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Hi.

I'm Mrs. Hudson.

And today I'm going to be teaching you a lesson called Light Microscopy: Observing and Drawing Cells.

This is a biology lesson and it comes under the unit that is titled Eukaryotic and Prokaryotic Cells.

The outcome of today's lesson is, I can use a light microscope to observe and produce scientific drawings of cells.

So we are going to be asking ourselves, how do you use a microscope and what do cells look like when you're viewed under a microscope? There will be some keywords that we'll need to be using in today's lesson, and they are light microscope, lens, magnification and focus.

So let's have a look at what those keywords mean.

A light microscope is a type of microscope that uses visible light and a system of lenses to generate magnified images of small objects.

A lens is a piece of glass or other transparent material with curved sides used in a microscope to magnify objects.

Magnification means making small objects appear larger in order to see in more detail.

It's really important that we don't use the word zoom in, and instead we use the word magnification.

And the final keyword is focus, which means a point to where light ray converge to form an image.

And again, with rather than saying to make an image clearer, we will always say to focus the image.

So today we're going to spit the lesson up into three parts.

And in the first part of the lesson, we're going to be looking at using a light microscope, and then we'll move on to observing and drawing cells, and then finally, we'll finish the lesson off by looking at magnification and the size of cells.

And in this part of the lesson we'll be doing a few calculations.

So let's get going with the first part, using a light microscope.

Now the first thing we need to be able to do before we can use a light microscope is understand the parts of it.

So this is a labelled diagram of a light microscope.

So let's have a look at the different parts that make it up.

So this is the eyepiece lens.

This is a part of the microscope that you look down when you view your specimen.

And you've also got the objective lens.

There are usually three objective lenses on a microscope.

You can only see two of them in this picture, but there will be a third one.

Now remember that lenses are curved pieces of glass or transparent materials, and they magnify images.

So there are two lenses on the microscope.

Then you've got the stage.

The stage has clips on, which is where you place the slide with the specimen on it.

And the stage is able to move up and down so that it can get closer or further away from the objective lenses.

And at the bottom you've got the light source.

Sometimes there'll be a switch, which you can switch on and off to turn the light source on or off.

And it's really important to make sure the light source is shining directly through the specimen so that you can view it.

Now, there are two parts of the microscope that focus the image.

The first part is the coarse focus wheel, and this is the initial focusing wheel.

It moves the stage towards or further away from the objective lens.

And once you've got the image in view, you will then use the fine focus wheel, which gives you a much more precise image so that you can focus much more clearly.

So these are all of the parts of the microscope, right? Let's check our understanding.

Add the missing labels to the parts of the light microscope.

Give it to go now.

I'm sure you're gonna do a great job.

Right.

Let's check how we did.

So we've got the fine focus wheel, which is at the bottom, but then the other focus wheel was the course focus wheel.

So well done if you manage to get that.

And then at the very bottom, it's the light source.

And then underneath the eyepiece lens, the other set of lenses are called the objective lens.

Really great job if you manage to identify those three missing labels.

We're going to watch a video now to see how to use a light microscope to observe the cells in a specimen.

It's really important that you listen carefully so that you know how to use a microscope.

Let's watch the video.

Okay, so let's have a little look at how to use our microscope.

So if you turn it round to face you like this, you should be able to see that curved piece of glass there in the top.

That's your lens, your eyepiece lens.

If we turn it sideways, it's easier for you to see all of the different parts.

Now, if you look at the back, there might be a light switch or you might have a mirror at the bottom that you need to angle.

But it's really important that that light passes through that hole in the stage in order for you to be able to view your specimen.

So this is your eyepiece lens that you're gonna look through.

And these are your objective lenses, which you can change in order to change the magnification of your image.

Then you have got your course focusing wheel, which gets in roughly the right focus, and then the fine focus in to make a real sharp image.

You can adjust the amount of light coming through if there's too much or too little light, and you're gonna place your slide with your specimen underneath those clips on the stage.

So when you're ready, have a little go at making sure that that eyepiece lens is facing you.

Different microscopes have them in slightly different ways.

And you're gonna get yourself a slide that's already prepared.

And you're going to put it on the stage underneath those clips.

Now it's really important when you do this that the specimen is over that little hole where the light is coming through so that you can visualise it.

You need to adjust your objective lens, so it's on the smallest one there.

And then you're gonna turn your course focusing wheel to bring the stage as close as you can to that lens.

And then you're gonna look through your eyepiece lens here, and you're gonna move that stage slowly away using the course focusing wheel until you can see a clear image.

You can sharpen up that image by just turning the fine focusing wheel there, which may be a separate wheel.

Once it's in focus, you can then increase the magnification by changing your objective lens.

And again, it should be in roughly the right place, but you can adjust your focusing wheel slightly whilst looking down the eyepiece lens to get it into view.

