In interference experiment the… - JEE Main 2026 Physics PYQ

JEE Main 2026 — Physics Optics

2026mcqmedium

In interference experiment the path difference between two interfering waves at a point $A$ on the screen is $\lambda/3$ , where $\lambda$ is the wavelength of these waves, and at another point $B$ the path difference is $\lambda/6$ . The ratio of intensities at points $A$ and $B$ is _______.

Official previous-year question

Held on 6 Apr 2026 · Verified 6 Jul 2026.

Options

A
$3$
B
$4$
C
$1/3$
D
$1/4$

Did you get this right?

Solution

The phase difference $\Delta \phi$ is related to the path difference $\Delta x$ by the relation $\Delta \phi = \dfrac{2\pi}{\lambda} \Delta x$ .

For point $A$ , the path difference is $\Delta x_A = \dfrac{\lambda}{3}$ .

$\Delta \phi_A = \dfrac{2\pi}{\lambda} \times \dfrac{\lambda}{3} = \dfrac{2\pi}{3}$

The intensity at point $A$ is given by $I_A = I_{max} \cos^2\left(\dfrac{\Delta \phi_A}{2}\right)$ .

$I_A = I_{max} \cos^2\left(\dfrac{\pi}{3}\right) = I_{max} \left(\dfrac{1}{2}\right)^2 = \dfrac{I_{max}}{4}$

For point $B$ , the path difference is $\Delta x_B = \dfrac{\lambda}{6}$ .

$\Delta \phi_B = \dfrac{2\pi}{\lambda} \times \dfrac{\lambda}{6} = \dfrac{\pi}{3}$

The intensity at point $B$ is given by $I_B = I_{max} \cos^2\left(\dfrac{\Delta \phi_B}{2}\right)$ .

$I_B = I_{max} \cos^2\left(\dfrac{\pi}{6}\right) = I_{max} \left(\dfrac{\sqrt{3}}{2}\right)^2 = \dfrac{3I_{max}}{4}$

The ratio of intensities at points $A$ and $B$ is:

$\dfrac{I_A}{I_B} = \dfrac{\dfrac{I_{max}}{4}}{\dfrac{3I_{max}}{4}} = \dfrac{1}{3}$

Answer: $1/3$

Your note

JEE Main Optics in other years

Optics 2025 Optics 2024 Optics 2023 Optics 2022 Optics 2021 Optics 2020 Optics 2019 Optics 2018 Optics 2017 Optics 2016 Optics 2015 Optics 2014

More JEE Main Optics questions

For a transparent prism, if the angle of minimum deviation is equal to its refracting angle, the refractive index $n$ of the prism satisfies.

2026medium

A parallel beam of light travelling in air (refractive index 1.0) is incident on a convex spherical glass surface of radius of curvature 50 cm. Refractive index of glass is 1.5. The rays converge to a point at a distance $x \mathrm{~cm}$ from the centre of the curvature of the spherical surface. The value of $x$ is $\_\_\_\_$ cm.

2026medium

A thin convex lens of focal length 5 cm and a thin concave lens of focal length 4 cm are combined together (without any gap) and this combination has magnification $m_{1}$ when an object is placed 10 cm before the convex lens. Keeping the positions of convex lens and object undisturbed a gap of 1 cm is introduced between the lenses by moving the concave lens away, which lead to a change in magnification of total lens system to $m_{2}$. The value of $\left|\frac{m_{1}}{m_{2}}\right|$ is $\_\_\_\_$.

2026medium

As shown in the diagram, when the incident ray is parallel to base of the prism, the emergent ray grazes along the second surface. ![JEE Main 2026 Physics, Ray Optics — question figure](https://prepforbharat.s3.ap-south-1.amazonaws.com/exam/615f0e999476412f48314daf/Physics/images/Ray_Optics/6983c1828bfdbb243563959e/question_1__q_6983c1828bfdbb243563959e__cdn-question-pool.getmarks.app__21S2_q_34_1__70267be84d_final_ppt_sync.png) If refractive index of the material of prism is $\sqrt{2}$, the angle $\theta$ of prism is.

2026medium

The size of the images of an object, formed by a thin lens are equal when the object is placed at two different positions 8 cm and 24 cm from the lens. The focal length of the lens is $\_\_\_\_$ cm.

2026medium

View all Optics questions

Keep practicing Optics

Work through every JEE Main Optics PYQ, year by year.

Browse all Physics topics