3 - LIGHT SOURCES AND SPECTRA

Question 1

light source

Answer 1

emits light

• sun = the world main light source
• direct sunlight = yellowish (early morning/evening)
• midday = whiter

illuminant = if the light source illuminates the surface

Question 2

isaac newton 1666

Answer 2

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

Question 3

spectrum

Answer 3

‘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)

Question 4

visible light wavelength

Answer 4

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

Question 5

skylight

Answer 5

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?

Question 6

tungsten lightbulbs

Answer 6

yellowish tint

more longer wavelengths

less shorter wavelengths

Question 7

fluorescent bulbs

Answer 7

look almost perfectly white

fluctuations at the different wavelengths of the visible range

Question 8

hard to tell what colour a light will be perceived as according to its wavelength

Answer 8

eg LED bulbs

Question 9

artificial coloured lights

Answer 9

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)

Question 10

helium neon laser

Answer 10

wavelength = 632.8nm

actually 632.4-633.2nm

Question 11

monochromatic source

Answer 11

emits only one wavelength

impossible to produce

so usually only a very narrow wavelength range

Question 12

what is the spectrum for white light?

Answer 12

• 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)

Question 13

warm white and cool white

Answer 13

yellow or blue tints to white light

Question 14

colour of monochromatic light

Answer 14

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)

Question 15

spectral colours

Answer 15

‘evoked by monochromatic light’ and can see in the rainbow

Question 16

non-spectral colours

Answer 16

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)

Question 17

illumination

Answer 17

‘light sources emit light that radiates out from the source location until it reaches non transparent surfaces

Question 18

absorbing light

Answer 18

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

Question 19

pigmented surface

Answer 19

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

Question 20

pigment

Answer 20

the substance that absorbs the light, and absorbs more of specific wavelengths than others

Question 21

illuminant spectra

Answer 21

• ‘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’

Question 22

distal stimulus = spectral reflectances

Answer 22

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