In Lloyd's mirror experiment, the central fringe is:

In YDSE, if the distance between the slits is made 4 times the original and the distance of the screen is halved, the fringe width becomes:

In YDSE, the source slit width is gradually increased. The interference fringes:

In Newton's rings experiment (reflected light), the center is: <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 280 130" style="max-width:280px;margin:0.5em auto;display:block;"> <title>Newton's rings — plano-convex lens on flat glass, dark center in reflected light</title> <rect width="280" height="130" fill="#faf9f5" rx="6"/> <line x1="40" y1="80" x2="240" y2="80" stroke="#14140f" stroke-width="1.5"/> <path d="M60,80 Q140,30 220,80" fill="none" stroke="#c6613f" stroke-width="1.5"/> <text x="140" y="25" text-anchor="middle" font-size="8" fill="#c6613f">Plano-convex lens</text> <text x="140" y="95" text-anchor="middle" font-size="8" fill="#14140f">Flat glass plate</text> <circle cx="140" cy="80" r="3" fill="#14140f"/> <text x="140" y="74" text-anchor="middle" font-size="7" fill="#14140f">t=0</text> <circle cx="140" cy="80" r="15" fill="none" stroke="#788c5d" stroke-width="0.8" stroke-dasharray="2,2"/> <circle cx="140" cy="80" r="25" fill="none" stroke="#788c5d" stroke-width="0.8" stroke-dasharray="2,2"/> <circle cx="140" cy="80" r="35" fill="none" stroke="#788c5d" stroke-width="0.8" stroke-dasharray="2,2"/> <text x="140" y="125" text-anchor="middle" font-size="8" fill="#14140f">Center dark (path diff = λ/2 from phase change)</text> </svg>

In YDSE, the intensity at the central maximum is I. The intensity at a point where the path difference is λ/3 is: