/* Ray Diagram Practice: an optics bench. Warm ivory paper, a squared grid in faint cyan, the principal rays in
   vermilion pencil, the lens and mirror in black ink, and a lacquered-brass accent like the fittings of a lens holder.
   Headings in a book serif, set above a principal axis with F and 2F ticks; the sheet has a brass edge. Print: black ink, rays in red, grids
   at true size (1 square = 1 cm), a problem never split across pages. */
[hidden] { display: none !important; }
.sr { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }
:root {
  --bg: #f2f1ea;
  --paper: #ffffff;
  --grid: #cfe3e6;
  --ink: #1d1b14;
  --muted: #57543f;
  --line: #dcd9cc;
  --edge: #8f8b78;
  --accent: #6a5a12;
  --accent-ink: #ffffff;
  --accent-soft: #eee8cf;
  --ray: #c4361a;
  --draw: #111917;
  --ok: #17663a;
  --ok-soft: #def1e5;
  --close: #7a5000;
  --close-soft: #f6ebd0;
  --bad: #a1251d;
  --bad-soft: #f8e1dd;
  --info: #24527a;
  --focus: #c25e00;
  --head: "Charter", "Bitstream Charter", "Sitka Text", Cambria, Georgia, serif;
  --sans: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --mono: ui-monospace, "SF Mono", Menlo, Consolas, "DejaVu Sans Mono", monospace;
  --r: 3px;
  color-scheme: light dark;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #14130f;
    --paper: #1d1c16;
    --grid: #22393b;
    --ink: #ece9dc;
    --muted: #bbb6a0;
    --line: #37352a;
    --edge: #77735e;
    --accent: #dcc66c;
    --accent-ink: #1d1806;
    --accent-soft: #332d14;
    --ray: #ff8a6b;
    --draw: #eef5f3;
    --ok: #8fdcaa;
    --ok-soft: #14301f;
    --close: #f0c56b;
    --close-soft: #352a12;
    --bad: #ff9d90;
    --bad-soft: #3a1c18;
    --info: #8ec3ea;
    --focus: #ffb366;
  }
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 17px/1.55 var(--sans); }
.wrap { max-width: 1000px; margin: 0 auto; padding: 0 16px; }
a { color: var(--accent); text-underline-offset: 2px; }
.skip { position: absolute; left: -999px; top: 8px; background: var(--paper); padding: 8px 12px; z-index: 10; }
.skip:focus { left: 8px; }
:focus-visible { outline: 3px solid var(--focus); outline-offset: 2px; }
sub { font-size: 0.72em; line-height: 0; }

.hero { padding: 22px 0 0; max-width: 760px; }
/* the principal axis under the heading: a line with ticks every 72 px, like F and 2F marks */
.hero::after { content: ""; display: block; height: 12px; margin: 12px 0 0; max-width: 520px;
  background: linear-gradient(var(--ink), var(--ink)) 0 5px / 100% 1.5px no-repeat,
    repeating-linear-gradient(90deg, transparent 0 71px, var(--ink) 71px 72.5px) 0 0 / 100% 12px no-repeat; }
h1 { font: 700 clamp(1.6rem, 4.3vw, 2.4rem)/1.12 var(--head); margin: 0 0 8px; }
.lede { margin: 0; color: var(--muted); max-width: 64ch; }
h2 { font: 700 1.3rem/1.25 var(--head); margin: 0 0 10px; }
.content h2::after { content: ""; display: block; width: 44px; height: 2px; margin-top: 5px; background: var(--ray); }

