L ight

1. Light By Miss Buicke

2. What we must know form the syllabus:

3. OP33 Recall that light is a form of energy and that it can be converted into other forms of energy OP34 Show that light travels in straight lines and explain how shadows are formed. OP35 Contrast luminous objects, which are themselves a source of light, with non-luminous objects, which are seen because light is reflected from them. OP36 Recall that white light is made up of different colours which can be separated by dispersion.

4. OP37 Produce a spectrum of white light using appropriate apparatus, and list the colours of the spectrum. OP38 Investigate the reflection of light by plane mirrors, and illustrate this using ray diagrams.Demonstrate and explain the operation of a simple periscope. OP39 (all higher level) Show the refraction of light as it passes from: air to glass, air to water, glass to air, water to air. Show refraction of light through a lens.Demonstrate the operation of a magnifying glass

5. Light is a form of energy. It can do work. Light is one of the forms of energy that we get from the sun. Light energy is often converted to other forms: • In photosynthesis light energy is changed to chemical • energy in growing plants. • Solar cells convert light energy to electrical energy. • Solar cells can power satellites, calculators and parking • meters in cities and towns.

6. Solar cells on a satellite

7. Speed of light: Light travels extremely fast. In scientific language the speed of light is 3 x 108 m/s. This means that light travels 300 000 000 meters in one second. If something could travel at the speed of light it could travel around the equator of the earth over seven times in one second. That’s fast Image of the sun.

8. Experiment: To show that light travels in straight lines. Equipment: Light bulb and three pieces of cardboard Method: Take 3 similar square pieces of cardboard Put a hole in the exact centre of each piece of cardboard Place the 3 pieces of cardboard between your eye and the bulb directly behind the pieces of card.

9. Result: • You will see the light from the bulb if the three holes are in a straight line • If you move one of the pieces of card the three holes are no longer in a straight line. You • will not see the light from the bulb now. • Conclusion: • Light travels in straight lines.

10. Shadows: • On a bright day you can see your shadow on the ground. • If you hold your hands in front of a lamp you can see • shadows on a wall. • Why are shadows formed? • Light travels in straight lines. • Light can not pass through certain materials, such as • opaque materials.

12. Experiment: Show how shadows are formed Equipment: Light source, white sheet of cardboard, meter stick. Method: Place a white sheet of cardboard 1 meter from the lamp or torch. Hold your hand between the torch and the piece of card. Result: A shadow is formed on the screen in the same shape as your hand. If you move your hand closer to the light, the shadow gets bigger, as your hand is blocking more light.

13. In a solar eclipse, a shadow is formed on the earth when the moon comes between the sun and the earth. The moon is the opaque material that blocks the light from the sun reaching the earth. Notice the central dark area and the less dark outer area of the shadow.

14. Luminous and Non-luminous objects. Luminous objects make and give out their own light. Lets look at some examples: The sun makes and gives out light all the time. When a light bulb is switched on electricity flows through the filament. The filament then becomes white hot. White hot substances make and give out their own light.

15. When you light a candle you are burning wax. This chemical reaction makes and gives out light. Therefore a candle is a luminous object.

16. Non – luminous objects Non-luminous objects reflect light from another source. The reason you can see this science book is because light from another source (lights in the room)is reflecting off the book and entering your eyes.

17. people are visible because we reflect light from other sources. If you were underground with no source of light you would not be able to see the person standing right beside you. How can a driver see where he/she is going at night on a dark country road? Light from the headlamps of the car reflect off the road and into his/her eyes.

18. Dispersion of light The light we get from the sun or from most light bulbs is a mixture of different colours of light. we call this mixture of colours white Light. White light is a mixture of the seven colours of the spectrum. Sometimes white light gets seperated into its seven different colours. Dispersion is the breaking up of white light into its seven colours.

19. Experiment : To demonstrate dispersion and produce a spectrum of white light. Equipment: Equilateral prism, ray box and a screen. Method: Darken the room as much as possible. Shine light from a ray box through a prism. Place a screen on the far side of the prism. Observe and identify the different colours visible on the screen.

20. Result: The colours you should see are: red, orange, yellow, green, blue, indigo and violet. Conclusion: Dispersion was demonstrated and a spectrum was produced.

21. When white light is split, the colours always follow the same order. Red, orange, yellow, green, blue, indigo, violet. RichardOfYorkGaveBattleInVain

22. A rainbow A rainbow is formed when sunlight is dispersed by raindrops in the air.

23. Have you noticed the different colours of light reflected off a CD?

24. Reflection Reflection is when light bounces off a surface Light can reflect off almost every surface. We are only concerned about light reflecting off plane or flat surfaces. The best know plane surface for reflecting light is the mirror.

25. When you see a reflection in a mirror it is laterally inverted. This means the left side is on the right and the right side is on the left. An ambulance will have the word ‘ambulance’ in mirror image painted on its front. That way a driver can read the word correctly when looking in a rear view mirror.

26. Experiment: to investigate the reflection of light by a • plane mirror • Equipment: ray box, white paper and protractor. • Method: • Darken the room. • Draw a straight horozontal line 10cm long on a sheet of • white paper. • Stand a plane mirror on this line on the white paper. • Shine light from a ray box onto the mirror. • You will see the ray of light on the white paper as it heads towards and reflects off the mirror

27. Mark two dots on the white paper along the ray of light heading for the mirror. • Mark two dots on the white paper along the ray of light reflecting off the mirror. • Result: • Join the dots, using a red pen draw a perpendicular line to the line representing the mirror. • Lable the angles iand r and measure using a protractor. • Conclusion: • Angle iand angle r are equal. • The angle at which the light comes in is the same as the angle at which the light reflects.

28. Angle i and angle r are the same

29. Experiment: To demonstrate the operation of a simple periscope. • Equipment: periscope ( made by you) • Method: • Position your eye as indicated in the picture • Light from a distance source hits the upper mirror. • This light reflects to the lower mirror and then reflects into your eye.

30. Light from a distance source • hits the upper mirror. • This light reflects to the • lower mirror and then reflects • into your eye

31. Applications of reflection • People use plane mirrors to see themselves brushing their • teeth, combing their hair. • The periscope is used to see over obstacles, such as a • submarine uses a periscope to see above sea level.

32. Refraction of light(Higher level only) Refraction of light is the bending of light as it passes from one medium to another. Have you ever noticed this happening?

33. Refraction from air to water and water to air The straw in the glass appears to bend. The bending appears to happen at the boundary between the air and the water. Of course the straw is straight. The light from the straw bends or refracts as it passes from air to water and water to air.

34. Refraction from air to glass and from glass to air.

35. The light bends as it enters the glass. • Note that the line bends towards the dotted line. • The light bends as it leaves the glass block. • Note that the light bends away from the dotted line • Remember which way the light bends as it enters the glass and then as it leaves the glass.

36. Lenses A lens is a piece of glass or plastic with a curved surface. There are two types of lenses: convex and concave convex concave

37. Converging lens • The type of lens is called a converging lens because the lens bends the light so that it converges to a point. • This bending of light is called refraction.

38. Uses of a convex lens • A magnifying glass is an example of a use of a convex lens. It uses • Refraction to produce a magnified image of an object. • Convex lens are use in glasses to correct long-sightedned, i.e a defect of the eye where only • Far-away objects can be seen. • Convex lens are also used in microscopes, cameras and telescopes.

39. Diverging lens • This lens is called a diverging lens because the diverging • lens bends the light so that it diverges outwards. • Again this bending of light is called refraction.

40. Uses of a concave lenses: • Concave lenses are used in glasses to cure short-sightedness, i.e where only objects close by can be seen.