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.

Which of the following best summarizes the distinction between physical blockers and chemical absorbers?

(A) Physical blockers darken their target light waves while chemical absorbers lighten them.
(B) Physical blockers convert their target light waves into radiation while chemical absorbers convert them into a different kind of radiation.
(C) Physical blockers disperse their target light waves while chemical absorbers convert them into light with a longer wavelength.
(D) Physical blockers scatter their target light waves while chemical absorbers convert them into radiation.
(E) Physical blockers prevent light waves from reaching the skin while chemical absorbers absorb them into the skin.



Based on the passage, which of the following can be inferred about the chromophores referred to in the final sentence of the passage?

(A) If exposed to light with wavelengths of approximately 300 nm , they will scatter the radiation.
(B) If exposed to light with wavelengths in the 290-320 nm range, they will lower the energy level of some of their constituent electrons.
(C) If exposed to light waves in the 290-770 nm range, they will absorb the photons and emit them as light of longer wavelengths.
(D) If exposed to light with wavelengths of approximately 300 nm , some electrons in their component molecules will switch to higher energy levels.
(E) If exposed to light waves in the 290-320 nm range, they will promote the discrete quantal amounts of energy that are required to excite electrons between energy levels.



Select the sentence in the second paragraph that explains the physical feature on the basis of which one could select a chromophore for a sunscreen that would protect against UVA radiation.