/* setup: a bench rail along the top of the tool */
.setup { display: flex; flex-wrap: wrap; align-items: flex-end; gap: 10px 12px; margin-top: 6px; padding: 12px 0; border-bottom: 1px dashed var(--edge); }
.setup label, .print-row label { display: flex; flex-direction: column; gap: 3px; font-size: 0.85rem; font-weight: 650; color: var(--muted); }
select, input { font: inherit; font-size: 16px; color: var(--ink); background: var(--paper); border: 1px solid var(--edge); border-radius: var(--r); min-height: 44px; padding: 6px 10px; }
select { max-width: 100%; }
.btn { font: inherit; font-weight: 650; min-height: 44px; padding: 8px 18px; border-radius: var(--r); border: 1px solid var(--accent); background: var(--paper); color: var(--accent); cursor: pointer; }
.btn.primary { background: var(--accent); color: var(--accent-ink); }
.btn.small { padding: 6px 14px; }
.btn:hover { filter: brightness(0.96); }
.link { font: inherit; background: none; border: 0; padding: 10px 0; min-height: 44px; color: var(--accent); text-decoration: underline; text-underline-offset: 2px; cursor: pointer; }
.seg { border: 0; padding: 0; margin: 0; display: flex; flex-wrap: wrap; gap: 0; flex-basis: 100%; }
.seg legend { font-size: 0.85rem; font-weight: 650; color: var(--muted); margin-bottom: 4px; padding: 0; }
.sg { position: relative; }
.sg input { position: absolute; opacity: 0; width: 1px; height: 1px; }
.sg span { display: inline-flex; align-items: center; min-height: 44px; padding: 6px 14px; border: 1px solid var(--edge); margin-right: -1px; background: var(--paper); cursor: pointer; font-weight: 600; }
.sg:first-of-type span { border-radius: var(--r) 0 0 var(--r); }
.sg:last-of-type span { border-radius: 0 var(--r) var(--r) 0; }
.sg input:checked + span { background: var(--accent); color: var(--accent-ink); border-color: var(--accent); position: relative; }
.sg input:focus-visible + span { outline: 3px solid var(--focus); outline-offset: 2px; position: relative; }
.opts-d { border-bottom: 1px solid var(--line); }
.opts-d > summary { cursor: pointer; padding: 10px 0; min-height: 44px; font-weight: 650; display: flex; align-items: center; color: var(--accent); }
.opts-body { padding: 0 0 12px; }
.rl { margin: 6px 0 0; padding-left: 1.3em; max-width: 78ch; font-size: 0.95rem; }
.rl li { margin: 0 0 6px; }
.rl .src { color: var(--muted); font-size: 0.88rem; display: block; }

.note { margin: 12px 0; padding: 10px 12px; border: 1px solid var(--close); background: var(--close-soft); overflow-wrap: anywhere; border-radius: var(--r); }
.note.info { border-color: var(--info); background: var(--paper); }
.note .link { min-height: 0; padding: 0; font-weight: 600; }

/* the worksheet */
.sheet { background: var(--paper); border-radius: var(--r); margin: 16px 0 8px; padding: 14px 18px 10px; }
.sheet:focus { outline: 2px solid var(--accent); }
.sheet-head { display: flex; justify-content: space-between; align-items: baseline; flex-wrap: wrap; gap: 4px 14px; }
.sheet-head h2 { margin: 0; }
.code { font-family: var(--mono); letter-spacing: 0.08em; color: var(--accent); }
.score { margin: 0; color: var(--muted); font-size: 0.95rem; }
.sline { margin: 4px 0 0; font-weight: 650; font-size: 0.95rem; }
.convd { margin: 4px 0 0; font-size: 0.92rem; }
.convd > summary { cursor: pointer; min-height: 44px; display: flex; align-items: center; color: var(--muted); font-weight: 600; }
.conv, .tol { margin: 0 0 6px; color: var(--muted); font-size: 0.9rem; max-width: 82ch; }
.qs { list-style: none; margin: 0; padding: 0; }
.q { border-top: 1px solid var(--line); padding: 16px 0 10px; margin-top: 12px; }
.q h3 { font: 700 1.12rem/1.3 var(--head); margin: 0 0 6px; display: flex; gap: 10px; align-items: baseline; }
.qn { font-family: var(--mono); color: var(--accent); font-weight: 700; }
.scen { margin: 0 0 10px; max-width: 72ch; }
.scen .do { color: var(--muted); }
.fig { background: var(--paper); border: 1px solid var(--line); padding: 6px; max-width: 100%; }
.fig svg { display: block; width: 100%; height: auto; margin: 0 auto; }
.fig.zoomed { overflow-x: auto; }
.fig.zoomed svg { margin: 0; }
.fighead { display: flex; justify-content: flex-end; margin-bottom: 4px; }
.zoom { min-height: 40px; padding: 4px 12px; white-space: nowrap; }

