---
title: "Sources of Light and Propagation"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 49
exercises: 12
source: https://one-course.com/books/physics/1/en/chapter/49-sources-of-light-and-propagation
---

# Chapter 49 — Sources of Light and Propagation

Why can you read this page? A lamp shines — but the page is no lamp, and yet your eye receives [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) from every printed line. Between the things that make [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and the things that merely hand it on lies a distinction optics cannot live without; and with it comes the full answer to the oldest question in this book: what does it take to see something?

## 49.1 Makers and forwarders

**Definition 49.1 (Primary sources).**

A *primary source* makes its own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source): the Sun and the [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star), flames, glowing filaments, the white-hot steel of the forge, fireflies, lightning. Its [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) exists whether or not anything else shines — switch off the world, and a primary source still glows.

**Definition 49.2 (Diffusing objects).**

Every other visible thing is a *diffusing object*: it receives [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and re-sends — *diffuses* — part of it in all directions. The page, the apple, the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon), your own face: none makes a glimmer of its own; each scatters received [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) generously to every side. A diffuser in the dark is simply invisible.

![The division of labor: the lamp makes light; the apple, receiving it, scatters some to every direction — which is why everyone around the table sees the same apple.](https://one-course.com/images/onecourse/chapters/physics-1/g7-light-sources-propagation/fig-4966ab8a61a8.svg)

*The division of labor: the lamp makes [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source); the apple, receiving it, scatters some to every direction — which is why everyone around the table sees the same apple.*

**Example 49.3 (Diffusion is not mirroring).**

Both [mirrors](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) and pages send received [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) back — but with opposite manners. The [mirror](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) bounces in *one* disciplined direction (equal angles — your old bounce rule), preserving the picture; the page’s microscopic roughness sprays [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) *everywhere*, destroying the picture but offering itself to every eye at once. That is why a [mirror](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) shows the lamp, while the page merely shows itself, from wherever you stand.

**Example 49.4 (The Moon’s file, closed).**

The childhood verdict now has its formal words: the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) is no [primary source](#def-g7-light-sources-propagation-primary); it is a [diffusing object](#def-g7-light-sources-propagation-diffusion) of planetary size — gray rock scattering sunlight in all directions, our way included. So moonlight is secondhand sunshine, and the “dark side” is simply the half whose file holds, at that moment, no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) to forward.

## 49.2 Seeing, settled

**Proposition 49.5 (The two conditions of vision).**

An eye sees an object exactly when

1. [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) *leaves* the object — made, if the object is a [primary source](#def-g7-light-sources-propagation-primary) ; diffused, if a forwarder — and
2. that [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) travels in a straight, unblocked line into the eye.

Break either condition — an unlit diffuser, or a wall across the sight line — and the object is unseen. Every visibility puzzle in this book unpacks into these two checks.

**Example 49.6 (The beam made visible).**

In clean [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), a torch [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) crossing the room cannot be seen from the side — [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) forwards almost nothing, and no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) from the [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) turns toward your eye. Shake a dusty cloth: the [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) leaps into view. Each dust mote is a tiny diffuser, catching [beam-light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and scattering some your way — thousands of specks drawing the [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray)’s straight line for you. Fog, mist and smoke perform the same service for lighthouse [beams](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) and cinema projectors.

## 49.3 Three coats for light

**Definition 49.7 (Transparent, translucent, opaque).**

Materials meet [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) in three manners:

1. *transparent* : [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) crosses nearly undisturbed, keeping its order — clear glass, still water, [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) ; [images](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#prop-g5-mirrors-reflection-image) pass intact;
2. *translucent* : [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) crosses, but scrambled — frosted glass, tracing paper, a thin curtain; brightness passes, [images](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#prop-g5-mirrors-reflection-image) do not;
3. *opaque* : [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) is stopped — absorbed or diffused back — wood, stone, metal, your hand; behind an opaque thing lies [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) .

**Method 49.8 (Sorting materials by torchlight).**

A dark room, a torch, a printed page, and the specimen between:

1. hold the specimen over the print with the torch behind it;
2. read the print through it? — [transparent](#def-g7-light-sources-propagation-coats) ;
3. see glow but no letters? — [translucent](#def-g7-light-sources-propagation-coats) ;
4. see darkness, and a [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) beyond? — [opaque](#def-g7-light-sources-propagation-coats) .

Test honestly: many materials change team with thickness — a sheet of water is [transparent](#def-g7-light-sources-propagation-coats), the deep sea is night-dark; one paper sheet is [translucent](#def-g7-light-sources-propagation-coats), the closed book [opaque](#def-g7-light-sources-propagation-coats).

