Hello and welcome to this lesson on convex or converging lenses.

This is from the unit called electromagnetic waves.

My name is Mr. Norris.

Now I'm sure you'll be familiar with lenses from lots of different applications, especially something like a magnifying glass.

So if I pop a magnifying glass up here, we can see it makes a lovely magnified image of what's on the paper.

But you might be less familiar with a magnifying glass creating an image like this.

Now, what's happened there to our person? So the magnifying glass has now distorted that image in a different way.

And so there are different kinds of lenses which can create different kinds of images in different conditions.

And these lenses are different.

Again, if I hold this up to my drawing, something like that, you can see it produces a different image again.

Actually has the opposite effect of the magnifying glass.

So lenses are very, very important.

They help people see, and they've got lots of other important applications too in cameras on smartphone cameras.

So let's look at how lenses work.

The outcome of this lesson is that hopefully by the end of the lesson you'll be able to use a converging lens to form an image on a screen and describe how the image can vary depending on the object distance.

Here are some keywords that will come up this lesson.

Diminished converging, principal focus, focal length and lens power.

Each word will be explained as it comes up in the lesson.

This lesson has three sections.

The first section looks at how we can create an image on a blank screen using a converging lens.

In the second section, we'll investigate the images formed by a converging lens, and in the third section, we'll draw some conclusions about how the object distance affects the image that conform.

Let's get going with the first section.

So on a bright day, the view outside a window can actually be projected onto a blank screen using only a lens.

So here's an example view from outside a window.

There's a nice shed in the background there.

And here is a projected image on a blank screen.

In this case, I'm just using a bit of wall in a room as my blank screen and the light passes through the lens.

I'm just using a magnifying glass, which has a lens in it, and the light passes through the lens and that creates this projected image on the wall, which is my blank screen, which is upside down and diminished, which means smaller than life size.

You can see the shed and the sky.

The shed is at the top of the image and the sky is at the bottom of the image.

So the image is upside down and the word for that is inverted.

Here's a closer view of the image that I created using that lens.

It's a full colour image of the view from outside my window.

So lenses can be different shapes.

This kind of lens is called a converging lens because it makes light rays converge, travel towards each other, so they meet or cross to create an image on a screen.

Whereas this kind of lens that's this shape is called a diverging lens because it makes light rays diverge, spread apart.

And this line is called the principal axis through the lens.

And diverging lenses also have a principal axis, the dash line that kind of passes through the middle of diagrams like these.

And each kind of lens has a symbol.

This is the symbol for a converging lens.

And you can kind of see if you were to join the two ends of the arrows together, it would trace out the shape of a converging lens.

And this is the symbol for a diverging lens.

And again, you can see it kind of matches the shape of a diverging lens.

So make sure you know the symbols for the lenses as well as their shapes.

Let's do a check.

Which of these are converging lenses? Choose as many as you think.

Off you go.

I'll tell you the answers now.

A, B, C, and F all represent converging lenses.

You might not have identified C, but C is a converging lens because it bulges outwards like converging lenses do.

They don't bulge inwards like diverging lenses do, like lens D and lens E.

So how do lenses work? Well, lenses work by refracting light, and lenses actually can refract rays of light twice.

So once at the air glass boundary, assuming the lens is made of glass, often they're not actually, they can be made of clear plastic or other materials, polymers.

So when light refracts at the air glass boundary, the light will refract towards the normal and then light will refract again when it leaves the lens.

So the glass air boundary, which is refraction away from the normal, but because of the shape of the lens, the light is refracted in the same direction both times.

So those two instances of refraction, they're often represented as just one single refraction that occurs at the centre line of the lens.

And we could also represent that using the symbol for this kind of lens as well, like that.

So how can this kind of lens form an image on a blank screen? Well, this is the starting point.

Light from the surroundings reflects in every direction from every point on an object.

It's important to remember that.

So that point on the object is reflecting light in all directions.

And so is that point.

And so is that point.

And so is that point.

And so is every point on that object.

So many rays of light from each point, will pass through the lens.

So from the top point on the object, all of those rays of light and the rays of light in between which we've not drawn, will all pass through the lens and the same from that point, all of those rays of light and the ones in between will pass through the lens.

