Question

In Young's double slit experiment, to increase the fringe width

• A. the wavelength of the source is increased
• B. the source is moved towards the slit
• C. the source is moved away from the slit
• D. the slit separation is increased
• E. the screen is moved towards the slit

Hint:

To get larger, clearer fringes: use Red light (longest visible $\lambda$), move the screen further away (increase $D$), and keep the slits as close together as possible (decrease $d$).

Answer 1

The Correct Option is A

Concept: Young's Double Slit Experiment (YDSE) demonstrates the interference of light. The fringe width ($\beta$) is the distance between two consecutive bright or dark fringes on the screen.
• Fringe Width Formula: The mathematical expression for fringe width is: \[ \beta = \frac{\lambda D}{d} \] Where:
• $\lambda$ is the wavelength of the light used.
• $D$ is the distance between the slits and the screen.
• $d$ is the separation distance between the two slits.

Step 1: Identify proportionalities from the formula.
From the equation $\beta = \frac{\lambda D}{d}$, we can derive the following relationships:
• $\beta \propto \lambda$: Fringe width increases if wavelength increases.
• $\beta \propto D$: Fringe width increases if the screen-to-slit distance increases.
• $\beta \propto 1/d$: Fringe width increases if the slit separation decreases.

Step 2: Evaluate the options based on these rules.

• (A) Increasing $\lambda$ directly increases $\beta$. This is correct.
• (D) Increasing $d$ decreases $\beta$.
• (E) Moving the screen towards the slit decreases $D$, which decreases $\beta$.
• (B) and (C) The position of the primary source relative to the slits affects fringe intensity and coherence, but does not change the geometric fringe width defined by the slits and screen.