![Three fates for a ray: through in good order; through but scrambled; stopped, with shadow behind.](https://one-course.com/images/onecourse/chapters/physics-1/g7-light-sources-propagation/fig-4544e69de90f.svg)

*Three fates for a ray: through in good order; through but scrambled; stopped, with [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) behind.*

**Example 49.9 (Architecture’s three teams).**

Builders deploy all three on purpose. [Transparent](#def-g7-light-sources-propagation-coats) window glass: view and [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) together. [Translucent](#def-g7-light-sources-propagation-coats) bathroom panes and frosted office partitions: daylight in, privacy kept — brightness without [images](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#prop-g5-mirrors-reflection-image) is exactly their job description. [Opaque](#def-g7-light-sources-propagation-coats) walls and shutters: darkness on demand. Lampshades are translucency in service of comfort: the blazing filament’s glare is scrambled into a soft, sourceless glow.

**Remark 49.10 (Nothing to forward, nothing to see).**

Riddle: an astronaut between the [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) looks *sideways* across a sunbeam-filled void. What does she see? Blackness — though sunlight floods past her eyes. Space carries no dust worth mentioning: nothing diffuses the passing [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) toward her, and [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) that does not enter the eye paints nothing. The daytime sky is blue precisely because our [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) *does* forward sunlight down to us — a [planet](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-planet)’s worth of gentle [diffusion](#def-g7-light-sources-propagation-diffusion) overhead. No [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), black sky, even at noon: the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon)’s astronauts stood in sunshine under a starless black dome.

## 49.4 Exercises

**Exercise 49.1 ★.**

Sort: candle flame; the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon); a firefly; this page; a red traffic [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source); the [planet](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-planet) Venus at dusk.

**Solution of Exercise 49.1.**

[Primary sources](#def-g7-light-sources-propagation-primary): the candle flame, the firefly, the red traffic [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). Diffusers: the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon), this page, Venus — the [planet](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-planet) shines by diffused sunlight, however star-like it looks.

**Exercise 49.2 ★.**

What does a [diffusing object](#def-g7-light-sources-propagation-diffusion) do with the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) it receives — and what becomes of it in a dark room?

**Solution of Exercise 49.2.**

It re-sends part of the received [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) in all directions. In a dark room it receives nothing, forwards nothing — and is invisible.

**Exercise 49.3 ★.**

State the two conditions of vision, and use them: why can you not see a perfectly clean [mirror](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection)’s *own* surface, only the things it reflects? (Careful — fine question; answer simply: what does its polish refuse to do with [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)?)

**Solution of Exercise 49.3.**

[Light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) must leave the object toward the eye. A perfect polish refuses to *diffuse*: it sends [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) on in one disciplined direction only, so no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) says “here is the surface” to every viewpoint — the eye receives only the re-directed pictures of other things.

**Exercise 49.4 ★.**

Why does everyone around the table see the same apple, while only one lucky eye position catches the lamp’s [reflection](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) in a spoon?

**Solution of Exercise 49.4.**

The apple diffuses to all directions at once — every eye gets a share. The spoon’s polish sends the lamp’s [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) in one direction only, and just one eye position sits on that single road.

**Exercise 49.5 ★.**

Why is a torch [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) invisible from the side in clean [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), and visible in mist? Name the process.

**Solution of Exercise 49.5.**

Clean [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) forwards almost nothing sideways, so no [beam-light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) enters an eye off the [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray)’s path. Mist fills the [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) with tiny diffusers, each scattering a share toward you: [diffusion](#def-g7-light-sources-propagation-diffusion) draws the line.

**Exercise 49.6 ★.**

Classify: window glass; tracing paper; a brick; shallow clear water; a frosted lightbulb’s envelope; aluminium foil.

**Solution of Exercise 49.6.**

[Transparent](#def-g7-light-sources-propagation-coats): window glass, shallow clear water. [Translucent](#def-g7-light-sources-propagation-coats): tracing paper, the frosted envelope. [Opaque](#def-g7-light-sources-propagation-coats): the brick, the foil.

**Exercise 49.7 ★.**

What job does translucency do in a bathroom window and in a lampshade? What passes, what is stopped?

**Solution of Exercise 49.7.**

Brightness passes; [images](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#prop-g5-mirrors-reflection-image) are stopped (scrambled). The bathroom window admits daylight and denies onlookers; the shade converts a blinding point-filament into a broad gentle glow.

**Exercise 49.8 ★.**

The [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) is sunlit — yet lunar astronauts saw a black sky at noon. Explain with [Remark 49.10](#rem-g7-light-sources-propagation-space), and say why Earth’s noon sky is bright blue instead.

