Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_7698 ( "Material" text, "Formula" text, "Refractive index(es) 589.3nm" text, "Hardness ( Mohs' scale )" text, "Density (g/cm 3 )" text, "State of the art" text )

### Question ###
Which formula has a refractive index 589.3 at 1.97?

### Accurate SQL ###

SELECT "Formula" FROM table_7698 WHERE "Refractive index(es) 589.3nm" = '1.97'