MCAT Chemistry Question — Photon
- Jun 26, 2017
- MCAT Question of the Day
In a phenomenon called the photoelectric effect, a photon striking the surface of a metal may be absorbed and lead to the emission of a single electron. The energy of the emitted electron is described by E = hf-φ, where φ is the work function, h is Planck’s constant (6.626 * 10^-34 J*s or 4.136 * 10^-15 eV*s) and f is the frequency of incident light. If 3 photons of 200 nm wavelength are incident on a filament of tungsten (φ = 4.5 eV), how many electrons are emitted?
Click for Explanation
Answer: D. 3 electrons will be emitted.
Kinetic energy is a scalar quantity that cannot be less than 0. The reader is left to make the inference that if the energy of emission is less than
zero, no electron would be emitted.
We can verify what the maximum wavelength of a photon is to stimulate emission from tungsten by setting E = 0 and rearranging E = hf-φ to solve for
frequency and then wavelength:
E+φ = hf
(E+φ)/h = f
Now recall that c = f*λ and therefore f = c/λ and substitute, to get:
(E+φ)/h = c/λ
λ*(E+φ)/h = c
λ = c*h/(E+φ)
Now let’s plug in our numbers (remember E = 0)
3*10^8 m/s * (4.136 * 10^-15 eV/s) / 4.5 eV = 2.757 * 10^-7 m
2.757 * 10^-7 m = 275.7 nm
The maximum wavelength of a photon to stimulate electron emission from tungsten is therefore 275.7 nm. Since the 3 incident photons are each 200 nm,
all less than the maximum wavelength, each photon will result in the emission of one electron.
Want more MCAT practice?
We’ve got options for every schedule and learning style!
From the best online MCAT course created by top instructors with 524+ MCAT scores to the most representative full-length practice exams and private tutoring, we can custom tailor your MCAT prep to your goals!
Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT expert using the form below. No obligation, just expert advice.
Search the Blog
Interested in our Online MCAT Course, One-on-One MCAT Tutoring or Med admissions packages? Set up a free consultation with one of our experienced Senior Student Advisors.Schedule Now
MCAT Blog What's on the MCAT?
MCAT Blog How to Review MCAT Full Lengths
Free MCAT Practice AccountNeed great MCAT practice?
Get the most representative MCAT practice possible when you sign up for our free MCAT Account, which includes a half-length diagnostic exam and one of our full-length MCAT practice exams.Learn More