All of those rays of light reflected from the object from that point will pass through the lens.

And all of the rays of light from that point on the object will also pass through the lens and all the rays of light in between those that we've not drawn.

And here's the important bit.

All of the rays from the same point on the object are refracted so that they meet at a set distance behind the lens, say here.

And if you place a screen at that distance, that will create a bright spot there from all of the rays of light from the original point on the object hit the screen there.

So there's a lot of light rays all from the same point on the object.

So they'll kind of all be like the same colour of light being reflected from the object hitting the screen there.

But light from every point on the object is focused to its own bright spot at the same distance where the screen is, where we've just put the screen behind the lens and that's what creates the lip image.

So from this point on, the object light is being reflected from that red apple that's in the tree.

So it's mainly red light being reflected.

So all of the rays of light that pass through the lens get refracted to there, creating that red spot on the screen.

And from this part of the object from this part of the tree is mainly green light being reflected.

So all of the rays of light, which pass through the lens, are going to be refracted to its own point on the screen there.

And all of the rays of light, which are reflected from this point on the object, that pass through the lens, they're all refracted to their own point at the same distance, which is where we've put the screen, creating a point of light there.

So you can see how the image is built up by all of these different points of light from every point on the object, like that.

And you end up with an image of the object on the screen.

But notice how the bottom of the object, light rays from the bottom of the object, they get refracted to the top of the image and light rays from the top of the object, the top of the tree, they get refracted to the bottom of the image.

So that's why the image ends up inverted when you create an image on a blank screen using this kind of lens.

Let's do a quick check of what we just explained.

Which of the following helped to explain how a converging lens produces a sharp, non blurry image on a blank screen behind the lens? A, B, C, and D.

Pause the video now.

Read through the options and decide which of those do you think helps explain how a converging lens produces an image on a screen.

Pause the video now.

Off you go.

Okay, I'm gonna go through the answers now.

So lenses form images by refracting light.

So A is correct, that's your basic idea.

B is false because converging lenses don't cause ray to spread out 'cause spread out is diverging, diverging lenses cause rays to spread out.

So B was false.

And then C and D do help to explain how a converging lens forms an image on a screen because all the rays from one point on the object arrive at a single point on the screen and the rays from the different points of the object arrive at corresponding points on the screen at the same distance from the lens where you've got your screen.

Well done if you've got all three of those.

So you've got to put the screen in the right place where the rays of light from every point on the object are focused to at the correct distance.

So this image shows a screen placed at the wrong distance because on this screen, rays from one point on the object don't arrive at the screen in the same place they arrive at the screen in different places.

So that means the image on this screen is blurred and the same for an image on this screen that's in this position.

So where do you need to put the screen? You need to put it in this position so that rays from one point on the object all arrive at the screen in one place and that's gonna create a sharp, non blurry image with the screen in the right position.

So how do you work out the right distance? Well, the correct distance between the lens and the screen for a sharp image, it depends on two things, the power of the lens, and secondly the distance of the object from the lens.

So if an object is too close to a lens, then the rays reflected from that object are diverging and the lens might not be powerful enough to make them converge.

Rays for objects which are slightly further from the lens are less diverging, and the lens might then be powerful enough to refract the rays enough to bring those rays together so they converge to a focus, a point where they cross.

However, rays from very distant objects are effectively parallel when they arrive at the lens.

So this can take a little moment to get your head round.

So have a look at the diagrams. So if those rays were to continue over a very large distance, then effectively that central ray, we could think of it as being made up of parallel rays when they reach the lens after a very large distance.

So rays that are parallel to the principal axis of a lens.

They're focused to a point called the principal focus of a lens, this point here.

So that's where parallel rays rays that are parallel to the principal axis are focused to the principal focus of the lens.

That's where parallel rays of lights are focused to.

And the distance from the centre of the lens to that principle focus where parallel rays of lights are focused to, that distance is called the focal length of the lens.

So an object can be classed as distant if it's basically very many times further away than the focal length of a lens.

