COLOUR YEAR 11 - UNIT ONE PHYSICS Mr. Bowen East Doncaster Secondary College

Introduction In Chapter One we looked at some of the properties of light associated with its straight line propagation and its ability to reflect off materials. We are now going to explore the nature and behaviour of light and develop explanations for the images and colours seen around us.

Summary By the end of this presentation you should know: the component colours of white light why objects look coloured the factors that effect the colour of an object the primary colours of light how colour filters work that visible light is just a small part of the electromagnetic spectrum

Component Colours of light (1) The “rays” that we used to indicate the path of light in Chapter 1 were a simplified representation to indicate direction only. They told us little about the nature of light. Light is actually a series of complex electromagnetic waves that are able to detected by our eyes. Light exhibits some wave-like properties. The “wavelength” of light (distance between consecutive wavefronts) determines its colour.

Component Colours of light (2) The range of wavelengths of light that our eyes are responsive to is known as the “visible spectrum”. Each variation in colour is caused by light of a different wavelength. The colours that make up the visible spectrum are (in order from longest to shortest wavelength): red, orange, yellow, green, blue, (indigo), violet. Acronym: ROYGBIV

Wavelengths of light (nm = x 10-9 m)

Seeing coloured objects “White” light contains roughly equal amounts of each of the colours of the visible spectrum. Sunlight, incandescent light and fluorescent light all produce the same general sensation (see fig. 2.2 page 35). When light falls on an object it may be reflected, transmitted or absorbed. We see light from luminous objects directly but we see most objects because light is reflected off them.

Viewing objects under white light An object’s colour is predominantly determined by the colour/s of the light reflected from it. White light incident on a material that reflects predominantly red light will appear red. The other colours are absorbed. white light in (ROYGBIV) OYGBIV absorbed red reflected

Black, white and grey objects An object appears white if all of the colours of the spectrum are reflected in roughly equal proportions. An object appears black if all of the white light falling on it is absorbed (i.e. ROYGBIV are all absorbed). An object appears grey if only a small proportion of the incident white light is reflected with all colours in roughly equal proportions.

Colours other than ROYGBIV When you see reflected colours other than the main ROYGBIV colours, the pigment in the material is reflecting back a range of wavelengths not just one predominant colour. red + yellow = orange red + blue = purple yellow + blue = green Most pigment reflection is not pure (one colour only ) but often a range of colours centered on the most dominant reflection.

Primary pigment colours When we mix paints, yellow plus blue gives green. This occurs because when they are mixed the yellow pigment reflects orange, yellow and green; and the blue pigment reflects green, blue and violet. Green is reflected by both and becomes the dominant reflected colour. The three primary pigment colours are red, yellow and blue. These three can be mixed to make all other colours but they cannot be produced by mixing other pigments.

Viewing objects under coloured light When an object is illuminated by white light it is said to be showing “true colour”. If the incident light is not white, the object may look completely different. The “impure” colour depends on the colour/s of the incident light and the colours that are reflected back to you. what colours are incident? reflected?

Example 1 white light (ROYGBIV in) pure green light incident Y G O G green reflected back - looks green G Y G O orange, yellow(dominant) & green reflected back - looks yellow

Example 2 The French flag under white light. The French flag under pure yellow light? (the yellow is absorbed by the red and blue, the white section reflects the yellow)

Colour Addition & Primary Colours When you combine coloured light sources you are performing colour addition. Red, green and blue light combined together on to a screen produce white light. These are known as the “primary colours” of light. You can combine red, green and blue light in varying percentages to produce all other colours of the spectrum.

RGB values for common colours Table: RGB values for some common colors

Primary & secondary colours If the three primary (p) colours partially overlap they produce white and three secondary (s) colours. blue (p) cyan (s) green (p) magenta (s) red (p) yellow (s) white

Complementary Colours Secondary colours Cyan = green + blue Magenta = red + blue Yellow = red + green Complementary Colours Colours that combine to produce white light are said to be complementary. cyan + red = white, magenta + green = white, yellow + blue = white (these are pairs of complementary colours)

Filters and colour subtraction A filter allows some colour(s) to be transmitted but all other colours are absorbed. A filter changes the colour of light through a process called “colour subtraction”. White light Red light transmitted OYGBIV absorbed Red filter

Summary In Summary, our perception of the colour of an object is not solely the result of the properties of light. There are many factors which influence the perceived colour of an object. Stick the photocopy of this presentation in your module and copy Worked Examples 2.1A, 2.1B and 2.1C into it. Also do 2.1 Questions Nos, 1, 2, 3, 5, 6, 7, 8 and 10. THE END