- Hey, all my name is Prashanth Kumar or PK
and I'm an MCAT tutor at Shemmassian Academic Consulting.
Today, we're gonna be doing a passage walkthrough
in the MCAT Chem/Phys section.
You should be taken roughly
six and a half minutes on this passage,
but today we're gonna be going more in depth
so you can see exactly what's going on in my head
as I tackle this section.
Let's get it.
(swoosh)
(electrical typing)
All right, great,
so I've gone ahead and put the passage on the left
and the questions are gone through on the right.
So feel free to take a moment and pause the video
and come back to us when you're ready.
If not, let's get right into talking strategy.
For me, I like to maintain the same strategy
going into any passage.
The reason for this is it keeps me grounded
and gives me something that boosts my confidence,
regardless of whether the passage is super easy
or super challenging,
I know I have something to come back to.
This strategy for me, looks like read a paragraph,
write a quick three to five word summary,
then go on to the next paragraph on, and on and on.
Doing this quick three to five word summary
provides a lot of accountability, making sure I understand
exactly what's going on in the paragraph
before moving on to the next one.
Oftentimes I'd find myself
sort of reading an entire passage,
getting lost, not really understanding what's going on,
but when you do that paragraph by paragraph break down
and understand with that summary, exactly what's going on,
the whole narrative makes so much more sense.
And so that's a high, high yield tip of reading a passage.
My second tip is if figures come up,
I don't like to spend too much time on them
on my first read.
The reason for this is, we don't know what questions,
if any will be asked about these figures
so why would we waste time beforehand
if we're eventually gonna have to come back
and look at the figures if they come up?
So with that, those are my main high yield tips
for how to break down a passage
and we can get right into reading this one.
Researchers are studying the reflection
of radioactive particles off of mirrors in a novel study.,
To generate radioactive beta particles,
a radioactive 15C isotope
with an 11 minute half life is used.
The radioactive particles are concentrated
into a single beam using a series of slits
in an electromagnetic field.
After concentration, the particles with the desired path
are focused toward a mirror at a range of angles.
All right.
So the question I always ask myself in these situations is,
what's happening, right?
And I don't want all the details,
there's always gonna be a bunch of details
but I want a big picture understanding
of exactly what's going on.
And so if you can boil it down
into three to five simple words, symbols,
whatever you need to do to get that message across,
that's all we need.
And so here, all we're looking at is radioactive particles
going to mirrors
with electromagnetic.
So that's what I'm saying.
It's a quick three to five word summary.
You can use abbreviation, symbols, whatever you need,
no one else other than you
is going to be reading these three to five words.
So make sure that you make sense to you, not anyone else.
And with that, let's go right onto the next paragraph.
The beta particles make contact with the mirror
and the angle at which the particles are reflected
is measured by observing the point of contact
of the beta particle with a counting surface
and the time it takes for the beta particle
to reach the counting surface.
In this experiment, the counting surface
contains a molecule that can react specifically
with the beta particle to detect its presence.
In addition, the counting surface
is located at a fixed distance away
from the initial point of contact along a vertical plane.
All right.
So again, big picture question, what's happening?
And here all we're doing
is beta particle measurements, right?
That's the big picture understanding.
I haven't seen most of these terms in my constant review
or my practice questions, right?
We've maybe seen beta particles,
because that's nuclear chemistry,
but we haven't seen all these other things
like counting surfaces,
or a fixed distance away from a vertical plane.
Like these types of things
haven't come up in our exact sort of content phase.
And so if we look at this paragraph,
we just identify it as beta particle measurements.
We can come back if we need to, but if not,
let's just go right into the next paragraph.
After conducting a series of measurements
using the radioactive 15C isotope,
researchers test several additional isotopes.
In experiment two, researchers wanna develop a system
to detect the reflection of alpha particles.
All right, so what's happening?
And here we have isotope testing.
And why are we doing the isotope testing?
To look at alpha particles in the future?
So that's the idea, is like we're setting it up,
we're talking about sort of an experimental setup,
we wanna look at how it relates to beta particles
and whether we can use this to relate to alpha particles,
that's the big picture
of the entire sort of narrative of this passage.
