Introduction to Light

1. Introduction to Light Electro-Magnetic Radiation

2. Today, we will look at… • What is light? • From where does light come? • How does it get here? • How do materials affect light? • For what do we use light? • How do we detect light? Simon Heppenstall, Park Lane College, Leeds

3. What is light? • A form of energy • Electromagnetic Radiation • Sometimes • A particle • A wave • Photon Simon Heppenstall, Park Lane College, Leeds

4. A form of energy • There are many forms of energy. • Energy can neither be created nor destroyed: just changed from one form to another. • Another form of energy associated with light is heat. • Sound is also a form of energy. • We use energy to do things. Simon Heppenstall, Park Lane College, Leeds

5. Electromagnetic Radiation • For us, this comes from the sun. • Short wave (ie <400nm) • Ultraviolet, X-rays, Gamma rays • Visible (400nm>x>750nm) • Colours from Violet to Red • Long wave (>750nm) • Infrared, microwave and radio waves Simon Heppenstall, Park Lane College, Leeds

6. Sometimes • A Particle • Newton believed this • No-one dare argue • A Wave • James Clerk Maxwell believed this • Discovered that electricity and magnetism travelled at the speed of light • The decided that light must be an electro-magnetic wave. Simon Heppenstall, Park Lane College, Leeds

7. A photon • A photon behaves like a wave in space • A photon behaves like a particle near matter • Light is not waves, nor is it particulate. It is photonic. Simon Heppenstall, Park Lane College, Leeds

8. From where does light come? • The Sun • The Stars • Artificial light sources Simon Heppenstall, Park Lane College, Leeds

9. The Sun • Source of most of our natural light • 15 million K at centre • Diameter of 864,000 miles • 99% total mass of solar system • Runs in 11 year cycles • “Day” of between 25 and 27 days • Further Information: • N.A.S.A. Site Simon Heppenstall, Park Lane College, Leeds

10. The Stars • A minor source of light for us because of their distance away • Help with navigation around the globe • Light takes between 4 years and 15 billion years to get to Earth from the stars • They could have all gone out 3½ years ago Simon Heppenstall, Park Lane College, Leeds

11. Artificial Light Sources • Second most important source to us now • Light pollution stops us from seeing the stars • Usually a conversion of electrical energy to light and heat energy • Sometimes a conversion of chemical energy to light and heat energy Simon Heppenstall, Park Lane College, Leeds

12. How does light get here? • Light travels in straight lines • It travels about 186,000 miles per second • Light speed is not relative, ie the light travelling from the front of a train is travelling at the same speed as light travelling from a tree • It takes 8 minutes for light to get from Earth to the sun Simon Heppenstall, Park Lane College, Leeds

13. How do materials affect light? • Refraction • Reflection • Diffraction • Transparent • Translucent • Opaque Simon Heppenstall, Park Lane College, Leeds

14. What is Refraction? • Materials have a property called the refractive index • If light moves from one material to another with a different refractive index, it bends • This bend is called refraction. • Different frequencies of light bend by different amounts. White light is split into colours Simon Heppenstall, Park Lane College, Leeds

15. What is Reflection? • Some materials are shiny (metals) • Shiny materials make some light bounce back • This is called reflection • The light from the moon is reflected light Simon Heppenstall, Park Lane College, Leeds

16. What is Diffraction? • This is complicated, but when light hits an edge or a corner, a tiny amount is bent round the corner. • The only place you are likely to see this effect is in the science lab. • You need a LASER and a mesh called a diffraction grating. Simon Heppenstall, Park Lane College, Leeds

17. What is a Transparent Material? • A transparent material allows light to pass through it • Transparent means see-through • Glass and water are transparent materials • Even coloured glass is transparent Simon Heppenstall, Park Lane College, Leeds

18. What is a Translucent Material? • A translucent material allows some light to pass through it • Usually, the view through a translucent material is not very clear, but you can make out shapes • Tracing paper is translucent Simon Heppenstall, Park Lane College, Leeds

19. What is an Opaque Material? • An opaque material does not allow light to travel through it • If you placed a light source next to an opaque material, you would get a shadow at the opposite side of the object to the light • Wood is an opaque material Simon Heppenstall, Park Lane College, Leeds

20. How is a shadow formed? • A shadow is formed when light is blocked • Umbra – full shadow (darkest bit) • Penumbra – half shadow (lighter) • Penumbra is formed when the light source is larger than the object or there are two light sources Simon Heppenstall, Park Lane College, Leeds

21. For what do we use light? • To see • To measure • To cut • To entertain Simon Heppenstall, Park Lane College, Leeds

22. To see • Our eyes have adapted to pick up nearly half the wavelengths that the sun emits • Light hits the retina in our eyes • Rods respond to dim light: no colour • Cones respond to coloured light • We see in shades of red, green and blue Simon Heppenstall, Park Lane College, Leeds

23. To measure • LASER stands for Light Amplification by Stimulated Emission of Radiation • Allows for measurement within a fraction of a millimetre Simon Heppenstall, Park Lane College, Leeds

