Q. Two point sources $A$ and $B$ of luminous intensities 1 cd and 16 cd respectively are placed 100 cm apart. A grease spot screen is placed between the two sources. For the grease spot to become indistinguishable from both the sides, it should be placed at
A) 80 cm from 16 cd lamp and 20 cm from 1 cd
B) 20 cm from the 16 cd and 80 cm from 1 cd
C) $\frac{400}{3\ }$ cm from 16 cd and $\frac{100}{3\ \ }$ cm from 1 cd
D) $\frac{100}{3\ \ }\ $ cm from 16 cd and $\frac{400}{3}\ $ cm from 1 cd
Explanation
(a) $\frac{1}{x^{2}} = \frac{16}{(100 - x)^{2}}$
Or $\frac{1}{x} = \frac{4}{100 - x}$
Or $5x = 100\ $or$\ x = 20\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.