Describe what is meant by the greenhouse effect in the Earth’s atmosphere.

The graph shows the variation with frequency of the percentage transmittance of electromagnetic waves through water vapour in the atmosphere.

(i) Show that the reduction in percentage transmittance labelled X occurs at a wavelength equal to approximately 5 μm.

(ii) Suggest, with reference to resonance, the possible reasons for the sharp reduction in percentage transmittance at a wavelength of 5 μm.

(iii) Explain how the reduction in percentage transmittance, labelled X on the graph opposite, accounts for the greenhouse effect.

(iv) Outline how an increase in the concentration of greenhouse gases in the atmosphere may lead to global warming.

effect caused by gas such as H₂O/NH₃/CH₄/CO₂/greenhouse gas in the atmosphere;
gas absorbs outgoing (long wave) radiation from Earth;
gas re-radiates some of the energy back to Earth;

(i) \(\frac{{3.0 \times {{10}^8}}}{{6.5 \times {{10}^{13}}}} = 4.6\left( {\mu m} \right)\);
≈5(μm)

(ii) water vapour molecules have a natural frequency of oscillation;
if this frequency of oscillation is \(6.5 \times {10^{13}}\) / reference to frequency at X;
due to resonance this radiation is readily absorbed by the molecules / the radiation matches the natural frequency of oscillation;

or

X is a natural frequency (of oscillation) of water molecule;
so resonance effects mean that molecules are excited at this frequency;
and energy is removed/less energy transmitted from electromagnetic waves at this (particular) frequency;

(iii) energy gained by absorption needs to be re-emitted (as molecules de-excite);
in other directions / some returns to Earth;

(iv) more greenhouse gases means that there is more absorption of outgoing radiation;
therefore more energy returns to Earth;
leading to a further/greater increase in the temperature of the surface (of Earth);