And finally, you may be able to use your highest magnification objective lens, again, lens slightly turning your course and fine focusing wheels to get it in view.

Before we can have a go at using a light microscope, let's just recap the method for using one.

So the first thing we need to do is to turn the objective lens to the lowest magnification.

Then we place the slide on the stage underneath the clips.

Number three, we have to turn on the light source.

Number four, looking from the side, turn the course focus wheel to move the stage up so it's close to the objective lens.

Number five, looking into the eyepiece, turn the course focus wheel to bring your specimen into focus.

Number six, turn the fine focus wheel to make the image clearer.

And number seven, the magnification can be increased by changing to a higher powered objective lens.

Now we know how to use a microscope.

Let's just check our understanding.

So put these statements in the correct order for using a microscope to view a slide.

A, use the fine focus wheel to make the image clearer, B, place the slide on the stage, C, turn to lowest power objective lens, D, use the COE focus wheel to focus on the specimen.

So number these 1, 2, 3, and 4 to put them in the correct order.

Let's see how we did with that one.

So the first thing we need to do is C, turn to the lowest power objective lens, and then we need to do B, place the slide onto the stage, then D, use the course focus wheel to focus on the specimen, and then finally, A, use the fine focus wheel to make the image clearer.

Amazing if you manage to get those in the correct order.

That's brilliant.

Well done.

Now it's your turn to use a microscope.

So in Task A, we're going to follow the method of using a like microscope to: magnify and focus on cells in a tissue specimen from a plant or animal.

And then once you've done that, you're going to identify what a single cell looks like.

Pause the video now and have a go at that task.

And remember to use the method to specify to make sure you manage to magnify your cells really well.

Hopefully you had fun doing that.

And also manage to view your specimen really clearly.

So here are some examples of images that may be similar to what you saw through a light microscope.

So we've got four different types of cells here, muscle tissue, onion tissue, skin tissue and root tissue.

And in each of those images you can see there are multiple cells of the same type.

So they are called tissues, but then you can see the labels above they're single cells.

So hopefully you saw an image that was similar to this where there were lots of cells, so a tissue under the microscope, but you did manage to identify what one single cell looked like.

Great job.

Now we know how to use a light microscope.

Let's move on to the second part of our lesson, observing and drawing cells.

Now, once we've managed to observe our cells under a microscope, we can then make a labelled scientific line drawing of cells.

Observing and drawing cells does not look exactly the same as the original image or the object.

It is a clear and simple representation of what you have seen in your microscope.

So you can see here that there's an image of a muscle cell, and this is what we call a labelled scientific line drawing of cells.

And they have labelled the nucleus and the muscle cell, and they've also put times 400, which is the magnification in there.

There are certain for producing a scientific line drawing of cells, and they are as follows, we only select a few cells.

Even though the sample has lots of cells in it within the tissue, when we do the drawing, we only select a few of those cells.

We make sure that the cells are large in size, so this is that we can see them clearly.

The lines that we use are smooth and joined up.

We use continuous lines and no sketching.

We include clear labels including the magnification.

The magnification is the number at the bottom there that says times 400.

And we also use no shading apart from stippling.

Stippling is just the action of using dots with your pencil.

Let's see what that actually looks like in reality.

So here are two examples of drawings from this microscope image, and that specimen is a plant root tissue viewed through a light microscope.

So we can see that there are many cells within that tissue, but we want to make a scientific line drawing of that specimen.

And we can see there are two scientific line drawings on this slide.

One of them is really good and the other one is not quite as good.

So let's start with the top one.

This is a non example.

This is something we wouldn't want to see in a scientific line drawing.

The first thing is there are sketch lines.

They're not smooth and continuous.

There's also shading, which we know is not allowed.

You have to use stippling.

There were also too many cells, and there's no labelling involved in this diagram either including no magnification at the bottom.

Whereas the second example is a brilliant example of a scientific line drawing.

Reasons why, we've used smooth continuous lines.

There's stippling but not shading.

There are only a few cells.

The cells are labelled and they include a magnification.

So when you do a scientific line drawing, you need to make sure that your diagram looks similar to the one at the bottom.

Let's just quickly check our understanding of that.

So choose two things.

A scientific line drawing should have, A, smooth continuous lines, B, sketching, C, labelling, D, shading.

Hopefully, here we got a smooth continuous lines and C labelling.

We should never have sketching or shading.

Great job if you got that right.

Now we're on to task B of the lesson.

We're going to produce a scientific line drawing of one of your microscope slides.

So this is when you use the microscope to view the tissue.

You are going to make a scientific drawing from that.

But we're going to follow these rules.

Select only a few cells, use continuous lines and no sketching, lines are smooth and joined up, no shading apart from stippling.

