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WHY ARE EMISSION SPECTRA USEFUL TO SCIENTISTS?

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WHY ARE EMISSION SPECTRA USEFUL TO SCIENTISTS?

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Emission lines are unique to each element.

So they are used to identify elements and substances.

A line spectrum is evidence of quantization of energy levels.

Showing that electrons exist in discrete energy levels.

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Atomic emission and absorption spectra are analysed to provide evidence of the existence of electron energy levels within atoms, and to identify elements. Today astronomers analyse the light from distant stars using telescopes to further understand what elements make up different stars. The Southern African Large Telescope (situated in the Karoo in South Africa) is one such telescope which is used to analyse the light from distant galaxies.

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EMISSION SPECTRA = The Signature of the Stars

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EMISSION SPECTRA = The Signature of the Stars

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By looking at a star’s spectrum, scientists can figure out what elements it is made of.

Like the star’s ID card.

The spectrum is called an element’s “fingerprint” — it’s a unique clue that tells us who the star is.

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Light is like a box of crayons, each a different colour 

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Light is like a box of crayons, each a different colour 

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Kirchhoff’s laws tell us how different objects, like stars or gases, use those crayons to make patterns of light.

Law #1: The Rainbow Maker (Continuous Spectrum)

Law #2: The Picky Painter (Emission Line Spectrum)

Law #3: The Colour Eater (Absorption Line Spectrum)

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KIRCHOFF LAWS - CONTINUOUS VS DISCREET SPECTRA

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KIRCHOFF LAWS - CONTINUOUS VS DISCREET SPECTRA

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Law #1: The Rainbow Maker (Continuous Spectrum)

Picture a super-hot, dense, squashed-together gas or thing: a glowing light bulb or a star’s core.

It’s so hot and packed that all the tiny atoms inside are super excited and bouncing around.

Like kids at a party - hot and energetic - throwing every single crayon colour out the box! Kids = gas (analogy).

The result: a continuous spectrum.

A smooth rainbow with no missing colours.

Law #2: The Picky Painter (Emission Line Spectrum)

Picture a hot, low density cloud of gas, like a neon sign or a glowing cloud in space.

Atoms in this gas are excited, but further apart - they pick out only certain colours of light.

Instead of throwing out all the colors, each kid at the party picks just a few favorite crayon colors to throw.

We see only certain bright lines of colour (e.g. red or blue lines) against a dark background.

This is called an emission line spectrum

Law #3: The Colour Eater (Absorption Line Spectrum)

Picture light from that hot light bulb (the full rainbow from Law #1) traveling through a cool cloud of gas, like sunlight passing through a misty cloud.

The atoms in the cool gas are like hungry kids at the party who grab only their favorite crayon colors as it passes through, and “eat” those colours

those colours disappear from the packet of crayons

leaving dark lines where they used to be.

So, you still see a rainbow, but it has dark gaps (missing crayon colors) where the gas absorbed the light.

This is called an absorption line spectrum.

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CONTINUOUS EMISSION SPECTRUM FROM WHITE LIGHT

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CONTINUOUS EMISSION SPECTRUM FROM WHITE LIGHT

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THIS IS WHERE IT ALL STARTED...

1666:

Isaac Newton discovered dispersion of white light using glass prisms

Spectrum produced = continuous spectrum

Continuous spectra = produced by HOT, DENSE GASES e.g. the Sun.

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The Light from Stars Contains Information 

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The Light from Stars Contains Information 

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- The core of a star is HOT (15×10⁶K), and GLOWS.
- Astronomers can learn about a star’s motion, temperature, composition by analyzing starlight that reaches Earth.
- A spectroscope is used to analyze light from stars and other sources:

It separates light into a spectrum.

The light directed from a telescope through a spectroscope produces an image called a spectrograph.

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Every element emits a unique range of colours called spectral lines.
Spectral lines:

identify atoms/molecules

identify the components of stars

are dark or bright lines in an otherwise continuous spectrum

result from emission or absorption of light in a narrow frequency range.

A spectrum is produced when light shines through a gas;

Certain colours, or wavelengths, are absorbed by the gas

which produces a pattern of colours and dark lines called an absorption spectrum.

The "photographic negative" of an absorption spectrum is an emission spectrum.

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What is the difference between absorption and emission spectra?

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What is the difference between absorption and emission spectra?

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Key Comparison

Absorption Spectrum: Continuous background with dark lines (light absorbed).

Emission Spectrum: Dark background with bright lines (light emitted).

They are inverses of one another.

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Scientists use spectra to analyze unknown substances. 

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Scientists use spectra to analyze unknown substances. 

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Absorption Spectra

When white light (continuous spectrum) passes through a cold gas, specific frequencies of light are absorbed by the gas.

Dark lines (missing colours) appear in the spectrum → called absorption lines.

These lines correspond to the exact photon energies needed to excite electrons from lower to higher energy levels in atoms.

Each element has a unique absorption spectrum → acts like a fingerprint.

