Find the V1 value interval (V1min, V1max) for which D2 emits light?
This looks like a homework question.  I will offer some suggestions and pointers but I won't solve it for you.  Modify your question as you work out the details and we will help you reach the answer.

1) Look closely at the resistors R1, R3, R5.  Simplify those to a single resistor.

2) Observe that neither D1 or D2 will conduct at below a certain input (V1) voltage.  Work out what that voltage is - that value is important.

Modify your question with those calculations.