Which device would produce a diffraction pattern with a central bright fringe and decreasing secondary fringes?

• A. Convex lens
• B. Concave lens
• C. Flat mirror
• D. Opaque card with a slit

Answer: (D)

The device that would produce a diffraction pattern with a central bright fringe and decreasing secondary fringes is the opaque card with a slit. This phenomenon occurs due to the wave nature of light. When light passes through a narrow slit, it spreads out rather than traveling in straight lines. This spreading creates an interference pattern that consists of a central bright fringe, which is the point where the light waves constructively interfere most strongly. On either side of this central bright fringe, the intensity of light decreases, resulting in dimmer secondary fringes.

The central fringe appears bright because the light waves from the slit are in phase at this point, and as you move away from the center, the conditions for constructive and destructive interference lead to the diminishing brightness of the secondary fringes. This is a fundamental demonstration of diffraction and interference patterns, which are characteristic of waves.

In contrast, a convex or concave lens would focus or diverge light but would not produce a diffraction pattern like that. A flat mirror reflects light and does not create fringes. Therefore, the opaque card with a slit is the only option that generates the described diffraction pattern.