science

Photoacoustic effect

Photoacoustic effect is a science 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 Photoacoustic effect rather than just read about it. In short: The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (modulated light) or as a single flash (pulsed light).

Key takeaways

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

Reference excerpt

The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (modulated light) or as a single flash (pulsed light). The photoacoustic effect is quantified by measuring the formed sound (pressure changes) with appropriate detectors, such as microphones or piezoelectric sensors. The time variation of the electric output (current or voltage) from these detectors is the photoacoustic signal. These measurements are useful to determine certain properties of the studied sample. For example, in photoacoustic spectroscopy, the photoacoustic signal is used to obtain the actual absorption of light in either opaque or transparent objects. It is useful for substances in extremely low concentrations, because very strong pulses of light from a laser can be used to increase sensitivity and very narrow wavelengths can be used for specificity. Furthermore, photoacoustic measurements serve as a valuable research tool in the study of the heat evolved in photochemical reactions (see: photochemistry), particularly in the study of photosynthesis. Most generally, electromagnetic radiation of any kind can give rise to a photoacoustic effect. This includes the whole range of electromagnetic frequencies, from gamma radiation and X-rays to microwave and radio. Still, much of the reported research and applications, utilizing the photoacoustic effect, is concerned with the near ultraviolet/visible and infrared spectral regions.

History The discovery of the photoacoustic effect dates back to 1880, when Alexander Graham Bell was experimenting with long-distance sound transmission. Through his invention, called "photophone", he transmitted vocal signals by reflecting sun-light from a moving mirror to a selenium solar cell receiver. As a byproduct of this investigation, he observed that sound waves were produced directly from a solid sample when exposed to beam of sunlight that was rapidly interrupted with a rotating slotted wheel. He noticed that the resulting acoustic signal was dependent on the type of the material and correctly reasoned that the effect was caused by the absorbed light energy, which subsequently heats the sample. Later Bell showed that materials exposed to the non-visible (ultra-violet and infra-red) portions of the solar spectrum can also produce sounds and invented a device, which he called "spectrophone", to apply this effect for spectral identification of materials. Bell himself and later John Tyndall and Wilhelm Röntgen extended these experiments, demonstrating the same effect in liquids and gases. However, the results were too crude, dependent on ear detection, and this technique was soon abandoned. The application of the photoacoustic effect had to wait until the development of sensitive sensors and intense light sources. In 1938 Mark Leonidovitch Veingerov revived the interest in the photoacoustic effect, being able to use it in order to measure very small carbon dioxide concentration in nitrogen gas (as low as 0.2% in volume). Since then research and applications grew faster and wider, acquiring several fold more detection sensitivity. While the heating effect of the absorbed radiation was considered to be the prime cause of the photoacoustic effect, it was shown in 1978 that gas evolution resulting from a photochemical reaction can also cause a photoacoustic effect. Independently, considering the apparent anomalous behaviour of the photoacoustic signal from a plant leaf, which could not be explained solely by the heating effect of the exciting light, led to the cognition that photosynthetic oxygen evolution is normally a major contributor to the photoacoustic signal in this case.

Physical mechanisms

Photothermal mechanism Although much of the literature on the subject is concerned with just one mechanism, there are actually several different mechanisms that produce the photoacoustic effect. The primary universal mechanism is photothermal, based on the heating effect of the light and the consequent expansion of the light-absorbing material. In detail, the photothermal mechanism consists of the following stages:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Photoacoustic effect

Start with the simplest possible case. Write down what Photoacoustic effect claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Photoacoustic effect 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 Photoacoustic effect 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 Photoacoustic effect

In research: Photoacoustic effect appears in science 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 Photoacoustic effect 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: Photoacoustic effect is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acoustics, Medical diagnosis, Spectroscopy, so understanding it makes those chapters shorter.
In everyday life: Look for Photoacoustic effect 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Photoacoustic effect in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Photoacoustic effect 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 Photoacoustic effect out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photoacoustic effect in simple terms?

Why does Photoacoustic effect matter?

Because it connects several science 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 Photoacoustic effect?

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 Photoacoustic effect.