Lesson video

Hello, my name's Dr.

George, and this lesson is called Focusing Light and it's all about how lenses make images.

It's part of the unit Making Images.

The outcome for the lesson is I can explain how light from an object can be focused by a converging lens to form an image.

And I'll help you to understand what this means and how it works by the end of the lesson.

Here are the key words for the lesson.

I'll introduce them and their meanings as we go along, but you can come back to this slide anytime if you want to check the meanings for yourself.

The lesson has two parts called focusing light to form an image and how a convex lens refracts light.

Let's get started.

A luminous object, that's an object that makes light, emits light from every point in every direction.

Here we have light rays showing how just three of those points are emitting light in some different directions.

We can't draw all the possible light rays because they cover the entire image.

And here's a non luminous object, an object that doesn't make light.

And every point on that object reflects light in all directions.

That kind of reflection we can call scattering.

Now if we have a lens, some of the light that's coming from objects can pass through that lens and it gets focused to form an image.

We'll see more about what that means soon.

And the more light passing through the lens, the brighter the image will be.

So remember, an image is a representation of the real thing.

It looks like the real thing but isn't.

And lenses are used in many different ways.

For example, a camera uses a lens to focus light, bring light together and form an image, and that image can be recorded.

A projector which makes a large image on a screen also uses a lens to do that.

And the eye itself uses a lens that's inside it to focus light from objects so that it makes an image at the back of the eye.

Let's say it's a bright day, we could hold up a lens facing the window, as this person is doing, and hold up a blank screen behind it and we can get an image of the view through the window on that screen.

You could say it's a live image.

It's showing what's happening right now.

It's not a recorded image that we can keep.

But this image is upside down.

We can use a word for that, inverted.

And it's in full colour.

And the reason the image is produced is because lens refracts or bends light rays.

And we'll look at that in more detail later on.

So now here's a short question.

How do lenses focus light to form an image? Do they reflect light, refract it, magnify it, absorb it, or emit it? With these short questions, I'll wait five seconds, but you may need longer.

In which case, press pause and press play when you've chosen your answer.

The correct answer is lenses refract light.

They change its direction as shown in this diagram.

A convex lens is a type of lens that refracts light rays so that they travel more towards each other, as shown here.

And these three light rays are being brought to a point on the right, and the point where they meet is called a focus.

So we're actually using the word focus in two different ways.

It can mean a point where rays meet, but we also use it to mean bring light together.

A lens focuses light.

A convex lens can also be called a converging lens.

Converge means come together or bring together.

There's another kind of lens which is a concave lens.

You can see it's a different shape and a concave lens spreads light rays apart.

It makes them move more away from each other.

And when this happens, the rays can't come together to form an image on a screen.

A concave lens doesn't bring light to a focus.

Another name for concave lenses is diverging lenses.

Diverge means move apart.

We're going to be thinking about convex lenses today.

And convex lenses can have some different shapes.

So on the left there's a circular convex lens.

It bulges out in the middle.

On the right is what's called a square convex lens, but it still bulges out in the middle and it can still make images.

And convex lenses can bulge on both sides or just one as you see in the bottom right picture.

So which of the following show convex lenses? Pick out the ones that do.

Press pause if you need to while you think and press play when you're ready.

And there are four representations of convex lenses here.

All of the ones that bulge out either on both sides or one side.

A and E are concave lenses.

Now a lens will focus light, bring light together from different objects to different places depending how far away each object is from the lens.

So you can see here that the further away the tree is, the closer the focus is to the lens.

For an object that's a long distance away, not much of the light from it will hit the lens, but the rays that do hit the lens will be parallel or very nearly parallel.

And the distance from the lens to the focus is shortest for those most distant objects.

That distance from the centre of the lens to where the lens focuses light from a distant object is called the focal length.

So we know that this light is coming from a distant object because it's parallel or very nearly parallel.

If we have a wider thicker lens, it refracts light more, more strongly than a thinner lens.

So it's bending the light through a greater angle.

That means the lens has a shorter focal length and we would describe it as a more powerful lens.

You can also make lenses out of different materials.

Not all lenses are made of glass.

The lenses in my glasses, for example, are actually made of plastic.

And some materials refract light more than others.

So there are materials that refract light more than glass.

And using these materials, lenses with the same focal length, can be made thinner.

And in fact, that's one of the reasons why the lenses in glasses are often made of plastic because they can be thinner than glass lenses would need to be.

