Colour and Polarisation

Announcements We will have a short quiz on Monday after 설날 23rd February It will be on chapter 12 and 13 including: Sound waves Harmonics and resonance Characteristics of light Mirrors and ray diagrams Magnification, colour and polarisation.

Goal of the class To understand magnification, colour and polarisation Question of the Day: What is the difference between additive and subtractive colours? Previous Answer: The focal point is the midpoint between the radius of curvature and the mirror. R=2f Previous question: How is the focal point related to the curvature?

Magnification Magnification is the measure of how much larger or smaller the image is compared with the object itself. Magnification is expressed as a ratio of the height of the image to the height of the object. Label all the things!

Concave Mirrors Same as before

Practice You use a magnifying glass to read some small text. If the magnifying glass has a magnification of 3x and the text appears to be 6mm high, how high is the original text?

Mirror Equation Can be used to find the location or sizes of the image or the object based on the equation:

Problem Find the location of the image and its magnification:

Colour The light we get from the sun is called white light. But this light is a mixture of different colours. These colours can be separated by using a prism: Pink Floyd’s Dark side of the moon album cover.

Spectrum We call this pattern of different colours the spectrum of light, where a rainbow is a common example. Violet is high energy and red is low energy.

Spectrum The spectrum is a continuum but is often said to be made of seven colours. Red Orange Yellow Green Blue Indigo Violet R O Y G B I V Richard Of York Gave Battle In Vain

Seeing Colour The three colour receptors in the eye allow us to see millions of different colours. The additive primary colours are red, green, and blue. We don’t see everything white because the strength of the signal matters. All the different shades of colour we can see are made by changing the proportions of red, green, and blue.

Primary Colours of Light Talk about primary and secondary colours Also complementary colours

Colours A complementary colour is formed by subtracting a primary colour from white light. Every secondary colour is the complement of a primary colour. Blue is complementary to yellow Red to cyan Green to magenta

Only red light is reflected Seeing Colour The colour an object appears depends on the colours of light it reflects. For example, a red book only reflects red light: Homework White light Only red light is reflected

Seeing Colour If we look at a coloured object in coloured light we see something different. For example, consider a football kit: Shirt looks red White light Shorts look blue

Seeing Colour In different colours of light this kit would look different: Red light Shirt looks red Shorts look black Shirt looks black Blue light Shorts look blue

Subtractive Colours Coloured fabrics and paints get colour from a subtractive process. Chemicals, known as pigments, in the dyes and paints absorb some colours and allow the colour you actually see to be reflected. Magenta, yellow, and cyan are the three subtractive primary colours.

Comparison

TVs TVs work by illuminating pixels of different colours. At a distance the colours blend and you can’t see the individual colours. TV sets can mix the three colours to get millions of different colours.

Polarisation Light is an electromagnetic wave that has oscillating electric and magnetic fields.

Polarisation Some materials can block electric fields of a certain orientation. This results in letting only polarised light through.

Polarising Sunglasses

3D Glasses When you watch a 3D movie you use polarised glasses so that each eye sees a different image. Your brain then combines them into 3D.

Homework Page 472 Questions: 13, 14, 20, 26, 34, 48