Cisco
CompTIA
ISC
EC-Council
EXIN
Fortinet
ITIL
Juniper
Microsoft
VMware
Avaya
SAP
Google
NVIDIA
Salesforce
Amazon
Palo Alto Networks
ISC2
Splunk
ServiceNow
All Vendors
  2. Home
  3. Exams
  4. Test Prep
  5. MCAT Test

Test Prep MCAT Test Exam
Medical College Admission Test: Verbal Reasoning, Biological Sciences, Physical Sciences, Writing Sample (Page 17 )

Updated On: 12-Jan-2026
Page 17 of 164
PDF with 811 Questions

There are two opposing theories of light: the particle theory and the wave theory. According to the particle theory, light is composed of a stream of tiny particles that are subject to the same physical laws as other types of elementary particles. One consequence of this is that light particles should travel in a straight line unless an external force acts on them. According to the wave theory, light is a wave that shares the characteristics of other waves. Among other things, this means that light waves should interfere with each other under certain conditions.
In support of the wave theory of light, Thomas Young's double slit experiment proves that light does indeed exhibit interference. Figure 1 shows the essential features of the experiment. Parallel rays of monochromatic light pass through two narrow slits and are projected onto a screen. Constructive interference occurs at certain points on the screen, producing bright areas of maximum light intensity. Between these maxima, destructive interference produces light intensity minimA. The positions of the maxima are given by the equation d sin= n, where d is the distance between the slits, is the angle shown in Figure 1, the integer n specifies the particular maxima, and is the wavelength of the incident light. (Note: sin tan for small angles.)

Figure 1

A beam of electrons can also produce an interference pattern. Which one of the following expressions gives a consistent definition of an electron's "wavelength" if it has a total energy given by E? (Note: h = 6.6 × 10-34 J·s is Planck's constant and v is the speed of the electrons.)

  1. hvE
  2. hE/v
  3. hv/E
  4. E/hv

Answer(s): C

Explanation:

The easiest way to solve the problem is to check the units of each of the answers, a process called dimensional analysis. Since wavelength is measured in meters, the correct answer will be in meters as well. First, note that has the units of J·s, and v, a speed, has the units of m/s. Then, choice C has the units of hv/E = (J·s)(m/s)/J h
= m, so it is correct. The other choices are incorrect.
Choice A has the units of hvE = (J·s)(m/s)(J) = J2m. Choice B has =

Choice D has the units of E/hv = J/[(J·s)(m/s)] = m-1
Another way to solve the problem is to recall that the photon energy E is given by E = hc/, where c is the speed of light, and is the photon's wavelength. Solving for , we obtain = hc/E.
To find the "wavelength" of an electron, replace the speed of light c with the speed of the electron v, and obtain = hv/E, where E is now the energy of the electron. Again, this is choice C.



One of the most common methods that scientists use to determine the age of fossils is known as carbon dating. 14C is an unstable isotope of carbon that undergoes beta decay with a half-life of approximately 5,730 years. Beta decay occurs when a neutron in the nucleus decays to form a proton and an electron which is ejected from the nucleus.
14C is generated in the upper atmosphere when 14N, the most common isotope of nitrogen, is bombarded by neutrons. This mechanism yields a global production rate of 7.5 kg per year of 14C, which combines with oxygen in the atmosphere to produce carbon dioxide. Both the production and the decay of 14C occur simultaneously. This process continues for many half-lives of 14C, until the total amount of 14C approaches a constant.
A fixed fraction of the carbon ingested by all living organisms will be 14C. Therefore, as long as an organism is alive, the ratio of 14C to 12C that it contains is constant. After the organism dies, no new 14C is ingested, and the amount of 14C contained in the organism will decrease by beta decay. The amount of 14C that must have been present in the organism when it died can be calculated from the amount of 12C present in a fossil. By comparing the amount of 14C in the fossil to the calculated amount of 14C that was present in the organism when it died, the age of the fossil can be determined.
If the global production rate of 14C were to increase to 10 kg per year:

  1. the number of 14C atoms decaying per minute would increase.
  2. the number of 14C atoms decaying per minute would decrease.
  3. the weight of 14C on the Earth would increase indefinitely.
  4. the amount of 14C ingested by living organisms would not change.

