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Light, optics & colour

Light, optics & colour. Vision – how do humans see?. Discussion and demonstrations Activity: Euclid’s model of vision What arguments did Euclid give for his model? How would you try to persuade Euclid that the eye too is a ‘receiving organ’?

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Light, optics & colour

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  1. Light, optics & colour

  2. Vision – how do humans see? Discussion and demonstrations Activity:Euclid’s model of vision • What arguments did Euclid give for his model? • How would you try to persuade Euclid that the eye too is a ‘receiving organ’? • How would you counter his first argument, about the difficulty of finding a pin? Summarise the conditions necessary for you to be able to see an object.

  3. Starting points Prior learning:Primary school children are introduced to the distinction between primary (luminous) and secondary (non-luminous) sources of light. They are also encouraged to think of light, rather than darkness, as an entity. • Some correctly use the terms translucent, transparent and opaque. Misconceptions:Students may equate light with its source (light is in a lamp, or in the Moon). • They may continue to think of both light and darkness as states of being, so they do not explain shadows in terms of light propagation. • ‘Common sense’ suggests that vision involves light (or something) emanating from the eye (rather than light entering the eye). • Many people think that magnifying lenses make light brighter behind the lens than in front of it, and that mirror images are located on their surface (not behindit).

  4. Teaching challenges What is often lacking is any notion of light travelling in space. Understanding virtual images, which are a kind of optical illusion, relies on a correct understanding of vision. Students are familiar with mixing coloured paints, but need to be persuaded this involves colour subtraction. By comparison they may have little practical experience of adding coloured lights or using coloured filters, which require different rules.

  5. A general model for radiation journey: may involve transmission, reflection, refraction, partial absorption detector: absorption at the journey’s end

  6. Refraction Ray of light changes direction as its speed changes at a boundary. Ratio of sini/ sinr is constant, called the refractive index. Class experiment: Plot a graph of sini against sinr to test data collected. This should produce a straight line through the origin. Equation for a straight line is y = mx + c Light travels more slowly in glass than in air i.e. wave, not particles.

  7. Dispersion of white light Speed of light in glass depends on frequency (colour).

  8. Dispersion of white light With a diffraction grating

  9. Light sources filament lamp a fluorescent lamp (these differ) • 700 nanometres • 0.7 thousandths of a millimetre • 400 nanometres • 0.4 thousandths of a millimetre Photo credit http://home.comcast.net/~mcculloch-brown/astro/spectrostar.html

  10. Astronomical spectroscopy Spectra of stars at different points in the sky

  11. When a source is moving … Moving source of sound - Doppler effect Local spiral galaxy rotating Cosmological redshift

  12. Beyond the visible detecting infrared radiation detecting UV radiation modern astronomy: collecting radiation from across the whole electromagnetic spectrum

  13. Modelling light The journey from light source to detector can be thought of in three different ways. • rays • waves • photons

  14. Pinhole camera This 1544 diagram shows how to safely view a solar eclipse.

  15. Shadows … and ray streaks Demonstrations and discussion about rays. In pairs: 1 Explain shadow formation using a ray model. Use words from this list: emit, transmit, absorb, reflect. 2 Sketch a diagram to show how ray streaks are made by a ray-box with slit openings. Write an explanation of why these are not rays.

  16. Reflection For light, angle of incidence i = angle of reflection r angles measured with respect to a ‘normal’ Waves and particles are reflected in exactly the same way.

  17. Total internal reflection At a boundary between two optical media, typically some light is reflected from the surface and some is refracted into ite.g. the multiple images seen in glass shop-fronts. Going from a slower medium (e.g. glass) to a faster medium (e.g. air), when the angle of incidence is greater than a critical angle, all of the light is internally reflected.

  18. Two kinds of lenses • Converging, convex, positive. • Diverging, concave, negative. focal length, f, and power, P, describe how much a lens bends light. Units of fmetres, of Pdioptres.

  19. Describing images real image: Converging rays arrive at the image position, so an image will be formed on a screen placed there. virtualimage: Diverging or parallel rays make light appear to come from the image position. The eye creates an image but it cannot be captured on a screen placed at the apparent position of the image. [To understand this you must first understand how the eye works.] Compared to the object: • upright or inverted? • larger, same size or smaller?

  20. What happens if … • the convex lens is removed? • a cardboard mask covers half of the lens? • the screen is moved forward or back?

  21. Converging lens, real image C21 ppt P7.3 Ray diagrams Virtual Physics Laboratory simulation Don Evans Refraction by lenses presentation A caution:For convenience, ray diagrams showing image formation use just 2 or 3 rays. Lenses act on a cone of light.

  22. Diverging lens, virtual image

  23. Human eye with 20:20 vision

  24. Long sight freezeray.com

  25. Short sight freezeray.com

  26. Seeing a real image

  27. Seeing a virtual image

  28. Virtual image with a plane mirror In pairs, Do McDermott experiment 2.4

  29. Practical session • Image formation with a lens, comparing short cameras and long cameras • Experiments with a fan of rays • Pinhole camera and lens camera • Depth of field for a camera • Law of refraction • Model eye demonstration with flask • The lens formula (all from the Practical Physics website)

  30. Astronomical telescope Practical Physics: Making a telescope

  31. Spherical aberration

  32. Arecibo radio telescope, Puerto Rico

  33. Reflecting telescopes use a parabolic mirror

  34. Two kinds of colour • Coloured lights (emission is additive) • Pigments and filters (absorption is subtractive) Demonstration and discussion. Virtual Physics Laboratory simulation. Phet simulation Color Vision, with accompanying question sheet.

  35. Colour – a teaching order In pairs: Given these three things to teach, in what order would you choose to present them? Discuss. colour filters and/or pigments the appearance of objects under different colours of light white light, pigments (primary, secondary colours) Keep encouraging students to use the terms transmit, absorb, reflect.

  36. Support, references talkphysics.org SPT 11-14 Light & sound 1 Seeing with light 2Modelling light with ray diagrams 3 Reflection and refraction 4 Colours of two kinds Practical Physics website, Optics topic Freezeray.com David Sang (ed, 2011) Teaching secondary physics ASE / Hodder

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