Exams › JEE Advanced › Physics

Two coherent point sources separated by a distance d = 5 micrometers emit light of wavelength lambda = 2 micrometers in phase. A circular wire of radius R = 20 micrometers is placed symmetrically around the two sources (sources lie along a diameter). Points A and B are at the ends of the diameter along the line joining the sources; points C and D are at the ends of the diameter perpendicular to the line joining the sources. Which of the following is correct?

1. (A) Points A and B are dark and points C and D are bright.
2. (B) Points A and B are bright and points C and D are dark.
3. (C) Points A and C are dark and points B and D are bright.
4. (D) Points A and C are bright and points B and D are dark.

Correct answer: (B) Points A and B are bright and points C and D are dark.

Solution

For two coherent sources separated by d = 5 um, wavelength lambda = 2 um. Path difference at angle theta from the source axis = d*cos(theta). At A (theta = 0): PD = 5 um = 2.5*lambda -> destructive (dark). At B (theta = 180): PD = 5*cos(180) = -5 um, |PD| = 2.5*lambda -> dark. At C or D (theta = 90): PD = 0 -> constructive (bright). So A and B are dark; C and D are bright.

Related JEE Advanced Physics questions

• What factor influences the visibility of fringes in an interference pattern?
• When the screen is moved further along the x-axis away from the source, which of the following statements is false?
• In the Young's double slit experiment using a monochromatic light of wavelength λ, the path difference (in terms of an integer n) corresponding to any point having half the peak intensity is
• Using the expression 2d sin θ = λ, one calculates the values of d by measuring the corresponding angles θ in the range 0 to 90°. The wavelength λ is exactly known and the error in θ is constant for all values of θ. As θ increases from 0°:
• Young's double slit experiment is performed in a medium of refractive index mu1 = 4/3. A thin transparent slab of refractive index mu2 = 3/2 and thickness t = 8 micrometers is placed in front of slit S2. The magnitude of the optical path difference (with respect to the liquid medium) at the central point O is:
• In a standard Young's double-slit experiment, point P on the screen is located at a distance D from the slits and shows the 3rd bright fringe at t = 0. The screen then moves away from the slits at a constant speed v along the axis joining the centre of the slits and the centre of the screen. At t = 1 s the intensity at P is (3/4) of the maximum intensity, and P lies between the 2nd and 3rd bright fringes. What is the speed v of the screen?

⚔️ Practice JEE Advanced Physics free + battle 1v1 →