Answer(s): A

Explanation:

The number of atoms decaying per unit time is directly proportional to the number of atoms present. To see this, recall that the definition of half-life is the time required for half of a sample to decay. The half-life is a constant for a given material regardless of the sample size. Therefore, if the initial amount of material is greater, then a larger amount of material will decay during each half-life. If the production rate of 14C increases, then there will be more 14C present. Hence, the number of 14C atoms decaying per unit time will increase. So choice A is correct and choice B is incorrect.
The second paragraph describes how a balance between 14C production and decay is reached, so that the total amount of 14C on Earth approaches a constant. If 14C production is increased, then the weight of 14C on Earth will initially increase. However, as this 14C begins to decay, the total amount of 14C will approach a new constant as a new balance is reached between the production and decay processes. Hence, choice C is wrong.
If the production of 14C increases, then a larger fraction of the carbon on Earth will be 14C, and living organisms will ingest a larger fraction of 14C. Therefore, the percentage of 14C in living organisms will increase, and choice D is wrong.



One of the most common methods that scientists use to determine the age of fossils is known as carbon dating. 14C is an unstable isotope of carbon that undergoes beta decay with a half-life of approximately 5,730 years. Beta decay occurs when a neutron in the nucleus decays to form a proton and an electron which is ejected from the nucleus.
14C is generated in the upper atmosphere when 14N, the most common isotope of nitrogen, is bombarded by neutrons. This mechanism yields a global production rate of 7.5 kg per year of 14C, which combines with oxygen in the atmosphere to produce carbon dioxide. Both the production and the decay of 14C occur simultaneously. This process continues for many half-lives of 14C, until the total amount of 14C approaches a constant.
A fixed fraction of the carbon ingested by all living organisms will be 14C. Therefore, as long as an organism is alive, the ratio of 14C to 12C that it contains is constant. After the organism dies, no new 14C is ingested, and the amount of 14C contained in the organism will decrease by beta decay. The amount of 14C that must have been present in the organism when it died can be calculated from the amount of 12C present in a fossil. By comparing the amount of 14C in the fossil to the calculated amount of 14C that was present in the organism when it died, the age of the fossil can be determined.
The method of carbon dating used to determine age depends upon the assumption that:

  1. the half-life of 14C changes when it is ingested.
  2. all ingested 14C is incorporated into the body.
  3. the half-life of 14C depends on the type of molecule in which it resides.
  4. the half-life of 14C does not depend upon conditions external to the 14C nucleus.

Answer(s): D

Explanation:

Carbon dating uses the fact that 14C is unstable and decays over time, while 12C is stable and does not decay.
Let's say the ratio of 14C to 12C in an organism is known at the moment of death. By measuring the ratio of 14C to 12C in a fossil, the age of the fossil can be determined because the 14C in the dead organism decays over time in a quantifiable way.
Although it is not explicitly stated in the passage, choice D is an assumption of carbon dating. It correctly states that the half-life of 14C does not depend upon conditions external to the 14C nucleus. This means that the half- life does not depend on the weather, the amount of 14C present, etc. Because the half-life is a constant with respect to conditions external to the nucleus, it can be used to measure elapsed time accurately. Since
measuring time is the goal of carbon dating, choice D is a necessary assumption. Choice A is wrong because ingestion is an event that is external to the nucleus.
Choice C is wrong because the type of molecule in which 14C is incorporated is also a condition external to the nucleus. Choice B is wrong because we know from biology that some carbon leaves the body as waste. This does not affect carbon dating, however, because what matters is the ratio of 14C to 12C left in the body, not the specific amount of 14C left in the body.



