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C1.3.5—Absorption of specific wavelengths

Photosynthetic pigments absorb specific wavelengths, with chlorophyll absorbing red and blue light strongly while reflecting or transmitting much green light.

Syllabus
First assessment 2025
Objective
C1.3.5
Level
SL

Exam analysis

Chance of appearing6%of analysed past papers
Latest appearanceNovember 2019
Most common paperPaper1
Typical marks1–3

Common command terms

  • Describe
  • Distinguish
  • Outline
  • Predict
  • Explain

Recent exam appearances

November 2019Paper3 ["SL"] · TZ03(c)[ 2 ]C1.3.5—Absorption of specific wavelengths
May 2018Paper2 ["SL"] · TZ14(c)[ 3 ]C1.3.5—Absorption of specific wavelengths
May 2018Paper3 ["SL"] · TZ21(c)(ii)[ 1 ]C1.3.5—Absorption of specific wavelengths
May 2017Paper1 ["SL"] · TZ211[ 1 ]C1.3.5—Absorption of specific wavelengths
May 2015Paper2 ["SL"] · TZ14(a)[ 2 ]C1.3.5—Absorption of specific wavelengths

Coverage 2011–2019 · Updated 15 Jul 2026

Pigments Absorb Selected Wavelengths, Not Every Colour

Absorption spectra show carotenoids, chlorophyll b and chlorophyll a absorbing different parts of the visible spectrum, especially blue and red wavelengths.

A photon is absorbed only when its energy matches an allowed change in a pigment's electrons. Absorption raises an electron to an excited state, beginning the energy transfers of the light-dependent reactions.

Chlorophylls absorb strongly in blue and red regions but weakly in much of the green region, so green light is reflected or transmitted. Accessory pigments absorb additional wavelengths and transfer excitation energy toward chlorophyll a.

Absorption and Action Spectra Answer Different Questions

Spectrum What is varied What the y-axis measures What it tests
absorption wavelength light absorbed by a pigment or extract which wavelengths pigments capture
action wavelength rate of photosynthesis, such as O₂ production or CO₂ uptake which wavelengths drive the whole process

Both normally show strong effects in blue and red regions and weaker effects in much of the green region. The curves are related, not identical: an action spectrum integrates all pigments, energy transfer and every later rate-limiting step.

To compare wavelengths fairly, keep incident photon supply, temperature, CO₂ availability and plant material constant; a coloured filter that also changes intensity confounds the conclusion.

SL Checkpoint: Read Photosynthesis as Matter, Energy and Evidence

light energy → excited pigment electrons → ATP and reduced NADP → CO₂ reduced using hydrogen from water → organic carbon compounds; photolysis of water releases O₂

  • A chromatogram separates pigments; Rf compares their movement under matched conditions.
  • An absorption spectrum measures captured wavelengths; an action spectrum measures whole-process rate.
  • A limiting-factor experiment changes one condition, controls the others and measures O₂ production or CO₂ uptake per unit time.

When a rate curve plateaus, identify which factor was relieved and which may now be limiting. When evaluating CO₂ enrichment, separate strong control from ecological realism and do not extrapolate beyond the tested conditions.

Absorption of specific wavelengths

[ 4 ]

Outline how plants make use of the different wavelengths of light.

Concept essentials

  • Chlorophyll absorbs blue and red light more strongly than green light.
  • Green light is often reflected or transmitted by chlorophyll-containing leaves.
  • Accessory pigments extend the range of wavelengths available for photosynthesis.
  • Absorbed light energy excites electrons and supports the light-dependent reactions.