(inspirational music)
- Since in this series, this is part three,
I thought, what a wonderful time for some poetry.
What is our subject of the week?
Of terms related to lenses, we shall speak.
In part one of index, we have already spoken.
In part two on abbey, it was light that was broken.
Today, with our usual formality,
we will jump right in to specific gravity.
A term complex and quite confusing,
it lends itself easy to misusing.
So, let's dig in and see if we can find
the best way for an optician, that it can be defined.
And if it helps you in any way,
be sure to tell others you learned it
from that guy, from Laramy-K.
(upbeat music)
Hello, I would like to welcome you
to the Laramy-K Optician Works Training Center
where today we have part three in our three-part series
on terms related to lens materials.
And you may have noticed I am not in my usual attire today.
It is rather chilly out here in the studio,
so let's just hop on over to the white board and get going.
Before we jump into specific gravity and lens materials,
let's talk about it's broader application.
And that really will help when
we switch gears to our next white board.
For our purposes, this is not always done this way,
we say that specific gravity is the relationship
between the density of a specific amount of a given material
when compared to the density of a specific amount of water.
We are not solving for anything.
We are simply rewriting the relationship between two things.
Are you starting to see a pattern here?
There are three terms that are commonly associated
with lenses and lens material.
Index of refraction, how does the speed of light in air
or vacuum compare to the speed of light in a given material?
V, or abbey, how does the index of refraction
of one specific wavelength compare to that of two others?
Specific gravity, how does the density of one material
compare to the density of water?
It's all we're doing here.
In the real world, specific gravity is most commonly used
for measuring the density of
liquids and gases, not so much solids.
And water is not always the thing being compared to.
It's often oils and alcohols since it's used in brewing
and other scientific applications.
It is easiest to picture or see
how we end up with a result like this
when we think big, when we upscale it.
If we think about steel,
now you've all picked up a plate at the gym
or a barbel somewhere along your life,
and you know steel is really, really heavy, right?
If I have a one meter by one meter by one meter cube,
roughly three feet by three feet by three feet,
of solid steel, I literally could hop up
and sit on this thing, it would have the density
of 8,050 kilograms per cubic meter or 17,747 pounds.
If I took the volume of a cube of one meter of water,
it weighs 1,000 kilograms.
The specific gravity for steel is 8.05.
Or we can say that steel
is eight times denser than water is, so it breaks down.
And last, there are no units assigned to specific gravity.
We are comparing apples to apples, kilograms per cubic meter
to kilograms per cubic meter in this case.
We're gonna do some kinda old-fashioned fraction reduction.
And when we wipe the white board
clean and move on, we'd start looking at lenses.
Because they're so lightweight,
we're going to take it down to grams per cubic centimeter
compared to the grams per cubic centimeter of water.
Now, the other thing you're gonna see
as soon as you dig in to specific gravity at all,
Googling stuff online, other books,
it's gonna show you the beaker,
and the beaker is gonna be filled with water.
Supposedly distilled water at 39 degrees Fahrenheit,
which is it's densest possible condition.
And it's gonna say that anything
that's put on top of the water and it floats,
is going to be a specific gravity of less than 1.99, .7, .4.
If it is greater than one,
it is going to sink down into the water.
- What also floats in water? - Bread!
- Apples! - Very small rocks!
- So yeah, if you wanna dig into that some more,
you know, go ahead and Google it.
You don't need me to revisit that.
You may remember in one of the very
first videos that we ever did,
we had the high-prescription wheel of terror, remember that?
(dramatic music)
- [Mature Woman] Don't you dare touch me, stay back!
(drill whirring)
(screaming)
- That was a good one, wasn't it?
I ended up getting caught in my own
index, weight, density, gravity,
volume, mass wheel of terror here.
All of these terms are so closely related
that it becomes so easy to get confused,
get off track, and start thinking
about them in ways that we shouldn't.
So I've narrowed this down to two concepts,
they're related to each other,
that I want you to take away from specific gravity.
Now, I said we were gonna reduce a fraction.
We went from kilograms to grams,
from cubic meters to cubic centimeters.
We're comparing the two together, we can cancel that out.
We've already reduced our fraction to one.
If we were working the relationship,
1.46 divided by one is 1.46, we've reduced
that as far as we can go, we don't need that anymore.
What is the specific gravity of 1.74, is simply 1.46.
From there, we simply have a chart
of the seven common materials that we use for lenses.
There is the specific gravity, there is the index.
Now, what can we take away, why do lens manufacturers
put a specific gravity on their lens spec sheets?
All that we can take away from this
is that it is of a specific amount of a material.
Here's my cubic centimeter.
All I can take away from these numbers
is that a cubic centimeter of trivex
weighs less than a cubic centimeter of poly.
Poly less than 1.60,
CR39 less than 1.67, down the line.
Concept number one is that a lens
is not a beautiful, perfect cubic centimeter.
It will vary, obviously, as you know,
by it's index of material,
of it's power requirements, and it's size.
So you cannot make generalizations,
you cannot toss around terms.
Saying that there's any relationship
between these numbers and the particular
weight or density or specific gravity of a lens,
you're only talking about material in this specific amount.
The other concept that I want you to take away from this
when talking about specific gravity
is that I guarantee you, 100%,
that if you walked into the mall this weekend
and you went into one of those really busy,
big optical places where there are five opticians
and five frame stylists, if you hung
around for a little while and listened,
you'd hear somebody oh, Mrs. Smith,
we're gonna take you from poly
and we're gonna put you in a 1.60
which is gonna be thinner and lighter.
- [Female Announcer] Discover our thinner, lighter lenses.
- Not necessarily, just go back to concept one.
The higher the specific gravity
of that given material by volume,
the heavier it is going to be.
Yes, a 1.74 is gonna be most likely
thinner than a poly, but it is not going to be lighter.
So stop making that mistake.
Again, don't talk about things that
are not actually related to one another.
All you can tell from specific gravity
is how much a particular amount weighs.
In other words, would I rather carry
10 blocks of trivex up the hill,
or would I rather carry 10 blocks of 1.74?
And why did I bother to put the
index of refraction up there?
Well, as a reminder from our first lesson
in this three-part series that specific gravity
is about density, but density has little to do
with index of refraction, although we sure do
like to think about it that way.
Remember our trivex, not very dense
index of refraction of 1.53, glass,
extremely dense index of refraction 1.52,
so keep that in the back of your mind as well.
What does it all come down to?
Well surprise, surprise, it all comes down to you.
You matter, as power goes up, we do need to think
about the index of refraction,
we need to think about the abbey,
we need to sort of think about specific gravity,
we need to think about power, frame size, frame shape,
how to get great measures, all of that good
optician stuff that we cover on the Optician Works website.
Thank you, I think we can put
specific gravity behind us now.
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please give us a like, make sure that every lens
regardless of it's specific gravity
comes from Laramy-K, and I will see you again next week.
(chanting in Latin)