The chromophores that most readily fit this requirement are

[eee062010]

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.

171. 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.


Here the answer is D. I am wondering why not E. Could someone please explain it to me?
Thanks in advance!

[tommywallach]

Hey EEE,

What is the source of this question. You've posted it in this forum, but I don't see it in that book...

-t

[eee062010]

Source: 5LB. Book of GRE practice problems(Manhattan).

Chapter - 5: Question no. 171

[tommywallach]

Thanks EEE,

I've changed the subject line so people know where it's from!

The problem is in the wording, the thing that gets "promoted" is the electron itself (from one energy level to another). The energy itself is not promoted. See the second sentence of that last paragraph: "They absorb photos...and use the energy to promote electrons between energy levels."

Hope that helps!

-t