Example: Hydrogen absorption spectrum shows characteristic dark lines in visible light.

Emission Spectra

Produced when excited atoms release energy as electrons drop from higher to lower energy levels.

Appears as a series of bright, coloured lines on a dark background.

Each line corresponds to a specific photon energy (or wavelength) emitted.

Generated in a discharge tube:

Elemental gas sealed at low pressure.

High voltage applied across electrodes.

Light observed through a diffraction grating.

Provides strong evidence for the quantisation of energy levels in atoms.

Each element has its own unique emission spectrum (signature).

Example: Hydrogen Balmer series (visible light) → transitions to n = 2 energy level.

Transitions to n = 1 are in the ultraviolet range.

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COMPARISON BETWEEN HOW ABSORPTION VS EMISSION LINE SPECTRA FORM

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COMPARISON BETWEEN HOW ABSORPTION VS EMISSION LINE SPECTRA FORM

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Super Hot, Dense Gas → Continuous Spectrum:

High density and temperature cause frequent collisions and thermal equilibrium,

producing a smooth spectrum across all wavelengths (e.g., blackbody radiation in stars).

Hot, Less Dense Gas → Emission Spectrum:

Lower density allows discrete atomic transitions to dominate,

with high temperature exciting electrons to emit light at specific wavelengths (e.g., nebulae).

Cold, Less Dense Gas → Absorption Spectrum:

Cooler, less dense gas absorbs specific wavelengths from a hotter background source,

creating dark lines (e.g., stellar atmospheres).

Temperature and density dictate the processes that produce the observed spectrum.

1. Density -

determines whether the spectrum is continuous (high density, many interactions)

or discrete (low density, fewer interactions)

with isolated transitions.

1. Temperature -

governs whether the gas emits energy (hot)

or absorbs energy (cold)

determines the energy range of the emitted or absorbed photons.

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Introductory Astronomy: Different Types of Spectra

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1.

Explain how atomic emission spectra arise and how they relate to each element on the periodic table.

Atomic emission spectra arise from:

Electrons dropping from higher energy levels to lower energy levels within the atom.

Photons (light packets) with specific wavelengths are released, by the formula f = ΔE/h  or c/λ = ΔE/h

The energy levels in an atom are unique to each element on the periodic table.

The wavelength of light emitted tells which element the light came from.

2.

How do the lines on the atomic spectrum relate to electron transitions between energy levels?

The lines on the atomic spectrum relate to electron transitions between energy levels.

Emission lines:

- the electron falls to a lower energy level.

- a photon is released.

- this results in an emission line.

Absorption lines:

- if the electron absorbs a photon, it rises in energy levels.

- an absorption line is seen on the spectrum.

3.

Explain the difference between atomic absorption and emission spectra.

The difference between absorption and emission spectra are that:

absorption lines are where light is absorbed by atom.

you see a dip in the spectrum.

emission spectra have spikes in the spectra.

due to atoms releasing photons at those wavelengths.

4.

What colour is the light emitted by hydrogen when an electron makes the transition from energy level 5 down to energy level 2?

(Use Figure 1 above to find released photon energy.)

ΔE  = E₅ − E2

= 21,0 × 10−19 − 16,3×10−19 J

= 4,7 × 10−19 J  (MUST be POSITIVE = EMISSION)

λ  = hc/ΔE

= (6,63×10−34)(3×10⁸) / 4,7×10−19

= 423 nm     Colour emitted is violet.

5.

A glass tube filled with hydrogen gas has white light shone onto it. The spectrum measured has an emission line at a wavelength of 474 nm. Between which 2 energy levels did the transition occur?

(Use Figure 2 in solving the problem.)

ΔE      = hc / λ

= (6,63×10−34)(3×10⁸ ) / 474×10−9

= 4,20×10−19 J         Transition from E-level 4 to 2.

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A Level Physics - Emission and Absorption Line Spectra

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What does each line represent?
How do emission spectra like this prove the quantum model of the atom?

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https://youtu.be/fdBupbpWMkY

REMEMBER:

IN EMISSION SPECTRA OUR HUMAN EYE OBSERVE ONLY THE SPECTRA EMITTED WITHIN THE VISIBLE LIGHT EM-RANGE, i.e. electron transitions from higher energy levels down to energy level 2! This produces the Balmer series of radiation.

There are also electron transitions all the way back down to energy level 1, but that process emits UV radiation. UV is out of the scope of our normal eye vision. We need special spectrographs to observe that. This produces the Lyman series of radiation.

There are also electron transitions down to energy level 3, but that process emits IR radiation. Infrared (IR) radiation is out of the scope of our normal eye vision. This produces the Paschen series of radiation.

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How to solve a Rydberg/Balmer problem with solution

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ANSWERS TO EXERCISE - OPEN DOC AND KEEP SCROLLING DOWN

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ANSWERS TO EXERCISE - OPEN DOC AND KEEP SCROLLING DOWN

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PHOTOELECTRIC EFFECT & EMISSION SPECTRA