A point source S is at distance d from screen A and produces intensity I₀ at the centre O. A perfectly reflecting mirror is placed at distance d behind S as shown. Treating the source and its image as incoherent, find the intensity at O.

Question

G1 Physics · Accepted Answer

The source answer key marks option D, i.e. 9I₀/8. However, using the printed geometry literally: the real source is at distance d from O and its mirror image is at distance 3d from O, so incoherent intensities would add as I=I₀+I₀(d/3d)²=I₀+I₀/9=10I₀/9, which is option B. Therefore this item contains a source-key inconsistency. The workbook keeps option D to remain faithful to the source answer key.