photo:light
light
Light:
source of visible light:
fluorescence:
phosphorescence:
in some materials, the molecules disturbed by the absorption of light do not immediately return to their original state, and the emission of light continues after the exciting radiation is removed, resulting in delayed fluorescence.
scattering of light by particles:
molecules in the air scatter light especially short wavelengths, and the scattered light is partly plane polarised
the Rayleigh effect:
this results in the short wavelengths being scattered most & thus a clear sky appears blue & as light near the horizon passes through more atmosphere, even more short wavelengths are scattered away from the rays & thus sunsets & sunrises are left with longer wavelengths visible resulting in orange sunsets & sunrises.
similarly, a projector beam when viewed from the side appears bluish.
Illumination:
luminous flux (F):
luminous intensity:
illuminance:
illuminance onto a surface (E) = luminous flux / surface area, (unit is lux = lumen per square meter = meter-candle)
efficiency of a light source:
efficiency = luminous flux / power consumption (ie. units are lumens per watt)
eg. tungsten lamps 10-16 lumen/watt; fluorescent lamps: 35-50 lumen/watt;
Polarisation:
light from ordinary sources is unpolarised - its waves run in all planes perpendicular to the direction of the light
a polarising filter can be imagined as a grid of vertical lines which only allows through the waves that run in a plane parallel to the grid lines & blocking all other planes
when unpolarised light is reflected or refracted through glass, the reflected or refracted rays are partly plane-polarised
the angle of incidence, p, called the polarising angle, for which the polarisation of the reflected beam is complete is related to the index of refraction of the medium
Brewster's law: tan p = index of refraction
at the polarising angle (57deg for glass), none of the vibrations that lie in the plane of incidence is reflected, thus the reflected beam is plane polarised, but of relatively low intensity since only ~8% of the incident beam is reflected at the polarising angle.
the transmitted beam is not completely plane polarised at the polarising angle, unless many plates of glass are used, which also increases the intensity of the reflected polarised beam.
intensity of polarised beam:
double refraction:
when text is viewed through a crystal of calcite, two images of the text are visible, this is double refraction, 1st observed by Bartholinus in 1669.
Huygen's in 1690, observed that the rays which produced the two images were plane-polarised, in mutually perpendicular planes
unlike most optical materials we have experience with which are isotropic, calcite's crystal structure is a rhombohedron and thus when light falls obliquely on its surface, the light is split into two parts & both are refracted, with one ray having a constant index of refraction (the ordinary ray), whilst the 2nd ray is regarded as being an extraordinary ray as its index of refraction changes with the angle of incidence.
Nicol in 1832, used an artifice to separate these rays to give a single beam of plane-polarised light - the Nicol prism.
dichroic:
certain crystals (eg. tourmaline), known as dichroic, produce two internal beams polarised at right angles to each other & in addition, strongly absorb one beam while transmitting the other, although the transmitted light is colored.
Herapth in 1852, discovered that dichroic crystals of quinine iodosulphate (herapathite) transmit a beam as plane-polarised light with transmission close to the ideal 50% for all wavelengths of visible light
Land in 1929 invented a practical method for embedding the tiny synthetic crystals of herapathite (~1011 per sq.cm) in a transparent cellulosic film 0.001-0.004“ thick in uniform alignment which acted like a single huge crystal - the Polaroid sheet, with its ability to be bonded to glass, had the advantage of large size, low cost & polarising effectiveness approaching that of the Nicol prism except at the extremities of the spectrum.
more recently, Polaroid materials have been prepared by aligning molecules (eg. polymeric iodine in polyvinyl alcohol or polyvinylene) rather than tiny crystals, which have greater stability & freedom from scattered light.
when two Polaroid light polarisers are held in the line of vision in the crossed position, no light gets through & the field of view is dark. If a crystal of quartz or a tube of sugar solution is placed between these polarisers, the light reappears as these are optically active substances and rotate the plane of polarisation.
polarisation by scattering:
when a strong beam of light is passed through a region containing no fine particles, it is not visible from the side. If, however, the beam is intercepted by fine particles such as smoke, dust, colloidal suspensions, the beam is partly scattered, and becomes visible.
in this Tyndall effect, the color & intensity of the scattered light depend on the size of the particles.
very small particles scatter chiefly blue light, as the particles are made larger, the longer wavelengths are also scattered until the scattered light appears white.
scattered light is partly plane-polarised (hence the effectiveness of a photographic polarising filter in making the blue sky darker, which is maximal at 90deg. from the sun)
optically active substances:
materials that have the property of rotating the plane of polarisation while transmitting polarised light are called optically active substances.
polarimeters are instruments for measuring optical rotation.
photoelasticity:
certain materials such as glass become doubly refractive under mechanical strain. & when the material is placed between a crossed polariser & analyser, patterns of interference fringes can be observed & can be used for detecting strains in glassware, plastics, etc.
photo/light.txt · Last modified: 2019/07/09 09:29 by gary1