What is Light?

1. What is Light? One of the greatest scientific mysteries of our time Light acts both like a wave and a particle! Dr Darren Reynolds Faculty of Applied Sciences

2. What is Light? In 1666 Newton discovered that white light is made up of all colours Dr Darren Reynolds Faculty of Applied Sciences

3. What is Light? Dr Darren Reynolds Faculty of Applied Sciences

4. What is Light? About the same time a Dutch physicist and astronomer suggested that light consisted of waves. Christiaan Huygens (1629-1695) Dr Darren Reynolds Faculty of Applied Sciences

5. What is Light? Maxwell’s Electromagnetic Theory broad spectrum of radiation ranging from cosmic rays (10 -14 nm) to radio waves(100 to 106 nm). James Clerk Maxwell (1831-1879) Dr Darren Reynolds Faculty of Applied Sciences

6. What is Light? According to Maxwell: Light is an electromagnetic field which is characterised by both frequency,, and wavelength,    = c Dr Darren Reynolds Faculty of Applied Sciences

7. What is Light? The unit of frequency is Hertz (Hz) but its dimensions are S-1 The frequency is a fundamental characteristic Dr Darren Reynolds Faculty of Applied Sciences

8. What is Light? 1900 Max Planck discovered E= h Energy Planck’s constant Frequency Max Planck (1858-1947) Dr Darren Reynolds Faculty of Applied Sciences

9. A ‘quantum’ is the amount of energy contained in a single photon. What is Light? 1905 Einstein used Planck’s work to propose that light is quantized Albert Einstein (1879-1955) Dr Darren Reynolds Faculty of Applied Sciences

10. 400 500 600 700 What is Light? Wavelength (nm) Dr Darren Reynolds Faculty of Applied Sciences

11. What is Spectroscopy? The analysis of the electromagnetic radiation emitted, absorbed or scattered by molecules and atoms Dr Darren Reynolds Faculty of Applied Sciences

12. Interaction of Light with atoms and molecules Three fundamental processes occur. • Scattering • Absorption • Emission Dr Darren Reynolds Faculty of Applied Sciences

13. Scattering Why is the sky blue? Dr Darren Reynolds Faculty of Applied Sciences

14. Scattering Red Sunsets Dr Darren Reynolds Faculty of Applied Sciences

15. Absorption Why is grass green………..? Dr Darren Reynolds Faculty of Applied Sciences

16. Absorption & Emission Dr Darren Reynolds Faculty of Applied Sciences

17. Line and Band Spectra Dr Darren Reynolds Faculty of Applied Sciences

18. Line and Band Spectra Dr Darren Reynolds Faculty of Applied Sciences

19. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception distal image proximal image COLOUR PERCEPTION

20. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception the average person can distinguish between 130 and 200 separate colours there are only a few different types of photoreceptors colour must be perceived through coding combinations among a few basic types of receptors COLOUR PERCEPTION

21. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception the fact that you see colour means that the photoreceptors in the retina of the eye are able in some way to differentiate among the various wave frequencies of light you do not see colour at night (rod vision) but you do see colour in daylight (cone vision) - colour perception must occur in the cone cells COLOUR PERCEPTION

22. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception cross-sectional view “yellow spot” zoom (next slide) COLOUR PERCEPTION

23. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception cross-section of the retina COLOUR PERCEPTION

24. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception most mammalian species are dichromatic - only have middle (green) and short (blue) wavelength sensitive cones primates (inc. humans) are trichromatic birds, reptiles and fish are pentachromatic humans have L-cones (red, maximally sensitive at 558 nm), M-cones (green, 531 nm) and S-cones (blue, 420 nm) rods cone COLOUR PERCEPTION

25. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception in the fovea there are no rods, only cones THERE ARE CONES IN THE REST OF THE RETINA, HOWEVER, ALONG WITH THE RODS BUT the distribution of L-, M- and S-cones is not homogeneous or the same as each other throughout the retina outside the fovea rods cone COLOUR PERCEPTION

26. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception RODS convey ability to see at night under conditions of very dim illumination RODS are capable of being stimulated by a single photon this very fine sensitivity is bought at a price: their response to light stimulation is much slower than that of a cone - signals may arrive as much as 1/10th secondlater that a simultaneous stimulation of a cone. rods cone COLOUR PERCEPTION

27. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception DISTRIBUTION OF CONES in the human eye S-cones can be differentiated morphologically from L- and M-cones (but not L- from M-) S-cones (blue) are at their LOWEST density in the middle of the fovea (5% of cones) and reach their max (15%) in the rest of the fovea. In the rest of retina they form only 8% of the cone population. L-cones (red) apparently constitute 33% of cones in the retina, M-cones (green) about 59% but in the fovea L- and M-cones constitute about 48% each rods cone COLOUR PERCEPTION

28. the eye - basics: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception seeing colours • We have fewer S-cones (blue) than L- (red) and M-cones (green) • We can see colour in great detail at the fovea - but red and green predominantly • But we can still see colour in all other parts of the retina (except the blind spot) • We cannot see anything at the fovea in very dim light as there are no rods • When concentrating on intense patches of colour, blue receptors are likely to become exhausted before red and green rods cone COLOUR PERCEPTION

29. the eye - colour deficiencies: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception deuteranopia protanopia “normal” vision COLOUR PERCEPTION

30. colour production: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour-television camera : dichroic mirror Audio signal Adding unit to form luminance signal lens Composite signal Colour encoder to form chrominance signal reflecting mirror vidicon tube Sync pulse COLOUR PERCEPTION

31. colour production: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour-TV transmission: Light reflected from the scene being televised is focused by lenses and split by means of chroic (colour-separating) mirrors into three separate images, one in each of the three primary colours - blue green and red. Each beam of coloured light is then directed into one of three identical vidicon tubes. The pattern of light falling on a photoconductive layer within each tube causes a varying pattern of electrical resistance; as an electron beam scans the photoconductive area from behind, a varying electric current is induced in a circuit connected to the conductive layer. The pattern of dark and light in each primary-colour image is thus converted into one of three varying electrical signals. A black-and-white luminance (brightness) signal is created in the adding unit, by combining information from each of the three colour signals. A the same time, the colour encoder produces a single chrominance signal, which defines the hue and saturation of each primary colour. the luminance and chrominance signals are combined into a composite video signal. Prior to transmission, the audio signal is incorporated, together with a synchronisation pulse (‘sync pulse’), which ensures that the electron scanning in the receiving system matches that of the transmitting system. COLOUR PERCEPTION

32. colour production: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour-television receiver: electron guns luminance signal deflector coils chrominance signal TV screen Colour separator electron beams Audio signal receiver Composite signal loudspeaker Sync-pulse separator frame pulse line pulse COLOUR PERCEPTION

33. colour production: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour-TV reception: The composite signal picked up by the receiving aerial is decoded, separating out the various constituent signals. The luminance signal controls the overall output of three electron guns in the cathode-ray tube of the receiving set, so determining the balance of light and shade in the final picture. The chrominance signal - now split into the three primary-colour signals - regulates the relative strength of each electron beam. The sync pulse, divided into line and frame components, controls the deflection of the beam across and down the screen. The television screen is coated with stripes of different phosphors, which glow red, blue or green when struck by electrons. Immediately behind the screen is a grille, or shadow mask, which contains many perforations. Travelling at slightly different angles as they pass through the perforations, the electron beams are caused to diverge before striking the screen, in such a way that the electrons from each gun can only reach phosphor stripes of the appropriate colour. Each image on the screen thus consists of stripes of varying brightness and colour that merge together to form the complete picture. COLOUR PERCEPTION

34. colour production: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour-photography: No matter how multicoloured prints of slides may appear, they are made of only the three secondary colours arranged in layers. When you look at a photograph, light passes through the layers and combines to give full colour. Developing a print film produces a colour negative, while in a slide, a process called colour reversal forms a positive colour image on the film. In a colour film each of three layers is similar to a black-and-white film, except the top layer is sensitive only to blue light, the middle layer to green and the bottom layer to red. The three layers detect the amounts of these colours in the image formed on the colour film by the camera lens. COLOUR PERCEPTION

35. colour production: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour-photography: Film developer chemicals for colour films contain dye couplers, which attach dyes to the silver that forms in the emulsion during development. The silver is then dissolved, leaving a layer of dye. The top layer becomes yellow, the middle layer magenta and the bottom layer cyan. It is then necessary to colour print the negative: if the middle and bottom layers have been exposed they will emit magenta and cyan light when the exposure light is shone. A dye emitting magenta light is absorbing green and emitting red and blue light . Cyan means red is absorbed and green and blue emitted. Therefore, if both these layers and emitting at the same time red and green will be absorbed by adjacent layers leaving only blue light, which is the colour printed. COLOUR PERCEPTION

