SQL & PL/SQL

When I use a numeric value against a column defined as varchar in the where caluse, Oracle is performing a full table scan though an index exist for that column. Is there any reason why Oracle behaves this way? I am using 10.2.0.3.0

create table employees (empid varchar2(3));

insert into employees values ('123');

create index emp_index on employees (empid);

select * from employees where empid = '123'; --- Index range scan

select * from employees where empid = 123; ---- Full Table scan

select * from employees where empid = to_char(123); ---- Index range scan

However, this is not the case when I use a string in where clause against a column defined as number.

create table managers (mgr_id number(3));

insert into managers values (123);

create index mgr_index on managers (mgr_id);

select * from managers where mgr_id = 123; --- Index range scan

select * from managers where mgr_id = '123'; -- Index range scan