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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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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...

In 1666, Isaac Newton discovered that white light can be separated into its component colours using glass prisms.

The spectrum produced is a continuous spectrum

Continuous spectra are produced by HOT DENSE GASES LIKE THE SUN.

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After Newton's discovery, scientists were using this new tool to analyze the light coming from several different light sources. 

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After Newton's discovery, scientists were using this new tool to analyze the light coming from several different light sources. 

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- Some scientists looked at hot objects and gases;
- Others looked at the stars.
- Observations were made and patterns detected.

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

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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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Scientists use spectra to analyze unknown substances. 

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Scientists use spectra to analyze unknown substances. 

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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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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 E = hf
So:             f = ΔE/h  or c/λ = ΔE/h
- The energy levels in an atom are unique to each element on the periodic table.
- Therefore the wavelength of light emitted can be used to determine 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 drops an energy level
- a photon is released
- resulting in an emission line.
Absorption lines:
- if the electron absorbs a photon it rises an energy level
- an absorption line is observed 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 has been absorbed by the atom
- thus you see a dip in the spectrum
- emission spectra have spikes in the spectra
- due to atoms releasing photons at those wavelengths.

4.
Describe how the absorption and emission spectra of the gases in the atmosphere give rise to the Greenhouse Effect.

The following needs to be in your answer:
- in what wavelength range the sunlight reaches the earth,
- the absorption of the sunlight
- and the re-radiation as infrared light = HEAT
- the scattering of the infrared light by the carbon-dioxide
- how this scattering contributes to the Greenhouse Effect.

5.
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 the energy of the released photon.)

ΔE      = E₅ − E2
= 21,0 × 10−19 − 16,3×10−19 J
= 4,7 × 10−19 J
(MUST GET POSITIVE ANSWER OUT FOR EMISSION)

λ       = hc/ΔE

= (6,63×10−34)(3×10⁸) / 4,7×10−19

= 423 nm
Colour of light emitted at 423 nm is violet.

6.
I have a glass tube filled with hydrogen gas and shine white light onto the tube. The spectrum I then measure has an emission line at a wavelength of 474 nm. Between which two 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 occurred from E-level 4 to E-level 2.

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A Level Physics - Emission and Absorption Line Spectra

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VIDEO EXPLAINING EMISSION AND ABSORPTION SPECTRA 

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VIDEO EXPLAINING EMISSION AND ABSORPTION SPECTRA 

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A Level Physics - Emission and Absorption Line Spectra

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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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PHOTOELECTRIC EFFECT & EMISSION SPECTRA