Which of the following diagrams correctly shows the principal focus labelled as pf and the focal length of the lens? Only one of these diagrams correctly shows the location of the principal focus of this lens.

Which diagram is it? A, B, C, or D? Pause the video.

Now make your decision.

The answer is A, and it's only A because the principal focus is where parallel rays of light are focused to and the rays of light are not parallel instant on the lens in B, C, or D.

The rays of light instant on the lens are only parallel in A, okay, so the rays are brought to a focus in all of these options, but it's only the principal focus in A, because the principal focus is where parallel rays of lights are focused to.

Let's talk about lens power now.

So this second lens on the screen is wider.

It has greater curvature as a fatter lens, and that increases its power.

That means it refracts light through greater angles.

So it's got a shorter focal length.

However, this lens has the same curvature as the first lens, but it's got a greater power and a shorter focal length because it's made of a different material that refracts the light more.

So a material where the speed of light is even slower.

So it's got, it can refract the light through greater angles even though the lens has got the same curvature.

So these are the two factors that affect the power of a lens, the curvature or fatness of a lens or the material that the lens is made from and how much that refracts light.

Let's do a check on that.

Use the words greater or smaller to complete the following sentences about the power of a lens.

The power of a lens is greater if, and then read the three bullet points and fill in each blank with either the word greater or smaller.

Off you go.

Okay, I'll go through the answers now.

Pause the video if you need more time.

So the power of a lens is greater if the material it is made from refracts light through greater angles.

The power of a lens is also greater if the lens has greater curvature, if it's fatter.

So it has to be more curved than if it's thinner.

And the power of a lens is also greater if the focal length is a smaller distance from the lens.

Well done if you've got all three of those.

Okay, we're ready to do the first task in the lesson now I would like you to make an approximate measurement of the focal length of a lens and the focal length is given the symbol F.

So you need to collect a converging lens, a ruler and a blank sheet of paper, and then dim the lighting in the room.

You need to identify a bright object like a lamp or an object outside a window on a bright day and make sure it's at least 1.

5 or two metres away.

So at least that distance.

So it should be many times further away than the focal length of the lens.

And the object therefore then counts as a distant object and rays of light are kind of almost parallel, which are, which enter the lens from that object.

So step three, hold up the lens between the distant object and the paper, like in the diagram.

Then adjust the distance between the lens and the paper until the clear image forms on the paper.

Because the object is distant, then the distance between the lens and the screen should be equal to the focal length of the lens.

Step four, ask another person to measure that distance, which to good approximation should be equal to the focal length of the lens.

So pause the video now and have a go at that task.

Now the exact result you'll get for that task will depend on the focal length of the actual lens that you used.

So here are some example results.

Here's the view from a window.

Here's the image of a window on paper with a ruler held up.

And here is the position of the lens compared to the screen.

So it's about 14.

4 centimetres from the screen, but we need to add on naught 0.

8 centimetres for the end of the ruler.

So 15.

2 centimetres, so about 15 centimetres for an approximate measurement of the focal length of this lens.

So that takes us to the second section of the lesson.

And what we're gonna do in this section is investigate the images that can be formed on a screen by a converging lens.

So a converging lens focuses rays from a distant object that's a parallel to the principal axis to the principal focus, the point marked pf on the screen.

And we've seen that the projected image is inverted, which means upside down and diminished so smaller than life size.

But you're gonna investigate the images formed when the object is closer to the lens.

So the objects will be a light bulb, a torch or a lamp.

What you're gonna do is you're gonna vary the distance of the object to the lens.

So you're gonna start with a large distance or a large object distance and then move the object closer to the lens.

And each time you're gonna work out or measure the distance from the lens at which a sharp image appears on a screen, and that's called the image distance.

You'll also record the nature of each image.

Is it magnified or diminished? And is it upright or inverted? So the following setup can be used in this experiment.

We've got a lens in a holder that's fixed in position, there's a bulb in a holder and that should be movable along that metre rule on the left to set the object distance.

You then should have a screen which is movable along that second metre rule to find where a sharp image forms and a box with a white side can work well.

And that's the image distance where a sharp image forms of your object.

Consider these two predictions.

Here's the first one.

