( ( The ( cooler ( , ( low-density gas ) ) ) ) ( ( acts ( to ( block ( ( the photons ) ( which ( ( ( have ( the ( right wavelengths ) ) ) , ) ( while ( ( the ( other photons ) ) ( travel ( through ( the ( gas unperturbed ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN cooler) (, ,) (NN low-density) (NN gas)) (VP (VBZ acts) (S (VP (TO to) (VP (VB block) (NP (NP (DT the) (NNS photons)) (SBAR (WHNP (WDT which)) (S (VP (VBP have) (NP (DT the) (JJ right) (NNS wavelengths)) (, ,) (SBAR (IN while) (S (NP (DT the) (JJ other) (NNS photons)) (VP (VBP travel) (PP (IN through) (NP (DT the) (NN gas) (NN unperturbed)))))))))))))) (. .)))
The cooler, low-density gas acts to block the photons which have the right wavelengths, while the other photons travel through the gas unperturbed.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))