Convert YYYYMMDD to DATE

I have a bunch of dates in varchar like this:

20080107
20090101
20100405
...

How do I convert them to a date format like this:

2008-01-07
2009-01-01
2010-04-05

I've tried using this:

SELECT [FIRST_NAME]
      ,[MIDDLE_NAME]
      ,[LAST_NAME]      
      ,cast([GRADUATION_DATE] as date)      
  FROM mydb

But get this message:

Msg 241, Level 16, State 1, Line 2
Conversion failed when converting date and/or time from character string.