Lucas suggests that the ray diagrams are always gonna be symmetrical because he's noticed that symmetry is important a lot of the time in physics.

So the closer the object to the lens, then the closer the image will form.

That's his prediction.

But Sophia suggests something different.

She suggests that rays from a closer object.

When the object is closer to the lens, she's noticed that they're more diverging.

So after the lens applies the same amount of refraction, she thinks the rays will meet and the image will form at a point further from the lens.

Whereas Lucas thought the closer the object, the closer the object distance, the closer the image distance.

Sophia thinks the closer the object distance, the further the image distance.

Whose prediction do you agree with? Take a moment to consider that now.

Or you might think something different.

Let's do a check on a possible sources of error in this investigation because this will show that you've understood what you're gonna do in the experiment.

So which of the following could be sources of error in this investigation? Pause the video, read through each option and decide which you think could be sources of error in this investigation.

Off you go.

I'm gonna give some feedback now.

I'll take each one in turn, it may be difficult to judge the position of the object, especially if you use a light bulb as the object because the bulb filament is inside the bulb.

So you'll have to judge that.

You might only be able to judge that to within half a centimetre, for example.

That's definitely a source of error because it's difficult to judge.

B, different parts of the object, different parts of the bulb filament may have parts at slightly different distances, making it hard to judge a single object distance.

That's another reason why your object distances might not be exactly what you want 'em to be.

So that's a source of error too.

Well done if you've got that, C, there may be a range of screen distances at which the image looks sharp causing uncertainty in the image distances.

That's true.

And the bulb could get hot.

Well that's not a source of error, that is a safety consideration that you need to be aware of.

So well done if you identified A, B, and C could be sources of error in this investigation.

Okay, so it's now time to do that investigation.

So step one, set up the equipment as shown, including a power supply and wires for the bulb if needed.

And then dim the room lighting.

Step two, place a screen at a distance equal to the focal length, which is given the symbol F of the lens.

And that's the focal length you found in task A.

So use the same lens that you used in task A, so you already know the focal length of this lens.

And then by placing the screen there, that should be the image distance where an image should appear if an object is distant from the lens.

Step three, using trial and improvement, adjust the position of the bulb to find the greatest object distance that produces a sharp, bright image on the screen when the screen is at a distance of or close to F the focal length.

You can adjust the screen slightly if you need to, but the image should appear somewhere close to F, if not exactly at your predicted F.

Step four, record that first result in a table as shown below.

Step five, slowly move the bulb closer to the lens, reducing the object distance until there is a measurable change in the image distance by say one centimetres or more.

So what that will involve doing is moving the bulb closer to the lens a little bit and then moving the screen a little bit to see which way the image moves.

And then keep on adjusting the bulb, the object distance to the lens and the screen distance to the lens until you've moved the bulb closer and you've moved the screen to a new position to get the image distance at the new position of the bulb.

Then step six, record the new results for object distance, image distance and the details of the image in the table.

And repeat step five until the object is as close to the lens as possible.

Ideally within one focal length.

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

So here are some results from this investigation.

And this was done for a lens with a 15 centimetre focal length.

But you might have used a lens with a completely different focal length.

So your results might look different, but hopefully the overall pattern should look roughly the same.

The first pattern that should be the same is that as the object distance decreased, the image distance should have increased.

So moving the object closer to the lens pushes the image further away from the end from the lens.

Another thing you should have seen is that the at the greatest object distance, so the first object distance you tested, you should have had a very small image, but as the object distance decreased, then the size of the image should have increased towards a similar size at a certain point.

But then beyond that point, if you decrease the object distance further, so made the object closer to the lens, then you would've found that the image became magnified.

So the image started as diminished when the object was very far from the lens.

But as the object moves closer to the lens, the image moves further away and gets bigger and bigger.

So it goes from diminished to the same size as life size and then becomes magnified and then continues to increase in size if the object distance is brought closer and closer to the lens.

Until that is that the object distance is brought so close to the lens that it's about only about a focal length away.

And at that point and beyond, no image can be formed.

So you shouldn't have been able to form an image on a screen if you brought your object to the focal length or closer to the lens.

