We cannot usually leave out a noun after an adjective.

She is a beautiful girl. (NOT She is a beautiful.)

But there are some exceptions.

The + adjective

The structure the + adjective is used to talk about certain well-known groups of people in particular physical or social condition. The most common expressions of this kind are:

The blind
The deaf
The mute
The dead
The injured
The old
The poor
The rich
The unemployed
The jobless
The young
The mentally ill

The government should do something for the handicapped.
The jobless are losing hope.
The rich should help the poor.
The dead leave their blessings upon the living.

Note that the expressions the dead, the poor, the rich etc are always plural. The rich means ‘all rich people’. Similarly, the blind means ‘all blind people’.

Note that these expressions cannot be used with a possessive ’s.

We say the problems of the poor or poor people’s problems and not the poor’s problems.

Adjectives are not normally used in this way without the.

The rich doesn’t care about the poor. (NOT Rich doesn’t care about poor.)

There are some exceptions to this rule.

Adjectives without the are possible after quantifiers like many and more. Adjectives can also be used without the after possessives and in paired structures with and or or.

An ideal society should provide opportunities for both rich and poor.