The Silver Halides

Three compounds do all the work. They contain the same metal, they are made the same way in the same kind of vessel, and they differ from each other by a factor of a million in how much dissolves and by tens of nanometres in what colour of light they can absorb. Those two differences decide which of them is coated on a film, which on an enlarging paper and which on a contact paper — and they decide how long you will stand at the fixer.

Making them: double decomposition

Two soluble salts are mixed; the ions change partners; one of the four possible combinations is insoluble and falls out. Kodak’s 1928 primer sets it out for bromide:

AgNO₃ + KBr → AgBr + KNO₃

Kodak, 1928: silver nitrate and potassium bromide

AgNO₃ + NaCl → AgCl + NaNO₃

The same reaction with a chloride: this is what salting paper does

AgNO₃ + KI → AgI + KNO₃

And with an iodide

The potassium or sodium nitrate stays dissolved and is washed out later; it never had a part to play. Written as a net ionic equation the spectators disappear and what is left is the reaction itself:

Ag⁺ + Br⁻ → AgBr

Net ionic: this is all that actually happens

Kodak adds the sentence that tells you what to expect in a test tube: if the solutions are at all concentrated, the silver bromide is thrown down as a thick, curdy precipitate. Abney had given the same equation in 1885 and noted that silver iodide is easily emulsified in gelatine but forms with much more difficulty in collodion — an early sign that the medium round the crystal is not a bystander.

Two rules of the mixing come from the same primer, and both matter as soon as you do it yourself. The silver nitrate goes in a little at a time, so the precipitate forms evenly rather than in lumps; dumping it in gives you a mass of large crystals where you wanted many small ones. And a material that is to be developed must be made with an excess of soluble halide and no free soluble silver left in it, because free silver ion meeting a developer reduces everywhere at once and fogs the whole surface. That is the first appearance of a rule this part keeps returning to: what is adsorbed on the surface of a crystal, and what is left dissolved around it, are as much a part of the material as its formula. Printing-out papers break the rule deliberately — Kodak’s Solio was made with an excess of silver nitrate — because a print-out image needs the free silver as a halogen acceptor and is never developed.

The solubility trend, and what each order of magnitude buys

Salt	Ksp at 25 °C	Dissolves to	Colour when fresh
Silver chloride, AgCl	1.6 × 10⁻¹⁰	1.93 mg/L	white
Silver bromide, AgBr	5.0 × 10⁻¹³	0.135 mg/L	yellowish, pale yellow
Silver iodide, AgI	1.5 × 10⁻¹⁶	0.0028 mg/L	light yellow

The solubility products are OpenStax’s Appendix J; the dissolved masses are the manufacturers’ and Merck Index figures on PubChem. Part III does the algebra that connects them and owns the common-ion effect; this page spends the result.

Down the column, each step costs you fixing time and buys you speed. Fixing is not dissolution in water — it is complex formation, which Part III also owns — but the solubility product is one of the two numbers multiplied together to get the combined constant, so a halide a thousand times less soluble is a halide the fixer has to work a thousand times harder against. That is why a chloride contact paper clears in under a minute, a bromide film wants several, and an iodobromide film wants longer still. It is also the reason a wet collodion plate, which is silver iodide, was fixed with cyanide rather than hypo.

Photography uses three of the four common halides. Silver fluoride is left out of this course entirely: it does not behave like the other three, and the course has read no source on it, so it states nothing about it beyond that fact.

Going the other way, the same trend is a lever an emulsion maker can pull. The most insoluble halide wins any competition for silver ion, which Ware states in exactly those terms: iodide added to a chloride paper converts the excess silver ions, and probably the unexposed chloride too, to silver iodide, “as a consequence of the insolubility of the iodide being relatively greater than that of the chloride”.

AgCl + I⁻ → AgI + Cl⁻

The more insoluble halide displaces the less

🧮How much silver bromide dissolves in a litre of pure water

Silver bromide dissolves as one Ag⁺ and one Br⁻, so if s is the molar solubility then Ksp = s².

s = √Ksp = √(5.0 × 10⁻¹³) = 7.1 × 10⁻⁷ mol/L

Molar solubility from a 1:1 solubility product

Multiply by the molar mass, 187.77 g/mol, and you get 1.3 × 10⁻⁴ g/L — 0.13 mg per litre, which is the figure PubChem gives from the other direction. Do the same for chloride and you get 1.8 mg/L; for iodide, 0.0028 mg/L. The ratio of dissolved silver ion from chloride to iodide is about a thousand to one, and the ratio of dissolved mass is about six hundred to one because iodine is heavier.

