PROPERTIES OF VISIBLE LIGHT

Section 4.2

Objectives

By the end of the lesson you should be able to:

• Explain the wave model of light

• Explain the difference between reflection and refraction

• Explain how and why different colours occur

WAVE MODEL OF LIGHT

• A model is a way of representing something in order to understand it better and to make predictions

• Scientists have developed a wave model of light to help explain light behaviour

• In this model, light is a type of wave that travels through empty space and transfers energy from one place to another (eg: from the Sun to Earth)

• Basically - visible light is a wave you can see

REFRACTION OF LIGHT

• If a light wave travels through empty space, what happens if it passes from one material to another – eg: from air into water??

• If the light wave is travelling at an angle and the speed that light travels is different in the two materials, the wave will be bent or refracted

• Refraction is the bending or changing direction of a wave as it passes from one material to another

• Example: if you were to try and spear a fish in a pond, you would have to aim slightly above where the fish appears to be…..why?

Refraction

Remember wavelengths and frequencies?

• White light, also known as sunlight, is made up of waves with different wavelengths and frequencies

• Think of a prism – a glass 3-D figure – if a light wave passes through it, it is refracted (it’s passing from air to glass)

• If the light wave passes through the prism, the different wavelengths bend by different amounts

• The longer wavelengths are refracted less than the shorter wavelengths and because of this, different colours are separated when they come out of the prism

Long wavelengths

Short Wavelengths

Colours of the Rainbow

• What else besides a prism refracts light?• Water droplets also refract light and the result different

wavelengths are bent by different amounts = rainbow• The range of colours of visible light is called the visible

spectrum• The spectrum ranges from colours with the longest

wavelength and the lowest frequency to those with the shortest wavelength and the highest frequency

Longest wavelength ------------- Shortest wavelength

Lowest frequency ------------ Highest frequencyRed Orange Yellow Green Blue Indigo Violet

Colours of the Rainbow

• RED has the longest wavelength – 700 nanometres (nm)

• VIOLET has the shortest wavelength – 400 nm

• The seven colours most easily seen in a rainbow are sometimes abbreviated:– ROY G BIV (Red, Orange, Yellow, Green,

Blue, Indigo, Violet)

Producing the Visible Spectrum

• Is colour picked up when light strikes a coloured object?

• Or does light itself contain colour??

• Sir Isaac Newton conducted an experiment to find the answer to these questions……

Newton’s Experiment

• He placed a prism so a thin beam of white light could pass through it

• When the light travelled through the prism, he saw bands of colour emerge

• Each band of colour refracted at a different angle

• He concluded: the prism was NOT the source of the colours, the colours must have already been present in the white light

Newton’s Experiment Part 2

• Newton then passed these colours through more prisms and this time only white light emerged!

• WHAT??• A prism causes white light to split into a

spectrum, if you add more prisms, they can recombine the colours, producing white light again

• BUT – if you remove one colour from the spectrum and pass the white light through the prisms, the recombined light is no longer white! (Fig 4.15, pg 147)

http://micro.magnet.fsu.edu/primer/java/scienceopticsu/newton/

Newton’s Conclusion

• Newton showed that colour was a property of visible light.

• He proposed that white light (like sunlight) is the result of mixing together all the different colours of light

Colour and Reflection

• What was REFRACTION again?• Then what do you think REFLECTION is?• Reflection occurs when a light wave strikes an

object and bounces off.• When sunlight strikes coloured clothing, some

colours are reflected while others are absorbed• ONLY the reflected colours can be seen• Your shirt is therefore absorbing which colours

and reflecting which colours??

Colour and Reflection

• Why does a bright red shirt look black when it’s in a dark room?

• Because the shirt does not produce its own light but only reflects light, it appears to be black when there is no source of light

• How many colours do you think you need to produce all the colours of the rainbow??

How many??

• ONLY 3 colours are needed!• Which ones are they?• RED, GREEN and BLUE• Adding these colours together in the correct

amounts will make white light they are called the additive primary colours

• If you combine the light of just two of these colours you will get a secondary colour – YELLOW, CYAN or MAGENTA (see fig. 4.17, pg 148)

Your turn

• Complete the reading check questions on page 149