CONTENTS 1
22．WHAT“WEIGHT”MEANS 1
23．BALANCING 7
24．CENTRIFUGAL FORCE 14
25．METALS 20
26．SPRINGS AND SPRINGINESS 25
27．BRIDGES AND ARCHES 31
28．ELECTRICITY 37
29．ELECTRIC CURRENT 43
30．MAGNETS—1 49
31．MAGNETS—2 56
32．LIGHT AND DARKNESS 62
33．LIGHT AND COLOUR 68
34．REFLECTION AND MIRRORS—1 73
35．REFLECTION AND MIRRORS—2 78
36．REFRACTION AND MAGNIFYING GLASSES 83
37．EYES AND EYESIGHT 90
38．OPTICAL ILLUSIONS 97
39．OPTICAL ILLUSIONS(COLOUR) 103
40．LIGHT AND HEAT WAVES 108
41．THE SHORTEST WAVES 113
42．SOUND WAVES 118
43．HOW SOUND BEHAVES 123
VOCABULARY 129