36. colour: how to describe it: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception • Colour has 3 attributes: • brightness • hue • saturation roughly, the intensity of light red, green, blue etc amount of white a colour appears to contain Black, greys and white are colours with zero saturation and no hue achromatic colours COLOUR PERCEPTION

37. colour: how to describe it: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception The colour circle - the psychological experience of hues GREEN BLUE-GREEN YELLOW-GREEN complementary colours on opposite sides of the circle saturation is represented as distance from the centre GREY BLUE YELLOW SATURATION ORANGE VIOLET RED PURPLE COLOUR PERCEPTION

38. Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception Colour mixing: Metameric pairs: colours which appear identical to the eye but are produced by different wavelength compositions such pairs demonstrate that the perceptual system cannot discriminate the component wavelengths that comprise a colour WHITE LIGHT is made up of all the visible spectrum frequencies and yet no individual colours are distinguishable by the eye COLOUR PERCEPTION

39. Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception Colour mixing: colours that look alike behave alike in mixing (the electromagnetic radiation composition doesn’t need to be considered) Projecting 2 coloured lights adds them: blue+yellow lights will give grey Using 2 coloured filters subtracts them: a blue filter will transmit blue and some green whereas a yellow filter transmits yellow and some green but will absorb any blue - together, one filter in front of the other, will give green (black if the filters were truly monochromatic) COLOUR PERCEPTION

40. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception Psychology - perceiving blue + yellow = grey [complementary] blue + yellow = green Painting - pigments COLOUR PERCEPTION

41. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception Schopenhauer: the opening sentences of “Über das Sehn und die Farben: Eine Abhandlung (On Vision and Colours: An essay), pub. 1816 “All intuitive perception is intellectual…[without this] we would stop short at the mere sensation that might possibly have meaning in reference to the will as pain or comfort; but for the rest it would be a succession of states devoid of meaning, and nothing like knowledge. Intuitive perception, that is, knowledge of an object, first comes about through the understanding that refers every impression received by the body to its cause.” COLOUR PERCEPTION

42. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour constancy: this shouldn’t happen! most objects are visible because of reflected light: they have no characteristic energy of their own and, therefore, strictly speaking, no colour of their own - they are always seen under variable conditions of illumination COLOUR PERCEPTION

43. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour constancy: However, under normal circumstances familiar objects are perceived with very little change in hue or saturation colour constancy prevails leaf & donkey sihouettes in same green material on white background illuminated in red light - use a colour disk to match each silhouette a breakdown in constancy can be induced by withholding information or providing false information BUT we are very good at maintaining constancy, probably because outdoors illumination is so variable: changes from summer to winter, clear or cloudy days, noon or twilight WE add stability to a changing environment COLOUR PERCEPTION

44. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception colour constancy: colour constancy prevails colour constancy is NOT divorced from other constancy mechanisms in visual perception - particularly size constancy: WE add stability to a changing environment COLOUR PERCEPTION

45. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception size constancy: lens pupil same image at time 2 image at time 1 eye 2 distal image proximal image 1 COLOUR PERCEPTION

46. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception objective-subjective colours: Objective colour is a misleading term - in colorimetry the observer is not acting like a well-calibrated machine Of prime importance to colour perception is our past experience, both with familiar objects and with the effects of illumination COLOUR PERCEPTION

47. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception objective-subjective colours: Objective colour ... Secondarily, the effect of alternating achromatic (black, grey or white) light is important can be produced by: • flickering of a stationary light off and on • looking at a light through a rotating disk with sectors removed • (like looking through a rotating fan) or • spinning a disk which is painted in an achromatic pattern COLOUR PERCEPTION

48. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception objective-subjective colours: Secondarily, the effect of alternatingachromatic light... if the alternation is performed below the critical flicker frequency (where the stimulus is fused into a solid expanse of grey) hues are perceived colours, often desaturated (washed out), can even be perceived in a close pattern of lines Subjective colour : it is assumed that small eye movements constantly alter the light reaching each colour receptor COLOUR PERCEPTION

49. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception subjective colour: Benham’s top COLOUR PERCEPTION

50. colour perception: Colour Perception Colour Perception Colour Perception Colour Perception Colour Perception subjective colour: if rotated at slow speed will produce colours BENHAM'S TOP the thin black curved lines appear as desaturated hues it is presumed that all varieties of colour receptors in the eye are excited concurrently by the many wavelengths presented. When the alternation rate is appropriate one type of receptor may fire slightly in advance of another, allowing the hue associated with that type of receptor to be perceived even if observed in other than white light the variation in persistence of receptor firing may also be a factor COLOUR PERCEPTION