Stippling, remember is just using dots.

Make sure our cells are a large size and we use clear labels including the magnification.

I think you're gonna do a brilliant job of this.

So pause the video, have a go, and then press play when you're ready for me to feedback.

Brilliant.

So well done if you've managed to do that.

Just check your diagrams against these two here.

These are examples of good scientific line drawings of onion and cheek cells.

And the reason that they're good, they've used smooth continuous lines, there's no shading, but instead there is stippling, there's only a few cells, and the cells are labelled including the magnification.

So just have a look at your diagrams now and check that you have included all of those things.

Great job.

We are now onto the final part of our lesson, which is magnification and the size of cells.

And we're going to be doing some calculations here.

So let's get going.

Magnification is a measure of how many times an object has been enlarged.

We can calculate magnification using the real size and the image size of a magnified image, and we use the following equation.

Magnification equals the size of the image divided by the size of the real object.

The image size is the picture or the drawing of the cells, whereas the real object is the actual size of the cells.

The real size will be much smaller than the image size.

When carrying out calculations of size from microscope images of cells, we sometimes need to convert units.

So in this table here you can see different units.

We've got metre, millimetre, micrometre and nanometer.

So let's look at how many there are in a metre.

So we'll start nice and easy with 1 metre.

There is 1 metre in a metre, there are 1,000 millimetres in a metre, there are a million micrometres in a metre, and there are 1 billion nanometers in a metre.

Now the important part of this is that we are able to convert between these units.

So if I wanted to go from a metre to a millimetre, I would times the metres by 1,000 to get the millimetres.

If I wanted to get from millimetres to micrometres, I would times by 1,000.

And if I wanted to go from micrometres to nanometers, I would times by 1,000.

However, what if I wanted to go from nanometers back to micrometres? Well, rather than times in by 1,000, I would divide by 1,000.

And if I wanted to go from micrometres to millimetres, again, I would divide by 1,000.

And go from millimetres to metres, divide by 1,000 again.

You might want to pause the video now and make a note of these unit conversions ready for the calculations we're about to do.

So this is our first example of a calculation.

Let's read through the question first.

Calculate the magnification of a red blood cell with a diameter of 8 micrometres in an image measuring 5.

6 millimetres.

So what they're asking us to calculate is the magnification.

So we know that we are going to use the equation, magnification equals image size divided by real size.

Now the image size is given to you as 5.

6 millimetres, but if you look at the real size of the cell, which is the diameter of the cell, it's given as 8 micrometres.

We need to make sure that we're using the same unit.

So therefore we have to convert the 5.

6 millimetres into micrometres.

So 5.

6 millimetres in micrometres, we have to times by 1,000, so it will be 5,600 micrometres.

Now we've got the two measurements in the same units.

We can plug them into the equation.

Remember, the image size is going to be bigger than the real size, so always check that.

So here the image size is 5,600 micrometres, and the diameter, the real size is 8 micrometres.

And when we divide that through, we get an answer of 700.

Then the magnification is times 700.

Let's see if you've understood that by having a go on a check for understanding.

So here says, calculate the magnification of an E.

coli cell with a length of 1.

7 micrometres in an image measuring 10.

2 millimetres.

You can see that they have actually converted the millimetres into micrometres for you already by times it by 1,000.

So see if you can plug into the equation the correct values to calculate the magnification.

So hopefully here you recognise that it was image size divided by real size.

The image size is 10,200 and the real size is 1.

7.

And when you divide those through, you get an answer of 6,000.

Really great job if you got that right.

Now, this is a very similar calculation, but rather than being given the image size and the real size, they've given you a scale bar with the real size, and then you have to measure the scale bar to calculate the image size.

So let's read the question.

The image shows a pollen grain seen through a scanning electron microscope.

Use a ruler to measure the length of the scale bar and then calculate the magnification.

So they have given you the scale bar with the real size, and you will need to use a ruler to measure that scale bar, which will give you the image size.

So here on that ruler, the image size is 2.

5 centimetres.

But remember, we have to have the same units.

They've given the real size in micrometres.

So let's convert our centimetres into micrometres.

Well, first of all, centimetres to millimetres 4 times by 10.

So 2.

5 centimetres is equal to 25 millimetres.

And then to get to micrometres, which times by 1,000.

So the image size is 25,000 micrometres.

And then we just use the same equation we did before, which is magnification is equal to image size divided by real size.

So your sum will be 25,000 divided by 10, which gives you an answer of 2,500, which is the magnification.

Let's check our understanding and give you one to do that's very, very similar.

So here, the image shows a micro fossil seen through a scanning electron microscope.

Use a ruler to measure the length of the scale bar and then calculate the magnification.

This is the ruler here, so you should be able to calculate the magnification now.

