The spacing of the interference fringes in Young's double slit experiment can be calculated using the formula:

$$\Delta y = \frac{\lambda L}{d}$$

where:

$\Delta y$ is the fringe spacing, i.e., the distance between adjacent bright or dark fringes on the screen,
$\lambda$ is the wavelength of the light used,
$L$ is the distance from the slits to the screen, and
$d$ is the separation between the two slits.

Given this formula, let's discuss how each option affects the fringe spacing:

Option A: Moving the screen away from the slits

If we increase $L$, the distance from the slits to the screen, according to the formula, $\Delta y$ would also increase, causing the fringes to be more widely spaced, not more closely spaced.

Option B: Using blue light instead of green light

The wavelength of light ($\lambda$) is a key factor in determining fringe spacing. Blue light has a shorter wavelength than green light. Decreasing $\lambda$ (by switching from green to blue light) would cause $\Delta y$ to decrease, according to the formula. This means the fringes will be more closely spaced, making this option a potentially correct answer.

Option C: Using red light instead of green light

Since red light has a longer wavelength than green light, switching to red light would increase $\lambda$, leading to an increase in $\Delta y$, according to the formula. This would make the fringes more widely spaced, not more closely spaced.

Option D: Reducing the separation between the slits

Reducing the separation between the slits ($d$) while keeping the wavelength and the screen distance constant would cause $\Delta y$ to increase, according to the formula, thereby spacing the fringes further apart, not closer together.

Based on this analysis, Option B: Using blue light instead of green light is the correct choice, as it would cause the observed fringes to be more closely spaced due to the shorter wavelength of blue light.

Wave Optics

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