• 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.

•  It separates light into a spectrum.
• The light directed from a telescope through a spectroscope produces an image called a spectrograph. 

• 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.

• 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.

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

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

• They are inverses of one another.

• 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.

• 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.

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.

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.

• Law #1: The Rainbow Maker (Continuous Spectrum)

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

  Law #3: The Colour Eater (Absorption Line 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.