

colours shift if viewed from different angle

light reaching your eyes incident at different angle, 

transmitted wave travels different distance

thicker walls cancel out longer wavelengths

as wall gets thinner, shorter wavelengths (blue) undergo

interference

eventually become too thin to cause interference, appears

colourless just before popping

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             Polarization

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plays important role in how light and materials interact

plane in which electric field oscillates as wave propagates

electromagnetic waves produced when charged particle

accelerates

vertical axis - vertically polarized light

linearly polarized light - waves polarize along single axis

electric field directed parallel to axis of polarization

unpolarized light - polarized along many different axes

ex: light bulb, candles, sun

electrons vibrating in all different directions

1938 - E.H. Land discover Polaroid, could polarize light

made of hydrocarbons in long parallel strands

absorb light polarized along axis of orientation, allow light

perpendicular to pass through

light reflected from nonmetallic surfaces partially polarized

polarizing angle - angle of incidence which reflected light

completely polarized parallel to surface of interface

depend on index of refraction on both sides\

named Brewster's angle, after Sir David Brewster,

Scottish physicist who discovered it

polarized sunglasses tend to have vertical polarization axes.

designed to block light reflected from horizontal surfaces

photographers use polarizing filters to reduce unwanted glare