= Solution
Healthy green vegetation has strong visible <absorption> in the blue, around $0.43$–$0.47\ \mathrm{\mu m}$, and red, around $0.65$–$0.68\ \mathrm{\mu m}$, mainly from <chlorophylls> and other <photosynthetic pigments>. Weaker absorption around green wavelengths produces a relative <reflectance> maximum near $0.55\ \mathrm{\mu m}$ and the familiar green appearance. The <red edge> is the steep reflectance rise around $0.68$–$0.75\ \mathrm{\mu m}$. In the <near-infrared>, weak pigment absorption and multiple scattering at internal leaf tissue–air interfaces produce high reflectance, commonly a broad plateau. This plateau is primarily a structural scattering effect, rather than chlorophyll reflecting the light.<Measuring vegetation by NASA Earth Observatory>[“Normalized Difference Vegetation Index”: contrast between chlorophyll absorption of visible light and strong near-infrared scattering by leaf structure.]{ref}
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2025/paper-3-q5-vegetation-reflectance.png]
{title=Qualitative reflectance spectrum of healthy green vegetation}
{description=Original synthetic leaf spectrum with a green maximum, red absorption, red edge, near-infrared plateau and shortwave-infrared water absorption troughs. The curve is illustrative rather than measured; shaded bands exemplify red and near-infrared measurements for a vegetation index.}
Leaf <water> produces absorption troughs in the <shortwave infrared>, especially near $1.4$ and $1.9\ \mathrm{\mu m}$, with weaker features near $0.97$ and $1.2\ \mathrm{\mu m}$. Atmospheric <water vapor> also strongly absorbs near $1.4$ and $1.9\ \mathrm{\mu m}$, so satellite bands usually avoid the centres of those deep troughs. Drying, senescence, pigment loss and changes in canopy structure alter the spectrum; measured canopy reflectance also includes shadows and the soil background.
To highlight vegetation, assign the <near-infrared> band to the red display channel, red to green and green to blue: vigorously vegetated areas appear bright red in this conventional <false-colour composite>. Alternatively calculate the <normalized difference vegetation index> from atmospherically corrected, spatially registered red and near-infrared <reflectances>:
$$
\mathrm{NDVI}=\frac{\rho_{\mathrm{NIR}}-\rho_{\mathrm{red}}}{\rho_{\mathrm{NIR}}+\rho_{\mathrm{red}}}.
$$
Large positive values indicate a strong vegetation-like red/NIR contrast. For example, $\rho_{\mathrm{NIR}}=0.50$ and $\rho_{\mathrm{red}}=0.05$ give about $0.82$. Mask clouds, their shadows and invalid low-signal pixels. Bare soil usually has a smaller contrast, while water often has negative values; fixed universal thresholds are inappropriate, and dense vegetation can saturate the index. A band ratio improves contrast but is not a direct count of plants or a unique measure of physiological health.<Measuring vegetation by NASA Earth Observatory>[“Normalized Difference Vegetation Index”: normalized red/near-infrared contrast and interpretation of green vegetation.]{ref}
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