Colour, and what happens when light lands

The colour of the fresh precipitate is the first evidence of the absorption edge, and you can see it without instruments. A white solid absorbs almost nothing in the visible. A pale yellow one absorbs at the violet and blue end. So the order white, yellowish, light yellow is the order in which the three compounds start reaching into the visible spectrum — chloride least, iodide most.

Light that is absorbed does the same thing in all three. In its bookkeeping form:

2 AgX + light → 2 Ag + X₂

Net photolysis, where X is Cl, Br or I

That equation is true and tells you nothing useful, which is why Ware stresses that the reaction is reversible. The halogen it liberates is an oxidiser sitting next to freshly made silver, and if nothing takes it away it re-forms the halide. Irradiate a pure crystal and you do not get a black crystal: the yield saturates, the particles stop growing at about 10 nm, and the limiting darkening is an optical density of the order of 0.02 — barely perceptible. Something in the surroundings has to accept the halogen. That something is a halogen acceptor, and the latent image page owns it.

🔬Why 'sunned' silver chloride is violet, not black

Ware’s explanation is one of the neatest pieces of chemistry in the course. The silver made inside a chloride crystal is about 10 nm across, and its plasmon absorption band would sit near 390 to 400 nm if it were in water. It is not in water: it is embedded in silver chloride, whose refractive index of 2.071 drags the band out to about 550 nm — the wavelength at which the eye is most sensitive. So the material looks strongly violet. Dissolve the chloride away with a fixer and the band snaps back to 400 nm, where the eye barely responds, and the same silver now looks pale yellow. The particles have not changed; their neighbourhood has.

The lattice, and the defect that makes photography possible

An ionic solid is an alternating three-dimensional array, and OpenStax’s picture of sodium chloride — each ion surrounded by six of the opposite sign — is the one this course draws for silver bromide. The same passage states the property that ought to end this page: ionic solids are poor conductors of electricity, because the strength of the ionic bonds prevents the ions from moving freely in the solid state.

If that were the whole truth there would be no photograph. The mechanism of the latent image requires a silver ion, inside the crystal, to travel to a particular spot and be reduced there. Something must be moving.

The Frenkel defect

The escape is a defect. LibreTexts, following Housecroft, defines it: a Frenkel defect (also called a Frenkel pair, or Frenkel disorder) is a defect in which an atom or ion — normally the cation — leaves its own lattice site vacant and instead occupies a normally vacant interstitial site, between the other cations and anions. Two things are created at once: a mobile ion that is not where it should be, and a hole in the lattice where it used to be.

Three properties of that definition matter here.

• It is possible only if the cation is small compared with the anion, and the solids that show it have relatively open lattices and low coordination numbers.
• It is intrinsic: its existence lowers the Gibbs energy of the crystal, so a real crystal at a real temperature has some, without anybody adding anything.
• Its equilibrium number rises with temperature, through an exponential in the formation enthalpy over 2RT — so a warm emulsion has more of them than a cold one.

That last point has a consequence you will meet twice in this part and use for the rest of the course. If the population of mobile silver ions rises with temperature, then everything that depends on a silver ion arriving somewhere depends on temperature too: how efficiently a latent image builds during a long cold exposure, how well it survives in a warm loft, and how fast a crystal ripens in a hot kettle. Part V turns the last of those into a procedure, and the next two pages of this part turn the first two into reciprocity failure and latent-image fading. None of them is a separate phenomenon; they are the same defect chemistry seen at three different times.

A Frenkel pair in a silver halide lattice

1. 1Silver ion on its lattice site — held by six halide neighbours in three dimensions
2. 2Halide ion — the larger of the two, and the one that stays put
3. 3Interstitial silver ion — sitting in a gap, not on a site; free to hop from gap to gap
4. 4The vacancy it left — a silver-sized hole that other silver ions can move into

Two dimensions of a three-dimensional lattice, drawn and not to scale. The definition of the defect is LibreTexts'; that the interstitial silver ion is what carries silver to a trapped electron is Ware's account of Gurney and Mott.

!What this course can and cannot tell you about the silver halide lattice

Three separate honesty notes, because this is the section where it would be easiest to sound more certain than the evidence allows.

