A point source S is placed in front of a perfectly reflecting mirror as shown. A screen is at equal distance a on the other side. The intensity at the centre of the screen is I with the mirror present. If the mirror is removed, what is the intensity at the centre?

Question

G1 Physics · Accepted Answer

Treat the real source and mirror image as incoherent. If the real source alone gives intensity Iₛ at O and its distance is a, the image is at distance 3a and contributes Iₛ/9. Thus the mirror-present intensity is I=(10/9)Iₛ, so without the mirror Iₛ=9I/10. Option C.