So here's a question for you.

Which properties make a lens more powerful? Looking it wider, more curved, thinner, less curved.

Which one of those then refracting light more or refracting light less, and longer focal length or shorter focal length.

So pick one of each pair.

Press pause and press play when you have your answers.

A more powerful lens is wider, refracts light more, and has a shorter focal length.

Well done if you pick those.

So here we have a lens bringing together three rays of light from a point at the top of this tree.

If we want a clear image of the object form on a screen, the screen needs to be placed where the rays from that object come to a focus.

So not here, not here, but here.

And that's so that all the light rays from a single point on the objects that go through the lens will meet at one single point on the screen.

So those three light rays from that point at the top of the tree are all meeting at one point on the screen.

And so will any other light rays coming from that point.

In fact, if you simply hold up a screen to an object, even if it's a bright object, and you don't use a lens, then you'll simply see brightness all over the screen because light from every point on the object is landing at every point on the screen and that doesn't form an image.

If you put the screen in the wrong place, what you'll see using a lens is you'll see a blurred image, a fuzzy unclear image, and that's because the light from a single point on the object is now spread over an area of the screen, as you can see with the three ways shown here.

And so light from that point on the tree will overlap with the light from many other points on the tree and we won't see a clear image.

Now, as I said earlier, every point on an object reflects light in all directions and in our diagrams we're only going to show some of those.

So some light from every point on the object hits every part of the lens surface, like this, from this point at the top of the tree, and like this, from this point at the bottom of the tree.

And every ray that passes through the lens from the same point on the object, e.

g.

, top of the tree, is refracted to the same single point on the screen.

So here's what's happening to light from that point at the top of the tree, it ends up here on the screen making a bright spot of light there that is the corresponding part of the image.

It's it's the point on the image where we're going to see what looks like the top of the tree.

And here is what happens to light from that point at the bottom of the tree, it's all brought to a point somewhere else in the image.

And now here's light coming from what looks like perhaps an apple in the middle of the tree.

That's also all brought to a point somewhere else.

And some light from the bottom of the leaves brought to a point over here.

And you may have noticed that we're going to get an image that is inverted.

It's upside down, because light from the top of the object is refracted to the bottom of the image, and light from the bottom of the object goes to the top of the image.

Now have a look at this picture.

We have a lens forming an image of a penguin on a white screen.

Choose the correct option to complete each sentence.

So there are two sentences to complete here.

So press pause while you read those and press play when you've chosen your answers.

The first sentence should say light from each point on the penguin moves in many directions towards the lens.

Non luminous objects scatter light in all directions from every point on their surface.

And the second sentence should say, a single point on the image is created by many rays of light that pass through the lens all from the same point on the penguin.

Well done if you picked those out.

If we use a large lens and a smaller lens and they have the same focal length, we find that the image is the same size for both lenses.

You don't get a smaller image if you use a smaller lens, but the image is not as bright with the smaller lens.

So lenses with the same focal length produce images that are the same size no matter how small the lens.

Both lenses refract light from the same point on the object to the same point on the screen, as shown here.

But the smaller lens makes a dimmer image than the larger lens because less of the light from each point on the object passes through the smaller lens, and so it is brought to the image on the screen.

Andeep makes an image on a screen using a convex lens.

What happens to the image if a smaller lens is used with the same focal length and nothing else has changed? Everything else is the same.

Will the new image be smaller? Will it show less of the object? Will it be dimmer? Will it form further from the lens? Press pause if you need time to think and press play when you're ready with your answer.

And the only correct statement here is the new image will be dimmer.

It will be less bright, but it'll be the same size as before.

It will still show the whole object and it will be the same distance from the lens.

Now it's time for a little experiment.

You're going to dim the lighting in the room, so switch off the light and stick a sheet of paper onto the wall opposite a window.

Or if the wall is plain, you could just use the wall itself as a screen.

Then use a convex lens to produce an image of the view that's outside the window landing on the sheet of paper.

So you'll have to move the lens to different positions until you can get a reasonably sharp, clear image.

Then you're going to predict what you think you'll see when half the lens is covered and explain why you think that's what you'll see.

And then try it.

Cover half of the lens with black paper.

Make an image again of what's outside the window and see what happens.

Now think about whether your prediction was correct and do you think now that your explanation was correct? And if not, write an improved explanation that explains what you saw.

So take as long as you need, press pause while you're doing this, and press play when you've finished.

