Explanation:

1. Reaction:
When Na2SO3 is added to an acidic solution of K2Cr2O7, the following reaction takes place:
2Na2SO3 + K2Cr2O7 + H2SO4 → Na2SO4 + Cr2(SO4)3 + K2SO4 + H2O

2. Reduction of Chromium:
In this reaction, the orange-colored K2Cr2O7 (potassium dichromate) is reduced to green-colored Cr2(SO4)3 (chromium sulfate). Chromium in K2Cr2O7 undergoes a change in oxidation state from +6 to +3, leading to the formation of green chromium sulfate.

3. Formation of Green color:
The green color observed is due to the formation of chromium(III) sulfate, which has a green color. The transition of chromium from the +6 oxidation state in K2Cr2O7 to the +3 oxidation state in Cr2(SO4)3 results in the change in color.

4. Absorption of Light:
The green color is produced because chromium(III) ions absorb certain wavelengths of light and reflect others, giving the compound its characteristic color.

5. Conclusion:
Therefore, the addition of Na2SO3 to an acidic solution of K2Cr2O7 leads to the reduction of chromium and the formation of green-colored chromium sulfate, resulting in the green color observed.