45

Given a prism with a refractive index n and a refracting angle of A = 45°, we will determine the incidence angle i needed for the ray to retrace its path.

Step 1: Using the formula for minima deviation in prisms, when light retraces its path, the angle of incidence i is equal to the angle of emergence e. Since we have a symmetric setup and the angle of the prism is 45° (where A is equal to the prism's angle), we can set the following relation:

$$ n = \frac{sin\left(\frac{i + A}{2}\right)}{sin\left(\frac{A}{2}\right)} $$

Step 2: For A = 45°, we have:
n = \frac{sin\left(\frac{i + 45}{2}\right)}{sin\left(\frac{45}{2}\right)} = \frac{sin\left(\frac{i + 45}{2}\right)}{sin(22.5)}

Step 3: The ray retraces its path means angle of incidence i = 45°. Hence, putting i as 45° back into our equation, we get 9 = 45°.

Step 4: Therefore, the value of 9 (in degree) is 45.