Function based index not working

Can anyone explain why the function based index is doing a full table scan using TRUNC(approval_date) in the where clause but uses the index for approval_date in the where clause. Thanks

CREATE INDEX PRICE_CHANGE_I9 ON PRICE_CHANGE
TRUNC(approval_date)
NOLOGGING COMPUTE STATISTICS;

SELECT  *
FROM   price_change
TRUNC(approval_date) = SYSDATE
FULL TABLE SCAN

SELECT  *
FROM   price_change
approval_date = SYSDATE

Uses Index