But with that, we've knocked out the passage,
let's get right into the questions.
All right.
Question number one.
Assuming that the reflected beta particles
follow the law of reflection,
which of the falling best represents the angle
that the beta particles will be reflected from the mirror
if the angle of incidents is 38 degrees?
All right.
So a lot of key terms that you need to focus in on
right here, right?
One of them is that we're looking at
reflected beta particles following the law of reflection.
And so what that means
is that if we have sort of a mirror surface right here,
we're gonna be sending this sort of light ray in
and what's gonna happen to it,
is that it's gonna be reflected.
And now the important thing of the law of reflection
is that we call this the incidence ray, right,
Right, when a light coming in is coming at a specific angle,
and we set this angle to be perpendicular to our surface
or our mirror.
And this angle, we'll say, theta
is called the angle of incidents, right.
And that's what's mentioned right here,
the angle of incidents is 38 degrees.
And so what the law of reflection says
is that when we're dealing
with a light ray that's being reflected, it's gonna reflect
at the same angle away from this.
I'll put that in red actually.
So you can see, all it's saying is that
this new reflected angle
is going to be the same exact thing.
In this case, theta equals theta two, right?
The black angle is the same as the red angle.
And that's a very, very important tenant
of the law of reflection,
when a light Ray is being reflected,
those angles are gonna be equal.
So in this case,
we know that the angle of incidents is 38 degrees
so our angle of reflection is going to be also 38 degrees.
Right.
If it goes in at one angle, it's reflected,
it's reflected at the exact same angle
as the angle of incidents.
And this is a really important fact.
So I want you to hammer that in.
And with that, we're one for one,
and we're going onto question number two.
All right, a researcher begins with one gram of 15C.
After 33 minutes,
how much 15C's does the researcher have remaining?
An A, 500 milligrams, B, 250 nanograms,
C, 150 nanograms or D, 125 milligrams.
All right,
right away from reading the answer choices, right,
we're gonna have to focus in on what?
Milligrams and nanograms.
We have different sort of units.
And so just always be careful of this.
With that, we need to reckon
that this has to do with some sort of time period
and half life that we've talked about earlier, right?
When we see something after a certain number of minutes
or when we begin with a certain amount of substance,
how much is left, we usually think half life.
And we are gonna look back
at where in our passage we talked about half-lives, right.
And here it says 15C,
an isotope with an 11 minute half life is used.
All right.
Great.
And so if it's an 11 minute half life,
this is an very important topic, half life, right.
So that means every 11 minutes,
we're basically gonna half whatever our mass is at the time.
Right.
And so the way I do this is
I usually write it out like a ladder, right,
we have one gram here at T0.
Okay, so at zero minutes, nothing has happened.
Now, if we're in 11 minute intervals,
we just write that out,
we have 11 minute, we have 22 minutes,
and then finally we have 33 minutes.
Okay.
Because it says 33 minutes, we draw that.
So at every step of this ladder,
sort of every rung in this ladder, we're gonna half.
And so let's do the first (indistinct) together.
We start at one gram, we go down half,
we're gonna have 0.5 grams, right?
We do that again.
We're gonna be at 0.25 grams, right?
Half of one is 0.5, half of 0.5 is 0.25.
We do that one more time, what do we get?
0.125 grams.
And so, while I said,
we needed to focus in on the milligrams, nanograms,
milligrams sort of units differences
between the answer choices, right,
we see the numbers only match one thing,
and that's this D, 125 milligrams.
So we don't even need to worry about the units in the end
because all of the answer choices are different.
And so all you do is follow the rung of the ladder.
See you go from here to here here to here, here to here,
that's three half lives.
And so if you're able to do that,
all you do is sort of half it one by one by one
and you get D.
All right, great.
Onward to the next question.
Question number three.
Which of the following best depicts the particle
used in experiment two?
Is it A, an electron with a negative charge,
B, an electron with a positive charge,
C, a gamma with a neutral charge,
or D, a helium with just a normal helium atom?
