Emission and Absorption Spectra

This lesson covers emission and absorption spectra as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand how hot and cool gases produce different types of spectra, why each element has a unique spectrum, and how spectra are used to identify elements — including in stars.

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Types of Spectra

There are three main types of spectra that you need to know:

1. Continuous Spectrum

A continuous spectrum contains all wavelengths (all colours) with no gaps. It appears as a smooth rainbow of colours from red to violet.

• Produced by hot solid objects (e.g. a filament lamp, the Sun's surface).
• Also produced by hot liquids and very dense hot gases.
• The colours blend smoothly into one another with no dark lines or bright lines.

2. Emission Line Spectrum

An emission line spectrum consists of bright coloured lines on a dark (black) background. Each line corresponds to a specific wavelength of light.

• Produced by hot gases at low pressure.
• When the gas is heated (or has electrical energy passed through it), the atoms become excited — electrons absorb energy and move to higher energy levels.
• When the electrons fall back to lower energy levels, they emit light of a specific wavelength (specific colour).
• Each element produces a unique set of lines — like a fingerprint.

3. Absorption Line Spectrum

An absorption line spectrum consists of dark lines (gaps) on a continuous (rainbow) background. The dark lines appear at the same wavelengths as the bright lines in the emission spectrum of the same element.

• Produced when white light (a continuous spectrum) passes through a cool gas.
• The cool gas absorbs light at specific wavelengths — the same wavelengths it would emit if heated.
• These absorbed wavelengths appear as dark lines in the continuous spectrum.

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Comparing the Three Types of Spectra

Type	Appearance	Produced By
Continuous	Smooth rainbow, all colours, no gaps	Hot solid, liquid, or dense gas
Emission line	Bright coloured lines on a dark background	Hot gas at low pressure
Absorption line	Dark lines on a continuous rainbow background	White light passed through a cool gas

flowchart TD
    A["Continuous Spectrum<br/>All colours, smooth rainbow<br/>No gaps"] --> D["Produced by hot solids<br/>and liquids"]
    B["Emission Line Spectrum<br/>Bright lines on dark background"] --> E["Produced by hot gas<br/>at low pressure"]
    C["Absorption Line Spectrum<br/>Dark lines on continuous background"] --> F["Produced by white light<br/>passing through cool gas"]

    style A fill:#f39c12,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#2c3e50,color:#fff

Exam Tip: The key word difference: emission spectra have bright lines on a dark background; absorption spectra have dark lines on a bright (continuous) background. An easy way to remember: emission = bright lines emitted; absorption = wavelengths absorbed (dark gaps).

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Why Each Element Has a Unique Spectrum

Every element has a unique set of energy levels for its electrons. When electrons transition between these energy levels, they emit or absorb photons of light with very specific energies (and therefore specific wavelengths/colours).

Since no two elements have exactly the same set of energy levels, no two elements produce exactly the same pattern of spectral lines. This means:

• Each element's spectrum is like a fingerprint — it is unique and can be used to identify the element.
• If you observe a spectrum and match its line pattern to a known element, you can confirm that element is present.

Energy Level Transitions

When an electron drops from a higher energy level to a lower one, it emits a photon of light. The wavelength of this photon depends on the energy difference between the two levels:

• A large energy drop → a photon with a short wavelength (towards the blue/violet end).
• A small energy drop → a photon with a long wavelength (towards the red end).

Exam Tip: You do not need to calculate energy level transitions at GCSE, but you should understand the principle: each line in a spectrum corresponds to a specific electron transition, and the pattern of lines is unique to each element.

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Applications of Spectra

1. Identifying Elements in Stars

We cannot travel to stars to analyse their composition, but we can analyse the light they emit. Here is how:

1. Light from a star passes through the star's outer atmosphere (which is cooler than the core).
2. The cool gases in the outer atmosphere absorb light at specific wavelengths.
3. This produces an absorption line spectrum with dark lines.
4. By matching the pattern of dark lines to the known spectra of elements (measured on Earth), astronomers can identify which elements are present in the star.

This technique is called spectroscopy, and it has been used to identify elements such as hydrogen, helium, sodium, calcium, and iron in stars.

Exam Tip: Helium was actually discovered in the Sun before it was found on Earth — its name comes from "Helios," the Greek word for the Sun. Astronomers identified it by its unique spectral lines in sunlight.

2. Chemical Analysis

Emission and absorption spectra are used in chemistry to identify unknown elements or compounds in a sample. This is known as spectral analysis or spectroscopy.

• A sample is heated (or light is passed through it) and the resulting spectrum is recorded.
• The pattern of lines is compared to a database of known spectra.
• This allows identification of elements even in very small quantities.

3. Forensic Science

Spectral analysis can be used in forensic investigations to identify substances found at crime scenes — for example, identifying the composition of paint, drugs, or fibres.

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Red-Shift and the Expanding Universe

What Is Red-Shift?

When astronomers observe light from distant galaxies, they find that the absorption lines in the spectrum are shifted towards the red end of the spectrum compared to where they would normally be. This is called red-shift.

What Causes Red-Shift?

Red-shift occurs because the galaxy is moving away from us. As the galaxy recedes, the wavelength of the light it emits is stretched (increased), shifting it towards the red end of the spectrum.

This is similar to the Doppler effect for sound — when an ambulance moves away from you, its siren sounds lower-pitched because the sound waves are stretched.

Key Observations

• All distant galaxies show red-shift — they are all moving away from us.
• The further away a galaxy is, the greater its red-shift — it is moving away faster.
• This relationship was discovered by Edwin Hubble and is known as Hubble's Law.

What Does Red-Shift Tell Us?

• If all galaxies are moving away from us (and from each other), the universe is expanding.
• If we "rewind" this expansion, all matter was once concentrated in a single point — this supports the Big Bang theory.
• The Big Bang theory states that the universe began from an extremely hot, dense point approximately 13.8 billion years ago and has been expanding ever since.

flowchart TD
    A["Light from distant galaxies<br/>shows RED-SHIFT"] --> B["Absorption lines shifted<br/>towards red end of spectrum"]
    B --> C["Galaxies are moving<br/>AWAY from us"]
    C --> D["Further away = greater red-shift<br/>(Hubble’s Law)"]
    D --> E["The UNIVERSE IS EXPANDING"]
    E --> F["Supports the<br/>BIG BANG THEORY"]

    style A fill:#e74c3c,color:#fff
    style E fill:#2c3e50,color:#fff
    style F fill:#8e44ad,color:#fff

Exam Tip: Red-shift questions are common on the Edexcel paper. Make sure you can explain the chain: red-shift is observed → galaxies are moving away → universe is expanding → supports the Big Bang. Do not just state "the universe is expanding" — explain how we know (from red-shift of absorption lines).

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Blue-Shift

Blue-shift is the opposite of red-shift — it occurs when an object is moving towards us. The wavelength of light is compressed (decreased), shifting it towards the blue end of the spectrum.