One of the most common methods that scientists use to determine the age of fossils is known as carbon dating. 14C is an unstable isotope of carbon that undergoes beta decay with a half-life of approximately 5,730 years. Beta decay occurs when a neutron in the nucleus decays to form a proton and an electron which is ejected from the nucleus.
14C is generated in the upper atmosphere when 14N, the most common isotope of nitrogen, is bombarded by neutrons. This mechanism yields a global production rate of 7.5 kg per year of 14C, which combines with oxygen in the atmosphere to produce carbon dioxide. Both the production and the decay of 14C occur simultaneously. This process continues for many half-lives of 14C, until the total amount of 14C approaches a constant.
A fixed fraction of the carbon ingested by all living organisms will be 14C. Therefore, as long as an organism is alive, the ratio of 14C to 12C that it contains is constant. After the organism dies, no new 14C is ingested, and the amount of 14C contained in the organism will decrease by beta decay. The amount of 14C that must have been present in the organism when it died can be calculated from the amount of 12C present in a fossil. By comparing the amount of 14C in the fossil to the calculated amount of 14C that was present in the organism when it died, the age of the fossil can be determined.
In determining the age of the galaxy, a technique similar to carbon dating is used on stars with the radioactive isotope 232Th, which has a half-life of 1010 years. 14C is less suitable for this application because:

  1. its half-life is too long.
  2. 14C is more abundant than 232Th is in stars.
  3. 14C is unstable.
  4. its half-life is too short.

Answer(s): D

Explanation:

The age of the galaxy is much greater than that of fossils on the Earth. Therefore, in determining the age of the galaxy, it makes sense to use an isotope with a longer half-life than 14C, which is used in determining the age of Earth fossils. This is because 14C will decay to undetectable quantities more quickly than an isotope with a longer half-life, like 232Th. Hence, choice D is correct, and choice A is incorrect.
Choice B is wrong because there is no reason to think that 14C is more abundant in stars.
And if it were true, 14C would be more suitable for this application because the larger abundance would be easier to measure. Choice C is wrong because the instability of 14C, and 232Th for that matter, is precisely what makes them useful as dating tools.



One of the most common methods that scientists use to determine the age of fossils is known as carbon dating. 14C is an unstable isotope of carbon that undergoes beta decay with a half-life of approximately 5,730 years. Beta decay occurs when a neutron in the nucleus decays to form a proton and an electron which is ejected from the nucleus.
14C is generated in the upper atmosphere when 14N, the most common isotope of nitrogen, is bombarded by neutrons. This mechanism yields a global production rate of 7.5 kg per year of 14C, which combines with oxygen in the atmosphere to produce carbon dioxide. Both the production and the decay of 14C occur simultaneously. This process continues for many half-lives of 14C, until the total amount of 14C approaches a constant.
A fixed fraction of the carbon ingested by all living organisms will be 14C. Therefore, as long as an organism is alive, the ratio of 14C to 12C that it contains is constant. After the organism dies, no new 14C is ingested, and the amount of 14C contained in the organism will decrease by beta decay. The amount of 14C that must have been present in the organism when it died can be calculated from the amount of 12C present in a fossil. By comparing the amount of 14C in the fossil to the calculated amount of 14C that was present in the organism when it died, the age of the fossil can be determined.
In generating 14C in the upper atmosphere, a 14C nucleus combines with a neutron to form a 14C nucleus and:

  1. a proton.
  2. an electron.
  3. a 4He nucleus.
  4. a neutron.

Answer(s): A

Explanation:

To answer this question, you have to balance a nuclear reaction. The question stem suggests the reaction

which has a nitrogen nucleus and a neutron on the left side and a carbon nucleus and the unknown particle on the right.
Two things must be balanced in a nuclear reaction: the charge of the nucleus (which corresponds to the number of protons), and the number of nucleons (which is the number of protons plus the number of neutrons).
Balancing nuclear charge, we obtain 7 + 0 = 6 + b, which implies b = 1. Balancing the number of nucleons, we obtain 14 + 1 = 14 + a, which implies a = 1. Thus, the unknown particle is a nucleon with a charge of +1. The only particle that fits both criteria is the proton, choice A. Choice B is wrong because an electron is not a nucleon and it has a charge of ­1. Choice C is wrong because a helium nucleus has 4 nucleons and a charge of +2. Choice D is wrong because a neutron has a charge of 0.



Viewing page 17 of 164
Viewing questions 81 - 85 out of 811 questions



Post your Comments and Discuss Test Prep MCAT Test exam prep with other Community members:

Join the MCAT Test Discussion