Conditional mandatory field

Good evening gurus:

I have two fields that are interdependent.
- Field1 is set up as a drop down with 4 values (V1, V2, V4, V4)
- Field2 is a text box.

What i would like to achieve is the following:
- If field1 selected value is V1, then filling in field 2 is mandatory and the user cannot leave the field blank (checking that the field is not filled in with spaces is also necessary)
- if field1 value is not V1 then filling in field2 is not mandatory

Any suggestion on how to setup the javascript code? Is it a field level or document level script?

Any direction on a source of information for required field values other than setting it up in the properties window?

Thanks for helping!

MK