9) You shine light of a wavelength of 420nm upon a mirror that reflects the light directly back along the path of the incident light. The mirror is 1080nm from the light source. Will the light interfere constructively or destructively when it arrives back at the light source? Use a calculation to back-up the answer.

9) You shine light of a wavelength of 420nm upon a mirror that reflects the light directly back along the path of the incident light. The mirror is 1080nm from the light source. Will the light interfere constructively or destructively when it arrives back at the light source? Use a calculation to back-up the answer.