/* the drawing (units are millimetres on paper) */
svg.rd .gr { stroke: var(--grid); stroke-width: 0.3; fill: none; }
svg.rd .ax { stroke: var(--draw); stroke-width: 0.45; }
svg.rd .lm, svg.rd .lo, svg.rd .mi { stroke: var(--draw); stroke-width: 0.7; fill: none; stroke-linejoin: round; }
svg.rd .lc { stroke: var(--draw); stroke-width: 0.25; stroke-dasharray: 1 1; }
svg.rd .ht { stroke: var(--draw); stroke-width: 0.35; fill: none; }
svg.rd .mk line { stroke: var(--draw); stroke-width: 0.4; }
svg.rd text { fill: var(--draw); font: 3.2px Arial, "Helvetica Neue", sans-serif; text-anchor: middle; paint-order: stroke; stroke: var(--paper); stroke-width: 0.8px; stroke-linejoin: round; }
svg.rd .sb { font-size: 2.2px; }
svg.rd .ob, svg.rd .ib { stroke: var(--draw); stroke-width: 0.8; }
svg.rd .iv { stroke-dasharray: 1.6 1.1; }
svg.rd .oh, svg.rd .ih { fill: var(--draw); }
svg.rd .rs { stroke: var(--ray); stroke-width: 0.45; }
svg.rd .rv { stroke: var(--ray); stroke-width: 0.4; stroke-dasharray: 1.6 1.1; }
svg.rd .ra { fill: var(--ray); }
@media (max-width: 600px) {
  .fig:not(.zoomed) svg.rd text { font-size: 4.6px; stroke-width: 1px; }
  .fig:not(.zoomed) svg.rd .sb { font-size: 3.2px; }
}

.qparts { display: grid; grid-template-columns: repeat(auto-fill, minmax(min(100%, 17rem), 1fr)); gap: 10px 20px; margin-top: 12px; }
.part { margin: 0; border: 0; padding: 0; min-width: 0; }
.part.ch { grid-column: 1 / -1; }
.prompt, .part legend { display: block; font-size: 0.95rem; font-weight: 600; margin-bottom: 3px; padding: 0; }
.ain { display: inline-flex; align-items: center; gap: 6px; }
.ain input { width: 8.5em; font-family: var(--mono); }
.unit { color: var(--muted); min-width: 1.6em; }
.part.is-right .ain input { border-color: var(--ok); box-shadow: inset 0 0 0 1px var(--ok); }
.part.is-wrong .ain input, .part.is-unread .ain input { border-color: var(--bad); box-shadow: inset 0 0 0 1px var(--bad); }
.part.is-close .ain input { border-color: var(--close); box-shadow: inset 0 0 0 1px var(--close); }
.opts { display: flex; flex-wrap: wrap; gap: 6px; }
.opt { display: flex; gap: 8px; align-items: center; min-height: 44px; padding: 8px 12px; border: 1px solid var(--line); border-radius: var(--r); background: var(--paper); cursor: pointer; }
.opt input { margin: 0; min-height: 0; width: 18px; height: 18px; accent-color: var(--accent); flex: none; }
.opt:has(input:checked) { border-color: var(--accent); box-shadow: inset 0 0 0 2px var(--accent); }
.fbwrap:empty { display: none; }
.fb { margin: 6px 0 0; padding: 6px 10px; border-radius: var(--r); max-width: 64ch; font-size: 0.95rem; }
.fb.right { background: var(--ok-soft); color: var(--ok); }
.fb.close { background: var(--close-soft); color: var(--close); }
.fb.wrong, .fb.unread { background: var(--bad-soft); color: var(--bad); }
.qfoot { padding: 12px 0 0; }
.work > summary { cursor: pointer; color: var(--accent); font-weight: 650; padding: 10px 0; min-height: 44px; display: flex; align-items: center; }
.keybox { padding: 4px 0; }
.keybox h4 { font: 700 1rem var(--head); margin: 10px 0 4px; }
.cap { font-size: 0.88rem; color: var(--muted); margin: 4px 0 8px; }
.kt { border-collapse: collapse; width: 100%; font-size: 0.93rem; margin: 6px 0 10px; }
.kt th, .kt td { border-top: 1px solid var(--line); padding: 6px 8px 6px 0; text-align: left; vertical-align: top; }
.kt th { font-weight: 600; width: 38%; }
.kt .ka { font-family: var(--mono); font-weight: 700; white-space: nowrap; }
.kt .kw { color: var(--muted); overflow-wrap: anywhere; }
@media (max-width: 640px) {
  .kt tr { display: grid; grid-template-columns: minmax(0, 1fr); border-top: 1px solid var(--line); padding: 6px 0; }
  .kt th, .kt td { border: 0; padding: 1px 0; width: auto; }
  .kt .kw { font-size: 0.88rem; }
  .kt .kw:empty { display: none; }
  .kt .ka { white-space: normal; }
}
.steps { margin: 0 0 10px; padding-left: 1.3em; font-size: 0.95rem; }
.steps li { margin-bottom: 5px; }
.sheet-foot { display: flex; align-items: center; flex-wrap: wrap; gap: 6px 16px; padding: 14px 0 4px; border-top: 1px solid var(--line); margin-top: 10px; }
.hint { color: var(--muted); font-size: 0.9rem; }
.ask { display: flex; align-items: center; flex-wrap: wrap; gap: 8px; }

.share { padding: 8px 0 0; }
.share .hint { margin: 6px 0 0; }
.print-row { display: flex; flex-wrap: wrap; align-items: flex-end; gap: 10px 12px; }
.share-out label { display: flex; flex-direction: column; gap: 3px; font-size: 0.9rem; font-weight: 600; margin-top: 8px; }
.share-out input { width: 100%; }
.recent { border-top: 1px solid var(--line); margin-top: 12px; }
.recent summary { cursor: pointer; padding: 11px 0; font-weight: 600; min-height: 44px; }
.recent ul { list-style: none; margin: 0; padding: 0; }
.recent li { display: flex; justify-content: space-between; align-items: center; gap: 10px; border-top: 1px solid var(--line); padding: 4px 0; }
.recent small { display: block; color: var(--muted); }

.toast { position: fixed; left: 50%; bottom: 16px; transform: translate(-50%, 160%); background: var(--ink); color: var(--bg); padding: 10px 14px; border-radius: var(--r); max-width: calc(100% - 32px); transition: transform 0.2s; pointer-events: none; z-index: 20; }
.toast.on { transform: translate(-50%, 0); }
@media (prefers-reduced-motion: reduce) { .toast { transition: none; } }

.content { padding: 28px 0 8px; max-width: 760px; }
.content h2 { margin-top: 26px; }
.content p, .levels { max-width: 70ch; }
.levels { padding-left: 1.3em; }
.faq details { border-bottom: 1px solid var(--line); }
.faq summary { cursor: pointer; padding: 11px 0; font-weight: 600; min-height: 44px; }
.faq details p { margin: 0 0 12px; }
.sources { font-size: 0.92rem; color: var(--muted); overflow-wrap: anywhere; margin-top: 22px; }
footer { border-top: 1px solid var(--line); margin-top: 20px; padding: 16px 0 28px; color: var(--muted); font-size: 0.92rem; }
.indep { margin: 0 0 10px; }
.sb-footer p { margin: 6px 0; }
.sb-more { line-height: 1.8; }
.sb-footer a { color: var(--ink); }
@media (max-width: 600px) {
  .sheet { margin: 14px -8px 8px; padding: 12px 10px 8px; }
  .setup .btn { flex: 1 1 auto; }
  .sg span { padding: 6px 10px; }
}

