Page 164 - ICSE Math 7

Ray

A ray is a part of a line which starts from a fixed point and moves endlessly in
the same direction. An arrow on one side of a ray shows that the ray extends A B
indefinitely in one direction only. A ray has only one end point known as its
initial point. In the adjoining figure, A is the initial point of ray AB.

Plane

A plane is a flat surface. It has infinite length and breadth and hence extends A p
infinitely. A plane consists of infinite number of points and lines. It is denoted
by small letters like p or q or by three points (not in a straight line) drawn C
on it. In the adjoining figure, a plane p or ABC is shown. B

Parallel Lines m

Two lines which lie in the same plane and do not meet even when extended
indefinitely are known as parallel lines. The distance between two parallel lines n
always remains the same. In the adjoining figure, lines m and n are parallel to
each other and are represented by m || n (read as m is parallel to n).

Intersecting Lines n
m
Two lines which lie in the same plane and have a common point are known as O
intersecting lines. The common point is known as the point of intersection.
In the given figures, lines m and n are intersecting with O as their point of m
intersection.
O
n
Perpendicular Lines m

Two intersecting lines are said to be perpendicular if they make an angle of
90° at the point of intersection. In the adjoining figure, line m is perpendicular
to line n and is represented by m ⊥ n (read as m is perpendicular to n). 90° n

Properties of Lines
l
m
n
Property 1: Infinite number of straight lines can be drawn through a point.
O q
p

Property 2: One and only one line passes through two distinct points
in a plane. A B

m p
Property 3: Two different lines in a plane are either parallel or n q
intersect at exactly one point.

Parallel lines Intersecting lines

150