More about 3-Dimensional figures (English subtitles available)

The nature of reflectional and rotational symmetry in plane figures is reviewed at the start of the programme and is then followed by the introduction of symmetry in 3-D solids. When a solid is divided by a plane into two parts which are of the same shape, same size and symmetrical to each other, the solid is said to have the characteristic of reflectional symmetry, and also the plane is a plane of reflectional symmetry of the solid. It is pointed out that a solid may have more than one plane of reflectional symmetry. When a solid is rotated around a straight line through 360 degrees and overlays exactly its original solid more than once, the solid is said to have the characteristic of rotational symmetry and the straight line is an axis of rotational symmetry of the solid. Also, it is point out that a solid may have more than one axis of rotational symmetry. The programme shows that a solid may have different paper nets when it is spread out flat, but not all paper nets can be folded to form a specific solid. A paper net of a solid has the following characteristics :(i) the number of plane figures is the same as the number of faces of the solid, (ii) the edges of the plane figures that join together to form the solid are equal in length, (iii) the plane figures do not overlap each other when the net is folded into a solid. The programme illustrates what a solid will look like when it is observed at different angles and also how to formulate the original shape of a solid with reference to its various plane figures as viewed from different angles. It introduces the three different views of a solid: front view, top view and side view. The front view gives the information about its length and height, the side view gives the breadth and height, the top view gives the length and breadth. By integrating all the information obtained from different views, a comprehensive idea of a solid’s configuration and shape can be developed.