/* ---------- print ---------- */
.print-root { display: none; }
@page { margin: 10mm 12mm; }
@media print {
  :root { --ink: #000; --muted: #333; --line: #888; --paper: #fff; --edge: #555; --draw: #000; --accent: #000; --grid: #9fc7cf; --ray: #d0201a; }
  body { background: #fff; color: #000; font: 10pt/1.35 Arial, var(--sans); }
  main, footer, .toast, .skip { display: none !important; }
  .print-root { display: block; }
  svg { print-color-adjust: exact; -webkit-print-color-adjust: exact; }
  table.pt { width: 100%; border-collapse: collapse; }
  table.pt > tbody > tr > td, table.pt > thead > tr > td { padding: 0; }
  .prun { font: 8pt Arial, sans-serif; color: #444; text-align: right; padding-bottom: 3pt !important; }
  section.psheet, section.pkey { break-before: page; page-break-before: always; }
  section:first-child { break-before: auto; page-break-before: auto; }
  .ph { border-bottom: 1.5pt solid #000; margin: 0 0 4pt; break-after: avoid; page-break-after: avoid; }
  .ph h2 { font: 700 14pt var(--head); margin: 0; }
  .ph p { margin: 0 0 3pt; font-size: 9pt; }
  .pmeta { display: flex; justify-content: space-between; gap: 12pt; margin: 4pt 0 5pt; }
  .prule { font-size: 8.5pt; margin: 0 0 4pt; }
  .pqs { list-style: none; margin: 0; padding: 0; }
  .pq { break-inside: avoid; page-break-inside: avoid; margin: 0 0 6pt; padding-top: 4pt; border-top: 0.75pt solid #000; }
  .pq h3 { font: 700 11pt var(--head); margin: 0 0 1pt; }
  .pscen { margin: 0 0 3pt; font-size: 10pt; }
  .pfig { margin: 2pt 0 3pt; flex: none; }
  .pbody.side { display: flex; gap: 5mm; align-items: flex-start; }
  .pbody.side > .pparts, .pbody.side > .pkr { flex: 1 1 0; min-width: 0; }
  .pbody.side .pparts { grid-template-columns: 1fr; }
  .pbody.side .pp > span:first-child { white-space: normal; }
  .pbody.side .pkq { width: 45%; } .pbody.side .pka { width: auto; }
  .pka small { display: block; font-weight: 400; font-size: 7.5pt; }
  .pfig svg { display: block; max-width: none; }
  svg.rd text { fill: #000; stroke: #fff; }
  .pwork { height: 30mm; border: 0.5pt solid #bbb; border-radius: 1.5mm; margin: 2pt 0 3pt; padding: 1.5pt 4pt; font-size: 7.5pt; color: #666; }
  .pparts { list-style: none; margin: 2pt 0 0; padding: 0; display: grid; grid-template-columns: 1fr 1fr; gap: 1pt 12pt; }
  .pp > span:first-child { white-space: nowrap; }
  .pp { display: flex; align-items: flex-end; gap: 4pt; font-size: 9pt; min-height: 7mm; }
  .pp.wide { grid-column: 1 / -1; flex-wrap: wrap; min-height: 6mm; }
  .pline { flex: 1 1 14mm; border-bottom: 0.5pt solid #555; min-width: 14mm; height: 1em; }
  .popts { display: inline-flex; flex-wrap: wrap; gap: 2pt 10pt; }
  table.pkt { width: 100%; border-collapse: collapse; font-size: 8.5pt; margin-top: 3pt; }
  table.pkt td { border: 0.5pt solid #555; padding: 1.5pt 3pt; vertical-align: top; }
  .pkq { width: 34%; font-weight: 700; }
  .pka { width: 22%; font-weight: 700; }
  .pkw { font-size: 8pt; }
  table.pkt.two .pka { width: auto; }
  .psteps { margin: 3pt 0 0; padding-left: 12pt; font-size: 8pt; line-height: 1.3; }
  .psteps li { margin: 0 0 1.5pt; }
}