I share an example of the sort of thing you might have seen.

Here's a view from a window and here's an image formed on the opposite wall by a convex lens.

You might be able to see that the image is upside down, it's inverted compared with the real thing.

And here's an example prediction and explanation.

Prediction, with half the lens covered, the whole image will still be visible, it will still be inverted and the size won't change either.

However, the image will be dimmer.

And how could you explain that? Some light from every point on the object hits every part of the lens.

All the rays that pass through the lens from a single point on the object are focused to a single point on the screen forming the corresponding part of the image.

With half of the lens covered, only half of the rays from before can now get through the lens.

So each part of the image still forms in exactly the same way as before, except from less light.

So well done if you made a correct prediction and got some of those points into your explanation.

And now for the second part of this lesson, refraction of light by a convex lens.

We're going to look more closely at what happens when light rays passed through lenses.

For that we need to remember what refraction is, is a change in direction of light when it passes from one medium, one material, to another.

On the left you can see a beam of light being refracted twice, once when it enters the glass and again when it leaves.

And you can draw imaginary lines, dash lines called normal lines, that are at 90 degrees to each surface where the light ray hits, as shown here.

On the right, a beam of light is going through the glass without being refracted.

There's no change of direction.

And that's because if light travels along the normal, then there is no refraction.

The greater the angle of a ray of light to the normal line, the more the ray is refracted.

Have a look at these three pictures and can you see in which one is there the greatest angle between the incident ray and the normal line? It's the picture on the right.

And can you also see that that's the one in which the ray is bent the most, both entering and leaving the glass.

So the greater the angle, the more the light is refracted.

Now have a look at these three blocks of glass.

There's a ray of light passing into each of them, which block is going to refract the ray the most? Press pause while you're thinking and press play when you've decided.

The answer is A.

And we can see that most clearly in this top view.

In C, the ray is actually travelling along the normal.

There'll be no refraction at all.

In B, there's only a very small angle between the ray and the normal.

So there's not much refraction.

In A, the angle between the ray and the normal is greatest, so the ray changes direction the most.

We can put together glass blocks like these to make something that looks a bit like a lens.

And each of these glass blocks will refract light differently, and together they can bring the rays of light to a focus.

The light actually refracts towards the normal when it enters the glass and away from the normal when it leaves the glass.

A convex lens can do the same thing.

A convex lens is just a cleverly shaped piece of material that will refract different rays in just the right ways to make them meet as a focus.

And here's the focal length, the distance between the centre of the lens and the point where the rays meet for rays that are coming into the lens parallel.

When parallel rays of light pass through a lens, light through the upper half of the lens is refracted downwards and light through the lower half of the lens is refracted upwards.

Check that you can see that in the picture.

And light through the exact centre of the lens isn't refracted at all.

Also, notice that the further from the centre of the lens, the more a ray of light is refracted.

And as a result, all of these rays can end up meeting at the same point, a set distance from the lens that we call the focal length.

Now, think carefully about this.

Why do wider lenses have a shorter focal length than thinner lenses? Is it because there's more lens to interact with the light? More light can pass through the lens, light hits the lens at bigger angles to the normal or light takes longer to move through the lens? Press pause and press play when you've decided.

The correct answer is light hits the lens at bigger angles to the normal.

If that's not obvious to you, don't worry.

You're going to see more about this in the next task.

So I'd like you to copy these diagrams and complete the rays of light through each of these lenses to show how they each focus the light.

So you're going to bring these rays to a point, and most of the rays should be refracted twice, once when they enter the lens and again when they leave it.

So while you're answering the question, press pause for as long as you need and press play when you're ready.

Here's an example of what you could have drawn.

Notice that each ray is bent twice except for the ray that goes right through the centre of the lens and that's not bent at all.

The second lens is more curved and it refracts the light more strongly, bringing the light to a focus closer to the lens.

We would say it's a more powerful lens.

And now we're at the end of the lesson.

So here's a summary.

Some of the light from objects can pass through a lens and become focused to form an image on a screen.

Some light from every point on an object passes through every part of the lens.

A convex lens refracts light rays so they travel more towards each other.

A point where rays meet is called a focus.

All the rays from a single point on the object are focused to a single point on the screen forming the corresponding part of the image.

Well done for working through this lesson.

There were some slightly tricky ideas in there, but I hope you found it interesting and I hope you now feel that you understand more about how lenses make images.

Bye for now.