All right,
so the important thing that I did with the answer choices is
I recognized what each of these are, right?
This is a negatively charged electron,
this is a positively charged electron,
this is a gamma particle.
And this is a helium atom.
Okay, a normal helium atom.
And so with that, you need to be able to identify
what types of decay are associated with these particles.
And so the first one I think the easiest is this one, right,
if we are dealing with a gamma particle,
we're gonna have gamma decay.
Now what about A and B, right?
We're dealing with beta decay.
And so remember, there's two types of beta decays,
there's beta negative decay, which matches this one,
and then there's also beta positive decay,
which matches this one, right?
If you're decaying into a positively charged electron,
then you're gonna have beta positive,
if you're decaying into a negatively charged electron,
beta negative.
All right, and then four, two normal helium atom,
what are we decaying into?
We're decaying into an alpha particle or an alpha,
this is an example of alpha decay, right.
And so having this foundational understanding
is very very important,
sort of what's happening in any decay is we start with X,
we end with Y plus this sort of decaying molecule
or particle.
Right, and that's gonna be alpha, beta, gamma, whatever.
Okay.
And so with that, now we look at experiment two,
right, it's talking about experiment two.
So let's go over here and look at experiment two.
Experiment two is talking about alpha particles, right?
It's talking about the reflection of alpha particles.
So we need to be able to say, okay, alpha particles,
only one answer choice makes sense, and that is D, right.
We're saying in experiment two,
researchers wanna develop a system
to detect the reflection of alpha particles,
and alpha particles are helium molecules.
Awesome.
Now we've crushed three outta three,
we're onto our final question.
All right, question number four.
A plane of light hits a mirror at an angle of 37 degrees.
What is this angle called?
A, Snell's angle, B, angle of incidents,
C, angle of reflection, or D, angle of refraction.
All right, so this is exactly that same diagram we drew
in that first question, right?
We needed to say, oh, we have an angle
that the light's coming in on,
it's going out or it's refracting in a certain way.
So if the plane of light
is hitting the mirror at an angle of 37, right,
that means that we're going like this.
We have our sort of diagram, we're hitting it at 37 degrees.
Right.
And so what did we say this angle was called?
The angle of incidents, right?
Angle of incidents.
So B is our standard answer,
but let's go through the other answer choices
just to make sure.
So first angle of reflection
was the other thing we talked about, right,
and that was, it's gonna go out this way
and our angle of reflection is this angle,
which is also equal to 37 degrees, right?
Input equals output of the reflection.
Now refraction is when you're changing mediums
with your light and the light sort of bends a little bit
into a new, I'll use a different color for this one,
it sort of bends a little bit
into a new sort of angle, right?
And so then we have our angle of refraction right here.
So I'm gonna write this as angle of refraction.
All right.
So those are the two angles.
And you'll note that the angle of incidents
does not have to equal the angle of refraction.
It will only equal that if the medium stays the same.
And so these answer choices are eliminated.
Finally, we have Snell's angle.
This isn't in a really commonly used term,
but you'll note that Snell's law is a very
very high yield formula that's seen on the MCAT
relating to refraction,
and determining angles based on medium.
And so with that,
you're not gonna have Smell's angle,
but I highly, highly recommend you reading into Snell's law,
that deals with the refraction
of light in different mediums.
And so with that, we've used our answer from question one
to get us the answer choice of question four
and we are four for four, we've crushed it.
All right, great work team,
we crushed another chem/phys passage.
Kudos for you for getting it done.
Lots of great stuff like nuclear chemistry,
angles of reflection and refraction, law reflection,
all of that type of stuff.
Great job.
If you like this video, give us a like down below
and subscribe to our channel and comment exactly
what type of videos you're looking forward to next
so we can cater them to you.
Additionally, check out Vikram's 528 Strategy series
where he goes into depth on all the strategies
you need to use to get your highest score possible.
And finally click the link in our bio
for the MCAT question of the day
so you don't miss out on any studying before test day.
Happy studying, and let's get it.
(upbeat ambient music)