The structure. This course draws silver chloride and silver bromide as a rock-salt lattice, the sodium chloride arrangement with six neighbours per ion, because that is the structure the emulsion literature assumes and because Kodak’s own patent describes the silver halide lattice structure as cubic with (1,0,0) and (1,1,1) faces. It has not read a crystallography source giving the structure type or a lattice constant, and quotes neither. Silver iodide is described in the literature as having a different structure again; the course has not verified which, and says nothing more about it than that.

The numbers. LibreTexts gives the form of the equation for how many Frenkel defects a crystal carries but no values, and it names no compound at all — in particular it says nothing about the silver halides. So this course states that silver halide crystals carry mobile interstitial silver ions, which every account of the latent image requires and which Ware states directly, and it gives no defect concentration, no formation enthalpy and no ionic conductivity, because it has read none.

The bonding. Part III deliberately left open how ionic the silver halides really are, and this page cannot close it either: no solid-state source has been read. What it can now say is why the question was worth asking. A textbook-perfect ionic solid is an insulator, and a silver halide crystal is not — it moves silver ions through itself, and it releases an electron when a photon lands. Both facts push it away from the textbook picture. The course teaches the ionic lattice with mobile cations, marks the bonding question open, and moves on, because nothing later in the course depends on the answer.

The absorption edge, and the honest state of the numbers

A material with an absorption threshold ignores every photon below that energy, however many arrive. Hamamatsu’s detector note gives the conversion in the compact form Part I already used:

λ = 1240 / E

Threshold wavelength from threshold energy

where λ is the longest wavelength that still works, in nanometres, E is the threshold energy in electronvolts, and 1240 is Planck’s constant times the speed of light in those units. Run it the other way and a 420 nm photon carries 2.95 eV; a 500 nm photon, 2.48 eV; a 550 nm photon, 2.25 eV.

This course does not quote a band gap for any silver halide. It has not read a solid-state source that gives one, and a plausible-sounding number in electronvolts is exactly the kind of thing the first rule of this course forbids. What it has read are statements about where sensitivity stops, made by people who measured it, and those are what the diagram below shows.

Where undyed silver halides stop responding, as the sources state it

• Silver chloride, pure (320–420 nm) — Ware: ultraviolet and violet, negligible above about 420 nm
• Silver iodide, pure (320–490 nm) — Abney 1885: ultraviolet, violet and blue only
• Silver bromide, ruby modification (320–520 nm) — Abney 1885: ultraviolet, violet, blue, a little green
• Silver bromide, grey-blue modification (320–660 nm) — Abney 1885: as above but stronger in green, and reached by yellow and red

Drawn from verbal statements, not from measured curves, and marked as such: none of these four edges is a published spectral sensitivity. The point of the diagram is the spread — the same compound appears twice, in two crystal states, with different ranges. The coloured strip approximates where the visible spectrum falls and is a reading aid only; the wavelengths in the labels carry the information. The bands and curves are drawn to show the relationship, not measured.

⁇Does an undyed silver bromide emulsion see red?

The modern working rule, which this course teaches and which safelight practice depends on, is that undyed silver halides respond only to blue and ultraviolet. Kodak’s safelight guide rests on it: ideally a safelight filter transmits only light lying outside the colour-sensitivity range of the material it is recommended for. But the same paragraph adds the qualification this section needs, and it is Kodak’s own: “the color sensitivity of most emulsions does not end abruptly at a particular wavelength in the spectrum — most emulsions are somewhat sensitive to colors outside the intended range”, so most papers and films have some sensitivity even to the light their own recommended safelight transmits. That is why a safelight recommendation carries a wattage, a distance and a time limit rather than a promise, and why the darkroom session at the end of this part treats the safelight as something to be tested rather than trusted.

Abney, in 1885, reported otherwise, and he was measuring rather than speculating. He describes silver bromide as existing in several molecular states — identical in composition, differing in particle size, “built up of a greater or less number of primary molecules, according to the rapidity of the formation of the solid, and its subsequent treatment by heat, ammonia, &c.” — which transmit ruby, orange, green, purple or grey light. He then reports that their spectral ranges differ: the ruby and orange state reaches the blue and a little of the green, but the grey-blue state is “able to be impressed by the yellow and by the red rays”, and the green state in collodion is sensitive “much more so to the red, and infra-red regions”.