Well done if your results broadly fit those patterns.

So let's draw some conclusions then about how the object distance affects the image.

So the results could be plotted on a graph like this.

It would need to be a line graph because the independent variable, which is object distance, is continuous.

The values that object distance can take run on a continuous scale of numbers.

It doesn't come in categories, it's not a categoric variable.

So we don't do a bar chart, we do a line graph 'cause the independent variable is continuous and the graph clearly shows one of the key trends from the results, which is that the greater the object distance, the shorter the image distance was.

Or if we looked at the other way around the way we did it in the experiment, the shorter the object distance, the greater the image distance.

So we saw in the experiment that when the object distance decreased, both the image distance and the image size increased, and that was until the object distance equaled the focal length.

We'll look at what happened when the object was brought to the focal length or closer to the focal length if you got that far in your experiment in a moment.

And what we also saw is that the image formed on screen by a converging lens, it's always inverted like we saw from the very start of the lesson.

So the image actually switches from being diminished to being magnified when the object distance is 2F.

And that's when the animation pauses.

It pauses at when the object distance is 2F because that is the object distance where the image is the same size as the object at 2F, double the focal length there.

So that's when the image switches from being diminished to being magnified.

And as I just said, when the object distance was at double the focal length, the image distance was also double the focal length.

So the object distance was equal to the image distance and also the object.

The image size was equal to the object size in real life.

So it was neither magnified nor diminished at that distance.

And then when the object distance decreased from 2F from double the focal length, the image distance and size both increased at an increasing rate.

They increased very quickly with tiny increases and distance towards the lens until the object distance reached the focal length.

So let's look at that now.

But when the object was at the focal length, the refracted rays became parallel.

And if the object is closer than the focal length, the refracted rays weren't converging anymore.

They were diverging here, diverging.

So in both of those cases with the object at the focal length or closer to the lens on the focal length, no image can form on a screen as the rays from a single point on the object, then never meet or cross after being refracted by the lens.

Okay, let's do a check on the conclusions from this experiment.

Identify the correct option from each pair of words.

Pause the video now and have a go at that.

I'm gonna go through the answers now.

So with the large object distance, the image was diminished and close to the principal focus of the lens.

Well done if you've got both of those, but then as the object distance decreases, then the image distance and the image size both increase.

Well done if you've got that.

Okay, so it is time for a task now to describe how the object distance affects the image.

So the task has two parts.

First part, complete the table to describe how the object distance affects the image formed on a screen by a converging lens where F stands for focal length.

And then part two is really just something to look out for in the task because there are two object locations, so two rows in the table where no image forms on a screen.

So there's actually nothing to fill in for those two rows on the table because no image forms on a screen.

But you should use those rows to explain why no image forms on a screen with the object in those two locations.

So pause the video now and have a go at completing that task with your best effort.

I'm going to give some feedback now on that task.

When the object is very distant, the image is found at the focal length at F, it's a diminished image and the image is always inverted, so I'm gonna miss out that column for now.

Then when the object is brought closer, but it's still further away than 2F, the image moves backwards from F towards 2F, the other side of the lens, the image is still diminished, but it's increasing in size as the object is brought towards 2F.

Then when the object is at double the focal length at 2F, the image is at 2F and it's not magnified or diminished is life size.

Then when the object is brought closer to the lens from 2F to F, the image becomes further and further away and it's magnified.

And then when the object is put at the focal length or closer than the focal length, then no image forms on a screen because after a fraction by the lens, the rays are parallel or diverging so they never cross or meet.

Well done if you've got answers along those lines, make any improvements to your work now.

Here's a summary of the lesson.

Converging lenses can produce an inverted image on a screen by refracting light.

Rays of light from distant objects, which are parallel to the principal axis are refracted to a point called the principal focus, which is located at a distance from the centre of the lens called the focal length, which is given the symbol F.

A greater power lens has a shorter focal length.

The image distance and size or magnification depend on the power of the lens and the object distance.

As the object distance decreases, the image distance and size both increase until the object reaches the focal length or closer.

At these distances, the rays leaving the lens are parallel or diverging.

So cannot form an image on a screen.