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_name_28 ( mean_free_path VARCHAR, vacuum_range VARCHAR )

### Question ###
What is Mean Free Path, when Vacuum Range is 'medium vacuum'?

### Accurate SQL ###

SELECT mean_free_path FROM table_name_28 WHERE vacuum_range = "medium vacuum"