The two positions are not reconciled on this page, and the course does not pretend they are. What it does is this: it uses the modern rule as the working rule, on Kodak’s own terms and with Kodak’s own caveat attached; and it takes Abney’s observation as evidence for something this part teaches anyway — that the crystal’s state, not just its formula, sets what it can absorb. Ware supplies an independent reason for caution in the same direction: a real print-out paper behind a filter cutting everything below 470 nm still fogged at half the rate, because the print-out silver absorbs visible light itself and passes the energy on. The spectral sensitivity page later in this part is where colour response is properly settled, and it starts from this unfinished argument.

Crystal habit: why the shape is not the lattice

A cubic lattice does not oblige a crystal to be a cube. Which faces a growing crystal shows depends on which faces grow slowest, and that depends on the solution around it. Kodak’s own double-jet patent is unusually blunt about it: silver bromide or chloride is ordinarily prepared as fine crystals with a cubic lattice structure, but the faces of these crystals “are generally (1,1,1)” — the octahedral faces — and grains presenting cubic faces “are not common and do not result from the usual conventional methods of making silver halide”.

Cubic and octahedral habit from the same lattice

1. 1Cubic habit: (1,0,0) faces — uncommon; the patent gets it by holding pAg between 8.6 and 9.2 at pH 4.0 or below
2. 2Octahedral habit: (1,1,1) faces — what ordinary precipitation gives, in the patent's own words
3. 3pAg, the silver ion concentration of the solution — the variable that switches between them, after Berry and Skillman 1962

Habit, not size, and drawn rather than measured. The same lattice underlies both shapes; only which faces survive the growth is different.

The patent’s own recipe holds the pAg between 8.6 and 9.2 and the pH at 4.0 or below, running silver nitrate and halide simultaneously into rapidly stirred gelatin, and gets cubic-regular grains of about 0.2 micrometres. It also gives the rule of thumb that matters for the experiment two pages from here: a large excess of halide ion is avoided if the grains are to be regular.

Two other pieces of that sentence are worth naming now, because Part V builds on them. The gelatin the salts are run into is called a peptiser — it keeps the crystals dispersed as they form instead of letting them clump and settle — and the gelatin added after washing is called a protective colloid. Ware supplies the other half of the picture: a precipitated silver halide is not stoichiometric, and what is adsorbed on its surface depends on the solution it sits in. With silver ion in excess the crystal carries adsorbed Ag⁺; with halide in excess it carries adsorbed halide. That is the same variable as pAg, seen from the crystal’s side.

Iodide in a bromide crystal

Real film is not silver bromide. Kodak’s primer describes negative materials as silver bromide “with a small addition of silver iodide”, and every practical account since agrees that the addition is small. How small, and why not more?

The why is well sourced. Abney ran the comparison in 1885 and published both sides of the argument with Eder: paired gelatine emulsions, one plain and one carrying about 10 grains of potassium iodide against 93 grains of potassium bromide. The iodide plates were “a little slower in coming out; but, on the other hand, they were certainly much brighter and cleaner”, and the ammonia-boiled emulsion without iodide fogged while the one with iodide “remained quite bright”. His conclusion was that except for experimental purposes he never omitted it, “believing it to be a sheet-anchor for obtaining good and unfogged pictures”. A little iodide costs a little speed and buys clean highlights.

The how much is a formulation figure rather than a physical limit, and the course is careful to say so. Davey and Knott’s 1952 Kodak patent for an internal-latent-image emulsion states that the silver iodide content “should preferably be at least 6% of the total amount of silver halide” and is best brought to 10 to 20 per cent of the total halide.

!The number this page does not give

The physical question — how much iodide the silver bromide lattice will accept as a solid solution before a separate silver iodide phase appears — is a real one with a real answer, and this course has not found it in a source it has read. It therefore quotes no percentage as the lattice limit. What it can say is that a limit exists, that it is above the 10 to 20 per cent Kodak’s patent calls best practice, and that nothing in this course requires the number. Where a page needs a figure and has none, it says so rather than borrowing one that sounds right.

Which halide for which job

The three halides and the materials they became

1. 1Chloride — contact papers, exposed by artificial light; fastest to fix, slowest to expose
2. 2Chlorobromide — development papers, including the gaslight papers; warm or cold tones at will
3. 3Iodobromide — films and bromide enlarging papers; the fast end, and the slowest to fix

Assembled from Kodak's 1928 primer and Wall's 1912 dictionary; the arrangement is the course's, the claims are theirs.

