GRE Reading Comprehension: Manhatton-GRE阅读Manhatton - I4131Q1X5814396TB$

The goal of a sunscreen chemical is simple – to prevent harmful UVB (and in some cases UVA) radiation from penetrating the skin. There are two main ways in which this goal can be accomplished – by reflecting (physically blocking) ultraviolet light or by absorbing UV light. Sunscreen chemicals are therefore put into two groups based on which method they employ; they are classified as either physical blockers or chemical absorbers. Physical blockers, the most common of which is titanium dioxide, scatter all radiation in the UVB and UVA range. Titanium dioxide reflects light waves in the 290-770 nm range. However, the vast majority of commercial sunscreens are chemical absorbers. Chemical absorbing sunscreens work on the principle of photo-excitation of electrons. They absorb photons of light of specific wavelengths and use the energy to promote electrons between energy levels. When the electrons later return to the ground energy state, they emit light at longer wavelengths (lower energies). Chemical species that exhibit this behavior are called chromophores. The specific wavelength absorbed by a given chromophore is determined by the discrete quantal amounts of energy that are required to excite electrons between the energy levels or its molecules. Since the primary objective of an absorbing sunscreen is to absorb UVB light (290-320 nm), the best sunscreens are those that absorb most heavily in this range. The chromophores that most readily fit this requirement are those with conjugated pi-bonding systems.