**Solution of Exercise 49.8.**

The [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) has no [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air): nothing above the astronauts diffused the passing sunlight toward their eyes, so the sky offered nothing to see — black at noon. Earth’s [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) forwards a share of sunlight from every direction of the sky: our bright blue dome is [diffusion](#def-g7-light-sources-propagation-diffusion) overhead.

**Exercise 49.9 ★★.**

A single sheet of paper glows warmly in front of a torch; the closed book stops the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) entirely. What does thickness do to a material’s team membership? Give a second example of a team-changer.

**Solution of Exercise 49.9.**

Each added layer scrambles and steals a little more: teams are thickness-dependent. Water is the chapter’s second example — [transparent](#def-g7-light-sources-propagation-coats) by the glassful, [opaque](#def-g7-light-sources-propagation-coats) by the ocean. (Thin paper [translucent](#def-g7-light-sources-propagation-coats), the closed book [opaque](#def-g7-light-sources-propagation-coats).)

**Exercise 49.10 ★★.**

Cinema trick: projectors’ [beams](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) are invisible in a clean hall, yet films show them slicing dramatically through the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). What must the set crew add to the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), and what physics are they buying?

**Solution of Exercise 49.10.**

Something for the [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) to strike: theatrical haze or smoke — crowds of tiny diffusers that catch [beam-light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and scatter a share to every seat. They are buying [diffusion](#def-g7-light-sources-propagation-diffusion), the only way a sideways eye ever sees a [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray).

**Exercise 49.11 ★★.**

You see a friend’s face by a campfire. Trace the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)’s full journey — source, [diffusion](#def-g7-light-sources-propagation-diffusion), straight lines — and count how many diffusers stand in the chain if you then *also* see that face reflected in the lake.

**Solution of Exercise 49.11.**

Fire ([primary source](#def-g7-light-sources-propagation-primary)) $\to$ straight line to the face $\to$ the face diffuses $\to$ straight line to your eye: one diffuser in the chain. Via the lake: face-diffused [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) travels straight to the still water, is mirror-bounced (not diffused — the lake is polished), and continues straight to your eye: still one *diffuser*, plus one mirror-bounce — the face, then the lake’s disciplined fold.

**Exercise 49.12 ★★★.**

Nature’s own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) show: on a bright day, the sky away from the Sun is blue, the Sun’s disc blinding white — and at [sunset](https://one-course.com/books/physics/1/en/chapter/4-day-and-night-the-sun#def-g1-day-and-night-daynight) the low Sun turns orange while the clouds above catch pink. Using only “[air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) diffuses a little of the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) passing through it, and more so the longer the path”, explain why the low sun is robbed of part of its [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) — and who receives the stolen share. (The full color story is next year’s spectra chapter; reason here only with more-path, [more-diffusion](#def-g7-light-sources-propagation-diffusion).)

**Solution of Exercise 49.12.**

Sunlight crossing [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) loses a share to [diffusion](#def-g7-light-sources-propagation-diffusion) all along its path — the longer the path, the greater the theft. At [sunset](https://one-course.com/books/physics/1/en/chapter/4-day-and-night-the-sun#def-g1-day-and-night-daynight) the Sun’s [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) grazes the [atmosphere](https://one-course.com/books/physics/1/en/chapter/36-air-pressure-and-weather#def-g5-air-pressure-weather-pressure) along its longest road, so the direct [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) reaches you robbed and weakened — the tamed orange disc. The stolen share has not vanished: it was scattered along the way, delivered to other skies as their daylight — and some, scattered late, paints the high clouds pink. (Which colors are stolen soonest is next year’s story.)

## 49.5 Problem: Lighting the Museum

**Problem 49.1.**

Weekend problem — the new sculpture hall; a lighting designer’s night shift; sources, diffusers and three kinds of glass

The museum’s new hall opens Monday: one marble statue, paintings on the walls, a glass display case, and a nervous curator. You assist the lighting designer.

**Part I — First principles on site.**

1. The curator asks why the statue, “so brilliantly white”, still needs lamps at all. Answer with the two conditions of vision.
2. List the [primary sources](#def-g7-light-sources-propagation-primary) and the diffusers in the hall once the lamps are on: lamps, statue, paintings, visitors, the case’s glass.
3. The marble seems to glow from everywhere, with no blinding spot. Which manner of returning [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) is at work, and why does every visitor position receive it?
4. One polished brass plaque, by contrast, throws one dazzling [reflection](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) of a lamp. Name the other manner, and state the rule governing where that dazzle goes.

