Consider four identical monochromatic wave sources $\mathrm{A}, \mathrm{B}, \mathrm{C}, \mathrm{D}$ (as shown in the figure) which produce plane waves of same wavelength $\lambda$. Sources $\mathrm{A}, \mathrm{B}$ and C lie in a line, with $B$ at the origin and $A, C$ being $\pm \frac{\lambda}{2}$ from the origin. Initially the source $D$ is at a distance $\lambda$ from the origin but later it is moved to a distance $\lambda / 2$ from the origin. Two receivers $R_1$ and $R_2$ are kept at equal large distance $r \gg \lambda$ from the origin. Select the correct option(s).

• A

  If source D is turned off, $R_1$ picks greater intensity signal than $R_2$.
• Correct answer
  B

  $R_2$ receives greater intensity signal when the source D is at distance $\lambda$.
• Correct answer
  C

  Signal intensity at $R_2$ decreases 4 times when D is moved from $\lambda$ to $\lambda / 2$.
• Correct answer
  D

  Signal intensity at $R_1$ is same whether D is at $\lambda$ or $\lambda / 2$.

Hence, the correct answer is options: 'B', 'C', 'D'.