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Light, light sources and light interactions

HISTORY - INTRODUCTION

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Table of contents1. Three factors2. Introduction3. Direct emission4. The dye and pigment technologist may use light:5. The Electromagnetic spectrum6. Newton’s Prismatic Hues7. Electromagnetic radiation discovery by the

astronomer Herschel (IR)8. German physicist J W Ritter,UV9. Scottish physicist and mathematician James Clerk

Maxwell In 186010.German physicist Heinrich Hertz-1887

RADIO WAVES

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Three factors

• Colour in a manufactured object is normally obtained by applying

• a colorant (dye or pigment) • to a polymer substrate,• such as textile, paper or paint

medium.• The appearance of such surface

colours depends on three factors

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Three factors

1. the nature of the prevailing illumination under which the coloured surface is viewed

2. the interaction of the illuminating radiation with the coloured species in the surface layers, particularly within the visible region of the electromagnetic spectrum

3. the ability of the radiation that is transmitted, reflected and scattered from the coloured surface to induce the sensation of colour in the human eye/brain system.

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INTRODUCTION

• Physically, for example, • The production of colour• by a dyed or pigmented surface is most

simply explained in terms of the process of selective absorption of

• certain wavelength portions of the incident white light coupled with – transmission,– reflection – and scattering of the non-absorbed radiation

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INTRODUCTION

• Colour can also be produced, however, by other types of light interaction:

– by interference and by diffraction, • for example. The process of light interferenceproduces coloured effects in

– peacocks’ feathers – and the wing-cases (elytra) of beetles,

• And Light diffraction is increasingly being used to produce interesting forms of coloured materialssuch as

• liquid crystal displays (LCD) • and polarisation colours.

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Direct emission

• Many modern light sources such as lasers, coloured light tubes

• (such as sodium vapour street lamps)• and video or television screens• emit coloured radiation directly, rather than

producing the colour by selective absorption.

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Direct emission(deactivation)• Lasers and sodium vapour lamps produce their

coloured radiation by – electronic excitation– and deactivation processes,

• the latter resulting in direct emission of radiation in appropriate narrow wavebands within the visible spectrum,

• giving rise to a particular hue which is characteristic of the wavelengths or combinations of wavelengths in the emitted radiation.

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The dye and pigment technologist may use light:

(a) to illuminate a coloured surface for purposes of assessing colour appearance

(b) to irradiate a coloured surface to assess its light stability

(c) in optical and spectroscopic instruments, to assess optical properties of a surface orto undertake chemical and other scientific studies of the colorants used.

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THE ELECTROMAGNETIC SPECTRUM

Visible light is a form of electromagnetic radiation, distinguished from other forms byits ability to be detected in the retina of the human eye.

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Newton’s Prismatic Hues

• The first real progress in understanding• the relationship between light and colour came in

1672, with the publication of Newton’s descriptions and explanations of the effect of passing white light from

• the sun through combinations of prisms.• In describing his experiments, carried out• in 1666, Newton used the word ‘spectrum’ to

describe the range of colours (hues) produced,• which he noted were in the same order as that

observed in a rainbow.

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Newton’s Prismatic Hues• He also appreciated that the colours were not a special

property of the light but merely a sequence• of sensations perceived by the human eye. In his own

words:

And if at any time I speak of light and rays as coloured or endued with colours, I would be understood to speak not philosophically and properly, but grossly, and accordingly to such conceptions as vulgar people in seeing all these experiments would be apt to frame. For the rays to speak properly are not coloured. In them is nothing else than a certain power and

disposition to stir up a sensation of this or that colour.

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Electromagnetic radiationdiscovery by the astronomer Herschel (IR)

• the heating effect of the rays beyond the red end of the visible spectrum, a region now referred to as– the infrared (IR).

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German physicist J W Ritter,UV• exploring the other end of the spectrum, found that

silver nitrate was darkened more• rapidly by rays beyond the violet than by blue and

violet radiation from the visible spectrum;

• he is credited with discovering ultraviolet (UV) radiation.

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Scottish physicist and mathematician James Clerk Maxwell In 1860• Developed a theory that predicted a whole

family of wave radiations• having associated electrical• and magnetic fields, • distinguished by having a common velocity in space• but differing• in wavelength and frequency• It was more than 25 years before his prediction

was confirmed. Proof was provided in 1887 by the German physicist

• Heinrich Hertz

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German physicist Heinrich Hertz-1887RADIO WAVES

• generating an oscillating current from the spark of • an induction coil, • produced and detected radiation of extremely long

wavelengths, now known as radio waves. • Subsequently other forms of electromagnetic radiation

were discovered,• leading to the realisation that the visible spectrum

(from violet to red) forms only a small part of the total electromagnetic spectrum known today.