Notice what is not in that diagram: speed. Kodak’s primer is explicit that the halide does not set the speed on its own — “the different degrees of sensitiveness are obtained by varying the temperature and the duration of heating which the emulsions undergo during manufacture, the most sensitive emulsions being heated to higher temperatures and for a longer time than the slower emulsions.” That heating is ripening, and it works by growing the crystals and by building sensitivity centres on them. So the halide sets the range a material can work in and the ripening sets where inside that range it lands, which is why a slow contact paper and a fast plate could be made from the same two salts. Part V is where you do the ripening yourself.

Kodak’s primer gives the two ends: bromide paper is a slow bromide emulsion on paper, used for printing and especially for enlargements, while “the less sensitive papers which are commonly used for contact printing by artificial light contain silver chloride in the place of silver bromide”. Wall’s 1912 dictionary supplies the middle. Under Alpha Paper he records one of the first commercial papers coated with a chlorobromide emulsion, suggested by Eder in 1883, “intended for development, and not printing out”, and singles out the property that keeps chlorobromide papers in the catalogue to this day: warm or cold tones can be obtained at will. Under Gaslight Paper he describes a very slow gelatino-bromide or chlorobromide emulsion whose whole point was that it could be handled at eight to ten feet from a gas flame without a darkroom.

📷The photography

That last sentence is the practical shape of everything on this page. A material’s halide sets its speed; its speed sets what light you can work under; and what light you can work under sets whether you need a darkroom or a dim corner. When you choose a paper in Part XII you are choosing a point on this scale, and the box will not tell you which halide is in it — but the contrast, the tone and the safelight recommendation will.

Recap

• Double decomposition makes all three: a soluble silver salt plus a soluble halide gives an insoluble silver halide and a spectator nitrate. Concentrated solutions give a thick curdy precipitate.
• Solubility products run 1.6 × 10⁻¹⁰, 5.0 × 10⁻¹³, 1.5 × 10⁻¹⁶ for chloride, bromide and iodide. Down that column, fixing gets harder; the most insoluble halide displaces the others from a crystal.
• Colour is the first evidence of the absorption edge: white, yellowish, light yellow.
• Photolysis is reversible. Without a halogen acceptor a pure crystal saturates at an optical density of about 0.02.
• The Frenkel defect is the pivot. An interstitial silver ion and the vacancy it left make an ionic solid conduct silver through itself, which is what lets the lattice deliver silver to a trapped electron.
• No band gap is quoted here, because none has been verified; the sensitivity limits that are sourced disagree with each other in an interesting way, and the spectral sensitivity page picks that up.
• Habit is set by the solution, not the lattice. Ordinary precipitation gives octahedral (1,1,1) faces; holding the pAg between 8.6 and 9.2 gives cubes.
• Chloride for contact papers, chlorobromide for enlarging papers, iodobromide for film — and a few per cent of iodide buys clean, unfogged highlights at a small cost in speed.

Check your understanding

Question 1. Rank silver chloride, silver bromide and silver iodide by how quickly a plain hypo fixer will clear them, fastest first, and say what the ranking follows from.

• Iodide, bromide, chloride — following the order of their molar masses
• Chloride, bromide, iodide — following the order of their solubility products
• Bromide, chloride, iodide — following the order of their colours
• All three clear at the same rate, because fixing is complex formation and not dissolution

Show the answer and why

Answer: Chloride, bromide, iodide — following the order of their solubility products

Fixing is complex formation, but the overall equilibrium constant is the solubility product multiplied by the formation constant, so Ksp is one of the two numbers that decide it. With formation constants the same for all three, the order of Ksp — 1.6 × 10⁻¹⁰, 5.0 × 10⁻¹³, 1.5 × 10⁻¹⁶ — is the order of clearing. That is a thousandfold spread from chloride to iodide, and it is why a wet collodion plate, which is silver iodide, was historically fixed in cyanide rather than hypo.

Question 2. A perfect ionic crystal is an electrical insulator, because its ions cannot move. Why is a silver bromide crystal not one?

• Because the silver-bromine bond is largely covalent, so electrons are shared and can move
• Because it carries Frenkel defects: some silver ions sit in interstitial gaps rather than on lattice sites, and can hop between gaps
• Because the gelatin surrounding it conducts
• Because light is always falling on it, and light makes it conduct

Show the answer and why

Answer: Because it carries Frenkel defects: some silver ions sit in interstitial gaps rather than on lattice sites, and can hop between gaps

A Frenkel defect is a cation that has left its own site vacant and taken up an interstitial position; both the interstitial ion and the vacancy can move, so charge can move. The defects are intrinsic — their existence lowers the crystal's Gibbs energy — and their number rises with temperature. The first option is the tempting one and the course explicitly does not claim it: how covalent the silver halides are is a question it has left open for want of a solid-state source, and the ionic conductivity does not need that answer.