**Part II — Taming the glare.**

5. Bare [bulbs](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) glare in visitors’ eyes. The designer sheathes each in a frosted globe. Which material team is the globe on, and what does it do to the filament’s [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) ?
6. The paintings must be lit, but no lamp may appear *reflected* in their varnish from the main viewing spot. Using the [mirror](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) rule, where does the designer place the lamps relative to painting and viewer?
7. The display case’s clear glass is nearly invisible — until a visitor’s white sleeve appears in it like a ghost. Why does clear glass both transmit and faintly [mirror](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) ? What does the designer’s black-walled niche behind the case achieve?
8. Dust motes sometimes drift through the spotlights’ [beams](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) , drawing bright threads in the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) . Curator: “Beautiful! Keep it.” Designer: “It means dirty [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) .” Who is right about the physics, and what are the threads?

**Part III — The night test.**

9. [Lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) out, shutters sealed: the guard reports the hall “absolutely black — not even the white statue”. Explain with the two conditions why whiteness is helpless in the dark.
10. A skylight is proposed: frosted glass overhead. What will it deliver by day, and what will it refuse to deliver (views of clouds, direct sun-dazzle)?
11. The curator fears sunlight fading the paintings and asks for “ [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) without any sun”. The designer smiles: by day, even the north-facing windows’ soft [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) *is* sunshine. Explain — what has forwarded it?
12. Write the designer’s report: three sentences — one on why [diffusion](#def-g7-light-sources-propagation-diffusion) is the museum’s best friend, one on where mirror-manner [reflections](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) must be watched, and one on the team of glass chosen for each duty.

**Solution of Problem 49.1.**

**1.** Marble makes no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source): seeing needs [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) leaving the object into the eye, and an unlit diffuser sends nothing. White is a promise about *forwarding*, worthless without a delivery. **2.** [Primary sources](#def-g7-light-sources-propagation-primary): the lamps, alone. Diffusers: statue, paintings, visitors — and the case’s glass faintly (mostly it transmits). **3.** [Diffusion](#def-g7-light-sources-propagation-diffusion): marble’s surface scatters received [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) to all directions, so every position in the hall is served and none is dazzled. **4.** Mirror-manner [reflection](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection): the polished brass bounces the lamp’s [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) at equal angles into one direction — the dazzle lands only where that single road goes. **5.** [Translucent](#def-g7-light-sources-propagation-coats): the globe passes the brightness and scrambles the [image](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#prop-g5-mirrors-reflection-image) — the filament’s point-glare becomes a soft ball of [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). **6.** By the bounce rule, a lamp shows in the varnish when lamp and viewer sit at equal angles on either side of the painting’s surface. The designer places lamps high and steep, so their bounced road dives to the floor in front of the painting — never into the main viewing spot’s eyes. **7.** Glass mostly transmits but always mirror-bounces a faint share — clear windows are weak [mirrors](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection), seen against darkness behind. The black niche gives the transmitted view a dark ground so the faint [reflections](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#def-g5-mirrors-reflection-reflection) cannot compete — ghost sleeves fade. **8.** Both: physically the threads are [diffusion](#def-g7-light-sources-propagation-diffusion) by dust motes — [beam-light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) scattered to the visitors’ eyes; and dusty [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) is indeed what makes them possible. The physics is with the designer; the poetry with the curator. **9.** No source, no leaving [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), nothing to receive: whiteness is a high forwarding rate applied to nothing. Condition one fails for every object at once. **10.** Soft, even, shadowless daylight over the hall — brightness without [images](https://one-course.com/books/physics/1/en/chapter/32-mirrors-and-reflection#prop-g5-mirrors-reflection-image): no cloud views, and no direct sun-dazzle on the marble. **11.** The sky itself: [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) diffusing sunlight. North [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) is sunshine forwarded by a hemisphere of [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) — gentle, sourceless, and still solar; only the direct [beam](https://one-course.com/books/physics/1/en/chapter/42-rectilinear-propagation-of-light#def-g6-light-propagation-ray) has been declined. **12.** For example: “[Diffusion](#def-g7-light-sources-propagation-diffusion) is our best friend: it hands every artwork to every visitor without a single dazzle. Mirror-manner surfaces — varnish, brass, case glass — must be watched, for they deliver lamps to eyes along exact, predictable roads. And each glass serves by team: [transparent](#def-g7-light-sources-propagation-coats) for seeing in, [translucent](#def-g7-light-sources-propagation-coats) for glare and skylight, [opaque](#def-g7-light-sources-propagation-coats) for the dark.”