Wavelength of light and partial pressure

Long Answer PROBLEM 1 (10 marks): HCl(g) can be made by the direct reaction of H2 and Cl2 in the presence of light. 3.00 g of H2 and 140 g Cl2 are mixed in a 10.00 L flask at 28.0 °C. What are the partial pressures of gases after the reaction has completed? Here is the data you'll need: Temperature and volume remains constant at 28.0 °C and Wavelength of light needed for the reaction is 340 nm I don't get how the wavelength of light is relevant. My class has only been taught how to use Pv=nrt to calculate partial pressure. Can someone please help me solve this??