Question 3. An emulsion maker adds a few per cent of potassium iodide to a bromide emulsion. Why do it at all, and why not add fifty per cent?

• It raises speed; more would raise it further but cost too much
• It gives cleaner, less fogged highlights at a small cost in speed; a large amount would make the emulsion far harder to fix and is beyond what the bromide lattice takes as a solid solution
• It shifts the absorption edge into the red; more would shift it into the infrared
• It hardens the gelatin; more would make the emulsion brittle

Show the answer and why

Answer: It gives cleaner, less fogged highlights at a small cost in speed; a large amount would make the emulsion far harder to fix and is beyond what the bromide lattice takes as a solid solution

Abney's 1885 comparison is the evidence for the first half: the iodide plates were "a little slower in coming out" but "much brighter and cleaner", and the ammonia-boiled emulsion without iodide fogged while the one with iodide stayed clear. The second half has two parts — silver iodide is about three thousand times less soluble than silver bromide, so an iodide-rich emulsion is punishing to fix, and there is a limit to how much iodide the bromide lattice will hold in solid solution. This course does not quote that limit, because it has not read a source that gives it; Kodak's own patent calls 10 to 20 per cent of the total halide best practice, which is well short of fifty.

Question 4. Two silver bromide emulsions are precipitated from identical solutions, but one is stirred into gelatin held at a pAg of 8.8 and the other at a much higher halide concentration. What is most likely to differ?

• The chemical composition of the crystals
• The crystal habit — which faces the grains show — and how regular they are
• The solubility product of silver bromide
• The colour of the fixer after processing

Show the answer and why

Answer: The crystal habit — which faces the grains show — and how regular they are

Composition and solubility product are properties of the compound and do not move. What moves is habit: Kodak's double-jet patent states that a change in pAg alone lets the same precipitation give regular cubes or octahedra, and that a large excess of halide ion has to be avoided if the grains are to be regular at all. Ware adds the reason from the crystal's side — a precipitated silver halide is non-stoichiometric, and whether it carries adsorbed silver ions or adsorbed halide ions depends on the solution it grew in.

Question 5. A page states that silver bromide has a band gap of 2.7 eV, so it cannot absorb light beyond 459 nm. What is wrong with using that sentence in this course?

• Nothing; the arithmetic 1240 ÷ 2.7 = 459 is correct
• The arithmetic is right but the course has verified no band gap for any silver halide from a source it has read, so the number would be borrowed rather than sourced
• Band gaps do not apply to ionic solids
• The relation λ = 1240 / E is only valid for silicon

Show the answer and why

Answer: The arithmetic is right but the course has verified no band gap for any silver halide from a source it has read, so the number would be borrowed rather than sourced

The arithmetic is fine and the relation is general — Hamamatsu states it for a detector, and the physics does not care which material. The problem is provenance. A number that sounds right, repeated from memory, is exactly what this course's first rule exists to stop, and a band gap is a measured quantity that belongs to a solid-state source. What the course does instead is quote the sensitivity limits its sources actually state, note that they disagree with one another, and say plainly that no band gap is being given.

Question 6. Why does a red safelight let you handle enlarging paper but not panchromatic film?

• Because paper emulsions are thinner than film emulsions
• Because the paper's undyed silver halide has an absorption edge in the blue and cannot use red light, while panchromatic film has been dye-sensitised to respond right across the visible
• Because the paper is coated on a white base that reflects red light away
• Because film is faster, and speed alone decides safelight tolerance

Show the answer and why

Answer: Because the paper's undyed silver halide has an absorption edge in the blue and cannot use red light, while panchromatic film has been dye-sensitised to respond right across the visible

It is a spectral argument, not a speed argument, although speed decides how long you get. The halide itself responds only at the short-wavelength end — Ware puts pure silver chloride at negligible above about 420 nm — so red light simply is not absorbed. A panchromatic film has had dyes adsorbed onto its crystals precisely to make it absorb where the halide cannot, and that removes the gap the safelight lives in. Note the limit of the argument: safelight recommendations still carry a distance and a time, because no material is entirely blind to anything, and because print-out silver in an already-exposed sheet absorbs visible light itself.

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