24. To cut • Again, Lasers allow us to cut things very accurately Simon Heppenstall, Park Lane College, Leeds

25. To entertain • Fireworks started light entertainment • Light Shows set to music • Lasers drawing images in smoke at a night club • Entertainers have to remember that light itself is invisible. You can only see the effect of light bouncing off something, eg smoke particles. Simon Heppenstall, Park Lane College, Leeds

26. How do we detect light? • Our Eyes • Shape and Shade • Colour • Instruments • Cameras • Sensors • Optical Instruments Simon Heppenstall, Park Lane College, Leeds

27. Our Eyes – Shape and Shade • Shape and Shade is detected by the rods in our retina • Only part of the brain visible from outside the body • Seeing: Cornea Lens  Retina Rods and Cones  Optic Nerve  Brain Simon Heppenstall, Park Lane College, Leeds

28. Our Eyes – Colour • Colour is detected by cones • A lot of light is needed for cones to start to work • Three types of cone detect • Red • Green • Blue light. Simon Heppenstall, Park Lane College, Leeds

29. Instruments – Cameras • Cameras behave in a similar manner to the eye • Lens  Retina (in the eye) • Lens  Film (in the camera) • Light can affect the mood of a picture • Photographers often use filters to enhance this affect Simon Heppenstall, Park Lane College, Leeds

30. Instruments – Sensors • Electronics are getting more complex • Light Dependant Resistors (LDR) are often used as sensor/switch devices • Made of Cadmium Sulphide • Resistance alters with light amount Simon Heppenstall, Park Lane College, Leeds

31. Optical Instruments • Galileo invented the telescope • Infrared instruments allow us to see in the dark Simon Heppenstall, Park Lane College, Leeds

32. Links to other sources of information on Light • Treating Cancer • Solar System Simulator • Optics • Kodak Info on Light • The National Geographic Presentation on Sight and Sound Simon Heppenstall, Park Lane College, Leeds

33. Safety With Light • Never look directly at the sun • Never look into a LASER • Use your common sense Simon Heppenstall, Park Lane College, Leeds

34. You will be set a series of questions about the subject, Light. You have to choose the best answer available. Click on the next to the answer you think is correct. If you are correct, you will score two points. To find your current score, click on To get help, click on (If you click help, you only score 1 point) Click here to start: Quiz Instructions Simon Heppenstall, Park Lane College, Leeds

35. What is the main source of light on Earth? The Sun The stars Artificial light Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

36. What are the three primary colours of light? Red, Yellow, Green Red, Green, Blue Red, Yellow, Blue Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

37. For what does LASER stand? Light Amplification by Stimulated Electromagnetic Radiation Light Amplification by Stimulated Emission of Radiation Light Amplitude and Stimulated Electromagnetic Radiation Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

38. From what is an LDR made? Silicon Oxide Cadmium Sulphide Silicon Hydroxide Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

39. Where in the eye would you find cones? The Cornea The Lens The Retina Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

40. What were the first light shows? Light and Sound Shows Laser Shows Fireworks Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

41. Which of these best describes the energy conversion in a light bulb? Chemical  Light Electrical  Light Electrical  Light & Heat Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

42. What detects shade and shape in our eye? The Lens The Cones The Rods Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

43. How fast does light travel? 186,000 miles per hour 186,000 miles per minute 186,000 miles per second Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

44. How does light travel? Generally, in straight lines Into the eye By splitting into single frequencies Get Help (Score Max = 1) Tell me my score Simon Heppenstall, Park Lane College, Leeds

45. Help One • There are three basic sources of light that we receive • Artificial light (bulbs etc) • Stars (millions of light years away) • Sun (or Sol) • Which of these affects us most? Simon Heppenstall, Park Lane College, Leeds

46. Help Two • Primary colours are there, not because of light, but because of our eyes. • Our cones have a peak sensitivity to red, green and blue light. (These are the colours they detect best) • Paint uses red, yellow and blue to mix to make all other colours Simon Heppenstall, Park Lane College, Leeds

47. Help Three • LASERS were invented by Charles Townes and Arthur Schawlow in the 1950’s • When an electron changes from a high energy state to a low energy state, its atom emits a photon. • Take a crystal and shine a light on it. The electrons get excited and jump up a level. • As they calm down, they move down a level and emit a photon. • This causes a chain reaction as these photons excite more electrons, which in turn jump up a level and then calm down again. More photons are given off. • The light is amplified as more light comes off than was put in. • That’s the theory. In practice, lasers are blasted with a small amount of radiation. Simon Heppenstall, Park Lane College, Leeds

48. Help Four • Light Dependant Resistors are used for turning sections of circuitry on or off. • The chemical formula for the material from which they are made is CdS • The Cd has a RAM of 112.41 and boils at 1038 Kelvin Simon Heppenstall, Park Lane College, Leeds

49. Help Five • The part of the eye that actually detects the light passed back to it by the lens is called the retina. • This has the light detection cells in it • The lens focuses the light coming into the eye Simon Heppenstall, Park Lane College, Leeds

50. Help Six • The Chinese were the first people to witness an artificial light show • LASERS were invented in the 1950’s byCharles Townes and Arthur Schawlow • The electric light bulb was invented by Thomas Edison Simon Heppenstall, Park Lane College, Leeds