The goal of a sunscreen chemical is simpleto prevent harmful UVB (and

Question

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 photoexcitation 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.    1 
 1. 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.

2  
 2. 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.

3  
 3. 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. 
  “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.”

Sajjad1994 · Answer

OE

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

Chemical blockers scatter, or disperse, light waves. Chemical absorbers use light waves to promote electrons which then release light waves with a longer wavelength as they return to their ground energy state. Thus,  choice (C)  is correct.

Choice (A)  makes a reference to lightening and darkening light waves, neither of which is mentioned in the passage.  Choices (B)  and  (D)  refer to converting light waves to radiation, which is not mentioned in the passage. And absorbers absorb the radiation into their molecular structure, not into the skin as in  choice (E) . 
 
  Answer: C

Sajjad1994 · Answer

OE

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

The chromophores absorb light in the 290–320 nm range and use it to promote (or move up) electrons between energy levels. Since light with wavelengths of 300 nm falls in this range, their electrons should move up in energy levels when exposed to it. Thus,  choice (D)  is correct.

In  choice (E) , the phrase “ they will promote the discrete quantal amounts of energy required”  is unjustified, as it is the electrons that get promoted, not the discrete quantal amounts of energy required. The discrete quantal amounts of energy simply establish the wavelength range that the chromophores will absorb.

Choice (C)  would be correct if not for the range given: the author does not indicate how chromophores react to light above 320 nm.

Choice (B)  is the exact opposite of what the question asks for.

Choice (A)  applies to physical blockers but not to chemical absorbers. 
 
  Answer: D

Sajjad1994 · Answer

OE

3. 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.

In order to select a chromophore for a particular sunscreen, you would need to know which light waves the sunscreen needs to block and which chromophore would block those waves. This sentence identifies which feature of a chromophore determines which light waves it absorbs. The next sentence in the passage might seem like a good match, but it only specifies how to select a chromophore that would absorb UVB radiation, not UVA radiation. 
 
  Answer:   “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.”

The goal of a sunscreen chemical is simpleto prevent harmful UVB (and

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