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2. Square Pyramid: A solid formed by connecting a square base and four
triangular side faces with a common vertex named apex is called a
square pyramid. Here, we have:
F = 5: ABCD, DAO, OCD, OBC, OAB.
E = 8: AB, BC, CD, DA, OA, OB, OC, OD.
V = 5: O, A, B, C, D.

Quick Check

Identify the number of faces, edges and vertices in the polyhedrons given below. Name them.

Types of Polyhedrons

Convex and Concave Polyhedron

If the line segment joining any two points within the surface of a polyhedron lies
completely inside or on the polyhedron, it is known as a convex polyhedron. Convex Polyhedron
For example, a cube, a hexagonal prism, etc.

If the line segment joining any two points on the surface of a polyhedron, lies
outside the polyhedron, then it is called a concave polyhedron.
Prism Concave Polyhedron

A polyhedron whose bases are congruent polygons and other A polyhedron is irregular
faces i.e., lateral faces are parallelograms is called a prism. Note: if one of the faces is not a
Terms associated with a prism are: regular polygon.

Base – a polygonal end on which a prism stands
Axis – the straight line joining the centres of the base and top of a prism
Height – the perpendicular length between the faces (base and top) of the prism
Lateral Faces – all faces except base

Lateral Edges – the lines of intersection of the lateral faces
Based on the number of faces, a prism can be of the following types.
1. Right Prism: When the lateral edges meet the base at 90°, it forms a
right prism.
Some of the properties of a right prism are:
(a) All the lateral faces are rectangular and are perpendicular to the base.

(b) Height of prism = Lateral edges

(c) Number of lateral edges = Number of lateral faces = Number of sides in the base.

221 Visualising Solid Shapes