What is Absorption Spectrum?

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Grade Level:

Class 12

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Definition

What is it?

An absorption spectrum is like a unique 'fingerprint' of a substance, showing which specific colours (or wavelengths) of light it takes in or 'absorbs'. When white light passes through a substance, some colours get absorbed, and the remaining light shows dark lines where those colours used to be.

Simple Example

Quick Example

Imagine you have a red dupatta. When sunlight (which has all colours) falls on it, the dupatta absorbs all colours except red, and reflects only red light to your eyes. So, the 'absorption spectrum' of the red dupatta would show that it absorbs blue, green, yellow, etc., but not red.

Worked Example

Step-by-Step

Let's say we have a gas in a tube and we shine white light through it.

1. White light, which contains all colours from violet to red, is passed through a sample of hydrogen gas.
---2. The hydrogen atoms in the gas absorb specific amounts of energy, which correspond to specific wavelengths (colours) of light.
---3. For example, hydrogen atoms absorb light at 656 nm (red), 486 nm (blue-green), 434 nm (violet), and 410 nm (violet).
---4. When the light that has passed through the hydrogen gas is then spread out by a prism or grating, we see a continuous spectrum of colours, but with dark lines at precisely 656 nm, 486 nm, 434 nm, and 410 nm.
---5. These dark lines represent the light that was absorbed by the hydrogen gas.
---Answer: The pattern of these dark lines is the absorption spectrum of hydrogen.

Why It Matters

Understanding absorption spectra helps scientists identify unknown substances, just like a detective uses fingerprints! This is crucial in fields like medicine for diagnosing diseases, in environmental science to detect pollutants in air or water, and even in space technology to find out what stars and planets are made of.

Common Mistakes

MISTAKE: Confusing absorption spectrum with emission spectrum. | CORRECTION: An absorption spectrum shows dark lines where light is absorbed. An emission spectrum shows bright lines where light is given out (emitted) by an excited substance.

MISTAKE: Thinking that a substance absorbs all colours of light equally. | CORRECTION: Every substance has a unique absorption spectrum because its atoms or molecules can only absorb specific, characteristic wavelengths of light.

MISTAKE: Believing that the absorbed light simply disappears. | CORRECTION: The absorbed light energy causes electrons in the atoms or molecules to jump to higher energy levels. This energy is later released, often as heat or light of a different wavelength (fluorescence/phosphorescence).

Practice Questions

Try It Yourself

QUESTION: If a solution appears blue, what colours of light is it most likely absorbing? | ANSWER: It is most likely absorbing red and yellow light, as blue light is being transmitted or reflected.

QUESTION: Why is an absorption spectrum called a 'fingerprint' of a substance? | ANSWER: Because each element or compound has a unique arrangement of electrons, it absorbs a unique set of wavelengths of light, creating a distinct pattern of dark lines that helps identify it, just like a human fingerprint.

QUESTION: A gas sample shows dark lines at 589 nm and 590 nm in its absorption spectrum. What common element is likely present in this gas? (Hint: Think about streetlights!) | ANSWER: Sodium (Na), as it strongly absorbs light at these two wavelengths (the sodium D-lines), which is why sodium vapour lamps emit a distinct yellow light.

MCQ

Quick Quiz

What is observed when white light passes through a cool gas and is then analyzed by a spectrometer?

A continuous spectrum with bright lines.

A continuous spectrum with dark lines at specific wavelengths.

Only bright lines on a dark background.

No light is observed.

The Correct Answer Is:

When white light passes through a cool gas, the gas absorbs specific wavelengths, creating dark lines on the continuous spectrum. Options A and C describe emission spectra, and D is incorrect as some light passes through.

Real World Connection

In the Real World

In India, absorption spectroscopy is used in many labs. For example, in a food testing lab, scientists use instruments to analyze the absorption spectrum of a food sample to detect impurities or check its nutritional content. In hospitals, blood tests might use this technique to measure levels of certain compounds, helping doctors diagnose conditions.

Key Vocabulary

Key Terms

WAVELENGTH: The distance between two consecutive peaks or troughs of a wave, determining the colour of light. | SPECTRUM: The range of different colours or wavelengths of electromagnetic radiation. | ABSORPTION: The process by which matter takes up light energy. | SPECTROMETER: An instrument used to measure the properties of light over a specific portion of the electromagnetic spectrum.

What's Next

What to Learn Next

Now that you understand absorption spectra, you should explore 'What is an Emission Spectrum?'. It's the opposite but equally fascinating concept, showing how substances release light, and together they give a complete picture of how matter interacts with light!