4.2 Understanding How Light Travels A model is a way of representing something in order to understand it better and to make predictions. The Wave Model of Light describes light as a type of wave that travels through empty space and transfers energy from one place to another (such as from the Sun to the Earth). A form of wave energy we can see is Visible Light. See pages 144 - 145 (c) McGraw Hill Ryerson 2007

What’s happening here? (c) McGraw Hill Ryerson 2007

When light passes through different mediums, its speed changes. Refraction of Light When light passes through different mediums, its speed changes. Refraction is the bending of waves due to a change in its speed (Frequency). Light waves refract (bend) when they pass from one material to another. Waves travel at different speeds in different materials. Eg. When light passes from air to water. (c) McGraw Hill Ryerson 2007

Activity Colour Wheel Play with prisms (c) McGraw Hill Ryerson 2007

What colour(s) do you see when the colour wheel spins? Regroup What colour(s) do you see when the colour wheel spins? What colour(s) do you see when you shine a light through a prism? (c) McGraw Hill Ryerson 2007

A light wave also refracts when it passes through a Prism. Refraction of Light A light wave also refracts when it passes through a Prism. (c) McGraw Hill Ryerson 2007

Longer wavelengths are refracted less than shorter wavelengths. Refraction of Light White light is made up of waves of different wavelengths and frequencies. When a light wave passes through a prism the different wavelengths are refracted by different amounts. Longer wavelengths are refracted less than shorter wavelengths. This causes different colours to be separated when they come out of the prism. (c) McGraw Hill Ryerson 2007

Colours of the Rainbow When white light is separated into its different colours, this band of colour is called the visible spectrum. The range of colours of frequencies of visible light . The seven most visible colours of the spectrum are: Red, Orange, Yellow, Green, Blue, Indigo, Violet. (ROY G BIV) R O Y G B I V See page 146 (c) McGraw Hill Ryerson 2007

Which colour has the longest wavelength? (c) McGraw Hill Ryerson 2007

The colour red has the longest wavelength and lowest frequency. Colours of the Rainbow The colour red has the longest wavelength and lowest frequency. The colour violet has the shortest wavelength and highest frequency. (c) McGraw Hill Ryerson 2007

At one time people believed that colour was something added to light. Story of Isaac At one time people believed that colour was something added to light. When white light struck a green leaf, people believed that leaf was adding green to the light. In 17th century, English scientist Sir Isaac Newton conducted a famous experiment. He placed a prism so that a thin beam of white light could pass through it. Story of Isaac. At one time people thought colour was something added to light. Green leaf added green to the light. (c) McGraw Hill Ryerson 2007

When white light travelled through the prism, he saw bands of colour. Isaac Newton When white light travelled through the prism, he saw bands of colour. He observed each colour refracted at different angles. He concluded that the prism was not the source of the colours, that the different colours were already present in the white light. (c) McGraw Hill Ryerson 2007

Next, Newton passed these colours through more prisms. Isaac Newton Next, Newton passed these colours through more prisms. This time only white light emerged. Newton showed that colour was a property of visible light. He proposed that white light such as sunlight is the result of mixing together all the different colours of light. (c) McGraw Hill Ryerson 2007

Reflection occurs when light bounces off an object. Colour and Reflection Reflection occurs when light bounces off an object. When white light strikes an object, some colours are reflected and some are absorbed. Only the reflected colours can be seen. For example, yellow cloth reflects yellow and absorbs all other colours. See page 148 Take the Section 4.2 Quiz (c) McGraw Hill Ryerson 2007

Yellow, Magenta and Cyan. Colour and Reflection Only three colours of light are needed to produce all the colours of the rainbow: red, green and blue. They are called the three additive primary colours because adding all three together in proper amounts will make white light. Yellow, Magenta and Cyan. (c) McGraw Hill Ryerson 2007

The three secondary colours are yellow, cyan, magenta. The light of two additive primary colours will produce a secondary color. The three secondary colours are yellow, cyan, magenta. (c) McGraw Hill Ryerson 2007