ᴏᴘᴛɪᴄs

                17 - 18

light more useful when redirected and focused

Optics - properties of light and how it interacts with matter
through reflection, refraction, diffraction

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        Ray Approximation

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light travels in straight line until it encounters boundary

with another medium

ray approximation - represent beam of light as rays

geometric optics - part of optics where ray approx is valid

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             Reflection

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occurs when light not absorbed or fully transmitted

some portion of wave changes direction

all substances absorb some incoming light

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      Law of Reflection

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incoming ray makes angle of incidence

normal - imaginary perpendicular line

outgoing ray makes angle of reflection

law of reflection: angle of incidence = reflection

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      Specular and Diffuse
            Reflection

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specular - from smooth surface

mirror like reflection

incident light rays remain parallel

diffuse reflection - from rough surface

reflected rays in different direction

most objects visible as result of diffuse reflection

smoothness relative to wavelength

height of irregularities small compared to wavelength,

surface appear smooth, specular refelction

same surface can appear rough and smooth with

longer wavelength

dish surfaces of large radio telescopes, 50m+

visible light diffusely reflected, specular reflector for 

radio waves

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              Refraction

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refraction - transmitted ray changes direction

refracted path taken by light entering different medium described

using Fermat's principle of least time

1662 - French mathematician Pierre de Fermat propose path

taken is one that minimizes travel time

exact angle the ray follows depends on speed which it travels

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        Law of Refraction

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index of refraction - factor that slows down light

n= speed of light in vacuum (c)/speed of light in material (v)

dimensionless number

always greater than or equal to 1

defined as 1 for vacuum

wavelength in medium = divided by n

Snell's law - relating angle of incidence with refraction

higher index of refraction = bend towards normal

lower = away from normal

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               Mirage

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mirage - naturally occurring optical illusion due to refraction

of light through atmosphere

refractive effect cause sun to be visible at horizon after it's set

atmosphere less dense at higher alt

least time path way is curved arc that favors higher alt, travels

more quickly

index of refraction changes layer by layer in atm

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             Dispersion

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light mediated by electron oscillators

most transparent materials resonate with UV light

violet light 1% more slow in ordinary glass than red light

undergo greater amount ot refraction

Dispersion - separation of visible light into spectrum of colours

due to independence of refractive index

illustrated with prism

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               Rainbows

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millions of raindrops acting as tiny prisms

reflected, return to front side, refracted again

each droplet produce full spectrum of colours

possible to view as full circle

double rainbow = more refraction

dimmer than first

total internal reflection - light traveling from region of high

rfr index to lower rfr index, completely reflected at boundary

angle of rfr gradually increases in at critical angel, oriented

at 90 degree relative to norma;

no refracted ray produced

explains why diamonds sparkle more

undergo many internal reflections