Give it your best go, and then we'll go through the answer together.

Right.

So here the ruler is measuring 2 centimetres.

So the image size is 2 centimetres.

We need to convert that now into micrometres.

So times it by 10 gives you 20 millimetres, times it by 1,000, gives you 20,000 micrometres.

The real size is 100 micrometres.

So now we use our equation, which is image size divided by real size, will give you 20,000 divided by 100, which is a magnification of times 200.

That is absolutely brilliant if you managed to get that right.

Well done.

We're going to do a third type of calculation now, we're using the same equation, but we're just changing the subject.

This time we're trying to calculate the real size.

So if we look at the question, calculate the real size of a mitochondrion when the image size is 3 millimetres and it is magnified times by 5,000.

So in our equation now, the real size is going to be the image size divided by the magnification.

So let's look in our question again.

The image is 3 millimetres.

So it's going to be 3 divided by the magnification, which is 5,000.

So 3 divided by 5,000 is 0.

0006.

And if we write this as standard form, it will be 6 times by 10 to the minus 4.

However, let's say we wanted to convert that back into micro metres, we would have to times by 1,000.

So 6 times 10 to the minus 4 times by 1,000 is equal to 0.

6 micrometres.

So in this example, we don't do the unit conversion until after we have calculated the real image size.

Let's see if you can have a go.

So calculate the real size of the coronavirus when the image size is naught 0.

84 millimetres and it is magnified 7,000 times.

In the first part of this, just calculate the real image size in millimetres.

Let's see if you can do that.

Okay, so the image size here is 0.

84 millimetres and the magnification is 7,000.

So you will use the equation, real size equals image size divided by magnification.

So 0.

84 divided by fat 7,000 is equal to 0.

00012 or 1.

2 times 10 to the minus 4 millimetres.

Second part of this, can you now convert that answer into micrometres? Okay.

So hopefully we just straightaway thought we need to times this by 1,000, which gives us 0.

12 micrometres.

Brilliant job.

Those are the three different types of calculations we're looking at today.

You've done an amazing job.

Let's have a look now at doing a task.

So your job now is to answer these questions.

Now my hint to you is to look at what it's asking you to calculate in the question.

Is it magnification or is it real size? And have a look at the units.

We need to make sure that the units are consistent.

I'm sure you're gonna do a great job.

Pause the video and then press play, ready for me to feedback on all the answers.

In the first example, we're calculating magnification and you've got a length of 0.

8 micrometres and an image measuring 14 millimetres.

So first of all, we need to make sure that the units are consistent.

So we convert 14 millimetres into micrometres, which makes 14,000, and then we use image size divided by real size.

So 14,000 divided by 0.

8 to give us a magnification of 17,500.

For the second example, we're calculating magnification again.

We have a length of 1.

2 millimetres in an image measuring 15 centimetres.

So in this example, again, we need to make sure that we convert our units from centimetres to millimetres.

So 15 centimetres times by 10 is 150 millimetres.

And then again, we do image size divided by real size, which gives us a magnification of 125.

For number 3, this time we're calculating the real size and they've given us an image size of 30 millimetres and a magnification of 400.

So we're going to do image size divided by magnification here.

The image size is 30, the magnification is 400.

It will give us an answer of 0.

075 millimetres.

In question 4, calculate the real size of the bacterium.

This time we're calculating real size again, they've given us an image size of 0.

25 millimetres and a magnification of 2,000.

So we're going to do the image size divided by the magnification again, which gives us 0.

000125 millimetres.

And written in standard form, that's 1.

25 times by 10 to the minus four millimetres.

If we did want to convert that to micrometres, it would be 0.

125 micrometres.

And then the final question, calculate the image size of a human hair strand with a real size of 20 micrometres and a magnification of times 500.

So we need to do a rearrangement of this equation.

Image size is going to equal real size times by magnification.

So we will do 20 micrometres times by 500, which is equal to 10,000 micrometres.

And if we wanted to convert this into millimetres, we would divide by 1,000, which would give us 10 millimetres.

An amazing job if you managed to get all of those questions right.

Remember to practise unit conversions and look at what it is that the question is asking you to calculate, and then you'll do really, really well.

Let's just summarise everything that we've learned today.

So in summary, a light microscope can be used to observe animal and plant cells.

Observations from a light microscope can be recorded in a labelled scientific drawing.

And we spoke about that drawing and the rules around it.

So you only use a few cells, smooth single lines, no shading, only stippling, make sure that your cells are labelled and that the magnification is included.

We can also calculate the magnification and real size of structures observed with a microscope, and we use that using the following equation, magnification equals the size of the image divided by the size of the real object.

And remember, that we might have to do some unit conversions as part of all of this.

You've done a really great job today.

Well done.

I'm looking forward to seeing you next time.