Atomic Spectra

When an atom emits or absorbs light it does so only at sharply defined wavelengths — producing a line spectrum rather than a continuous one. The PMDC MDCAT 2026 syllabus reduces this chapter to a single subtopic with one learning outcome: describe and explain atomic spectra / line spectrum. Expect 1-2 high-yield MCQs.

PMC Table of Specifications. The single subtopic listed here is Atomic Spectra / Line Spectrum, and the single learning outcome (15.1) is to describe and explain atomic spectra / line spectrum — continuous vs line spectra, emission vs absorption, and why the lines are discrete.

Atomic Spectra and Line Spectrum

A spectrum is the range of EM wavelengths emitted or absorbed by a substance. Hot solids emit a continuous spectrum; rarefied gases excited by an electric discharge emit a line emission spectrum — a set of bright lines on a dark background. A cool gas placed between a continuous source and a spectrometer absorbs the same wavelengths, producing a line absorption spectrum (dark lines on bright background, e.g. Fraunhofer lines in sunlight).

Why the spectrum is a set of lines

An atom can only possess certain discrete amounts of internal energy — its energy levels. The atom cannot hold an energy between two levels.

When an electron drops from a higher level of energy E_i to a lower level E_f, the surplus energy leaves as a single photon: hf = E_i − E_f.
Because only certain energy differences are possible, only certain frequencies (and therefore wavelengths) can be emitted — so the spectrum is a set of sharp lines, not a continuous band.
Every element has its own unique set of energy levels, so its line spectrum is a fingerprint used to identify the element in a source such as a star or a flame.

Continuous vs line spectrum

A hot, dense source (a glowing filament, molten metal, the Sun's interior) emits all wavelengths — a continuous spectrum with no gaps. A low-pressure gas of free atoms emits only the wavelengths allowed by its energy-level differences — a line spectrum. The difference is the state of the source, not the type of light.

Emission vs absorption spectra

Emission: excited atom drops from a higher to a lower level, releasing a photon. Bright line on dark background.
Absorption: a cool gas absorbs photons of specific wavelengths from a continuous source, lifting electrons to higher levels. Dark line on bright background.
Both spectra share the same wavelengths for a given element — a chemical fingerprint.

Worked MCQs

Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — it's where the deeper concept lives.

Q1. A bright-line emission spectrum is produced by:

A white-hot tungsten filament
A rarefied gas excited by an electric discharge
Molten iron in a furnace
A cool gas placed in front of a continuous source

Only free atoms in a low-pressure gas give discrete lines. Hot dense sources (filament, molten iron) give a continuous spectrum; a cool gas in front of a continuous source gives a dark-line absorption spectrum.

Q2. Atomic spectra consist of sharp discrete lines rather than a continuous band because:

Light is a transverse wave
An atom can only have certain discrete energy levels
Photons have zero rest mass
Atoms are electrically neutral

Only certain energy differences E_i − E_f are available, so only certain photon frequencies can be emitted or absorbed. Discrete energy levels are the whole explanation of a line spectrum.

Q3. An atom emits a photon when it falls from an energy level of −3.4 eV to a level of −13.6 eV. The energy of the emitted photon is:

3.4 eV
13.6 eV
10.2 eV
17.0 eV

hf = E_i − E_f = (−3.4) − (−13.6) = 10.2 eV. Subtract, never add, the two level energies — adding gives the 17.0 eV distractor.

Q4. A dark-line absorption spectrum of an element occurs at:

Longer wavelengths than its emission lines
Shorter wavelengths than its emission lines
Exactly the same wavelengths as its emission lines
Wavelengths that depend on the temperature of the gas

The same pair of energy levels is involved whether the atom absorbs or emits, so the wavelengths coincide. This is why the line spectrum works as a chemical fingerprint — e.g. the Fraunhofer lines in sunlight.

Q5. The wavelength of a spectral line produced by a transition of energy 2.0 eV is approximately:

310 nm
620 nm
1240 nm
2480 nm

Using hc = 1240 eV nm, λ = 1240/E = 1240/2.0 = 620 nm, in the orange-red part of the visible spectrum. Larger energy gaps give shorter wavelengths.

Quick Recap

Continuous spectrum = all wavelengths, from a hot dense source; line spectrum = discrete wavelengths, from free atoms in a gas.
Emission spectrum: bright lines on a dark background. Absorption spectrum: dark lines on a bright background.
Both occur at the same wavelengths for a given element — a chemical fingerprint.
Atoms have discrete energy levels; a transition emits or absorbs one photon of energy hf = E_i − E_f.
Only certain energy differences exist, so only certain wavelengths appear — hence sharp lines.
Useful: λ (nm) = 1240 / E (eV).

Test yourself. Take a timed Atomic Spectra quiz or browse all Physics MCQs to lock these concepts in.