Reflecting the Double Slit Pattern

Abstract

This research was conducted to note the changes in the interference patterns, by different means of reflection. For this, we have used a double slit, different mirrors, and a screen to obtain the image. The hypothesis behind this was that double slit produces a fringe patten on a screen when a coherent light source is passed through it, but what if we put a mirror after the slits and obtain an image on a screen. Not just that, what if we put different kinds of mirrors in between the slit and the screen. We conducted this research in an environment with negligible sources of light except the non-coherent light source, and with negligible vibrations which could affect the image obtained. When this research was conducted with a flat mirror, fringes were roughly the same as predicted by mathematical calculations. But when this research was conducted by a convex mirror, the fringes seemed to be virtual . When this same study was conducted with a concave mirror, the fringes seemed to be either concentrated at a single point, or they seemed to be have enlarged with a virtual image however the ratio of magnified image to the actual image that have been obtained if there was no mirror, remained the same. Whereas in the case of a convex mirror, the image was smaller. This property is useful as it can be used to observe the pattern more precisely by magnifying it and the actual numbers, i.e.- when mirror would not have been used can also be calculated because the magnification is the same.

Introduction

This study explores how placing mirrors—particularly a concave mirror—in the Young’s Double Slit Experiment (YDSE) affects the magnification and visibility of interference fringe patterns, demonstrating the dual nature of light as both a particle and a wave.

Introduction

The YDSE demonstrates light’s wave nature through interference patterns formed when coherent light passes through two narrow slits. However, observing these patterns precisely is often difficult and costly. To improve visibility, the study investigates the use of plane, concave, and convex mirrors at the screen’s position to analyze how reflected rays influence fringe magnification.

Materials and Methods

Equipment included:

• Convex, concave, and plane mirrors (20 cm focal length)

• 632 nm laser light source (diameter 2 mm)

• Double-slit setup with 80 µm slit separation

• White screen and measuring instruments

The experiment was conducted under vibration-free, dark conditions. The fringe spacing formula

β=λDdβ = \frac{λD}{d}β=dλD?

was used, where λλλ = 632 nm, DDD = slit-to-screen distance, and ddd = slit separation.

Results

• Plane mirror produced a virtual image of equal size (no magnification).

• Concave mirror created a virtual, erect, magnified image when the slit was placed between the pole and focal point. Magnification increased as the slit approached the focus, up to the point where the pattern became blurry.

• Convex mirror produced diminished fringes, consistent with theoretical expectations.

A proportional relationship was found:

β′=M×ββ' = M × ββ′=M×β

where β′β'β′ is the observed fringe spacing and MMM is magnification.

Discussion

Results confirm that a concave mirror magnifies the interference pattern without altering wavelength or coherence, as it forms a virtual, erect image that serves as a secondary light source. The convex mirror, conversely, reduces fringe size. Observed changes match theoretical predictions from the mirror formula and interference equations. Minor measurement errors may arise from misalignment or imperfections in mirror curvature.

Conclusion

Using a coherent 632 nm laser, the study effectively illustrates how various mirror curvatures affect the magnification of interference fringes in a double-slit arrangement. A concave mirror created an enlarged virtual picture when the object was positioned between its pole and focus, increasing the fringe spacing, whereas a plane mirror maintained the fringe spacing and served merely as a reflective surface. The convex mirror, on the other hand, created a smaller virtual picture, which resulted in a smaller fringe spacing. These findings demonstrate that, without changing the coherence or wavelength of the light, mirror geometry directly alters the effective slit separation and, thus, the observable interference pattern. The relationship between wave interference and geometrical optics is strengthened by the experimental results, which closely match predictions from the mirror and magnification equations. Overall, this experiment confirms and evalutes how the mirrors manuplate the interference pattern at different distances

Copyright

Copyright © 2025 Himansh Bainsla. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.