The equation for the reaction between sulfur dioxide and acidified potassium dichromate(VI) is shown: $3\text{SO}_2 + \text{Cr}_2\text{O}_7^{2-} + 2\text{H}^+ \rightarrow 3\text{SO}_4^{2-} + 2\text{Cr}^{3+} + \text{H}_2\text{O}$. Which substance is oxidised, and which species acts as the oxidising agent?