physics

Wood's glass

Wood's glass is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wood's glass rather than just read about it. In short: Wood's glass is an optical filter glass invented in 1903 by American physicist Robert Williams Wood (1868–1955), which allows ultraviolet and infrared light to pass through, while blocking most visible light. History Wood's glass was developed as a light filter used in communications during World War I.

Key takeaways

Wood's glass belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wood's glass to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wood's glass from memory before moving on to harder problems.

Reference excerpt

History Wood's glass was developed as a light filter used in communications during World War I. The glass filter worked both in infrared daylight communication and ultraviolet night communications by removing the visible components of a light beam, leaving only the "invisible radiation" as a signal beam. Wood's glass was commonly used to form the envelope for fluorescent and incandescent ultraviolet bulbs ("black lights"). In recent years, due to its disadvantages, other filter materials have largely replaced it.

Composition Wood's glass is special barium-sodium-silicate glass incorporating about 9% nickel oxide. It is a very deep violet-blue glass, opaque to all visible light rays except longest red and shortest violet. It is quite transparent in the violet/ultraviolet in a band between 320 and 400 nanometres with a peak at 365 nanometres, and a fairly broad range of infrared and the longest, least visible red wavelengths.

Properties and uses Wood's glass has lower mechanical strength and higher thermal expansion than commonly used glasses, making it more vulnerable to thermal shocks and mechanical damage. The nickel and barium oxides are also chemically reactive, with tendency to slowly form a layer of hydroxides and carbonates in contact with atmospheric moisture and carbon dioxide. Most contemporary "black-light" bulbs are made of structurally more suitable glass with only a layer of a UV-filtering enamel on its surface; such bulbs, however, pass much more visible light, appearing brighter to the eye. Due to manufacturing difficulties, Wood's glass is now more commonly used in standalone flat or dome-shaped filters, instead of being the material of the light bulb. With prolonged exposure to ultraviolet radiation, Wood's glass undergoes solarization, gradually losing transparency for UV. Photographic filters for ultraviolet photography, notably the Kodak Wratten 18A and 18B, are based on Wood's glass.

Health effects Bulbs made of Wood's glass are potentially hazardous in comparison with those made of enameled glass, since the reduced visible light output may cause observers to be exposed to unsafe levels of UV, because the source appears dim. The low output of black lights is not considered sufficient to cause DNA damage or cellular mutations, but excessive exposure to UV can cause temporary or permanent damage to the eye.

See also Black light Dichroic filter Wood's lamp

References

Further reading Wood, R. W. (1919). "Secret communications concerning light rays". Journal of Physiology. 5e (IX). Margarot, J.; Deveze, P. (1925). "Aspect de quelques dermatoses lumiere ultraparaviolette. Note preliminaire". Bulletin de la Société des sciences médicales et biologiques de Montpellier (in French). 6: 375–378. Williams, Robin, Prof.; Williams, Gigi. "Prof. Robert Williams Wood". Pioneers of invisible radiation photography. Archived from the original on 2011-04-07.{{cite web}}: CS1 maint: multiple names: authors list (link)

Worked examples

Example 1 — a first encounter with Wood's glass

Start with the simplest possible case. Write down what Wood's glass claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wood's glass before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wood's glass ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wood's glass

In research: Wood's glass appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wood's glass in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wood's glass is common in secondary-school and first-year university syllabi. It links to neighbouring topics American inventions, Glass compositions, Glass trademarks and brands, so understanding it makes those chapters shorter.
In everyday life: Look for Wood's glass outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wood's glass in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wood's glass means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wood's glass out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wood's glass in simple terms?

Wood's glass is an optical filter glass invented in 1903 by American physicist Robert Williams Wood (1868–1955), which allows ultraviolet and infrared light to pass through, while blocking most visible light. History Wood's glass was developed as a light filter used in communications during World W…

Why does Wood's glass matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wood's glass?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wood's glass.