3 - LIGHT SOURCES AND SPECTRA Flashcards
light source
emits light
- sun = the world main light source
- direct sunlight = yellowish (early morning/evening)
- midday = whiter
illuminant = if the light source illuminates the surface
isaac newton 1666
splitting light in a prism
demonstrated that a spectrum of colours (rainbow) could be reassembled back to white light again using a lens and second prism
showed sunlight is a mixture of different lights (which are seen as different colours) = SPECTRUM
spectrum
‘a plot of the wavelength composition’
- continuous curve
vertical axis = quantity of light (intensity and power)
horizontal axis = wavelengths (400-700nm)(violet-red visible light)
visible light wavelength
all wavelengths between 400-700nm
but contains more of some wavelengths (more in the middle of the spectrum and lowest is 400nm)
see page 6 (week 2) for diagram
skylight
sunlight scattered around the earths atmosphere
has all wavelengths between 400-700nm
- but higher in the 400-500nm range (shorter wavelengths)
- lower in the longer wavelengths
maybe why the sky looks pale blue?
tungsten lightbulbs
yellowish tint
more longer wavelengths
less shorter wavelengths
fluorescent bulbs
look almost perfectly white
fluctuations at the different wavelengths of the visible range
hard to tell what colour a light will be perceived as according to its wavelength
eg LED bulbs
artificial coloured lights
created by putting filters (thin coloured film/glass) over a source of white light
filters wavelengths out (absorbed by pigments)
the wavelengths that pass through are then seen as coloured
or LASERS (only emit one wavelength)
helium neon laser
wavelength = 632.8nm
actually 632.4-633.2nm
monochromatic source
emits only one wavelength
impossible to produce
so usually only a very narrow wavelength range
what is the spectrum for white light?
- there isn’t one set spectrum
- many can look white and be indistinguishable
- white metamers
- perfectly white light = flat straight line when plotted on an intensity x wavelength plot
- same amount of each wavelength in the visible range
- has to be made artificially (probably impossible)
warm white and cool white
yellow or blue tints to white light
colour of monochromatic light
400-420 = violet 420-450 = indigo (rich blue) 450-500 = blue (480) 500-520 = blue-green 540 = green 580 = yellow 610 = orange 632.8 = red (helium neon laser)
spectral colours
‘evoked by monochromatic light’ and can see in the rainbow
non-spectral colours
not in the rainbow or monochromatic light
eg:
- pinks
- mauves
- browns
- teals
- hot pinks and purples
- mixture of wavelengths producing a mixed stimulation?
- ‘usually a mixture of an infinite number of wavelengths’
purple = mixture of short (blue) and long (red)
illumination
‘light sources emit light that radiates out from the source location until it reaches non transparent surfaces
absorbing light
when the illuminating light reaches a surface, some of it is absorbed and this depends on the material of the surface as the absorption differs between surface types
- dark surfaces = absorbs a lot of light
- light surfaces = absorbs less
- black surfaces = absorb all or nearly all of the light that falls on it
pigmented surface
absorbs more of specific wavelengths than other wavelengths
- a pigmented surface will reflect light that is of a different spectrum than the illumination that fell upon it, due to the absorption
pigment
the substance that absorbs the light, and absorbs more of specific wavelengths than others
illuminant spectra
- ‘varies significantly from place to place over time’
- ‘illuminant spectra depend on not only the spectrum of the light source but also on the effects of other surfaces’
- ‘light reflected from one surface illuminates others’
- and also when light filters through transparent objects (eg leaves)
- ‘since there are substantial variations in illuminant spectra, there will be substantial variations in the light reaching the eye (proximal) from the same surface in different places and different times’
distal stimulus = spectral reflectances
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