Lesson video

Hello, welcome to this lesson on the reflection of light.

This is from the unit called Electromagnetic waves.

My name's Mr. Norris.

Now the reflection of light can produce some interesting effects, even just in a simple mirror.

Oh, so of course a mirror creates a virtual image of whatever's in front of it, and that image is inverted.

So when I hold up my left hand, the reflection of my hand looks like an opposite hand with the thumb on the other side.

And if you move an object towards a mirror, then the reflection moves towards that object, and you can high five your own reflection.

But it's not just mirrors that reflect light.

all kinds of different surfaces reflect light in different ways.

And of course, that's essential to how we see.

So understanding reflection is pretty key to our experience of the world.

So let's look at how light reflects.

The outcome of this lesson is, hopefully, by the end of lesson you'll be able to recognise, draw, and describe how light and other waves can reflect from different surfaces using rays and wavefronts.

Here are some key words that will come up in today's lesson, specular reflection, diffuse reflection, ray, normal, and wavefront.

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

The lesson is divided into three sections.

The first section looks at reflection of light from different surfaces in different ways.

The second section looks at light as waves and light being represented by rays.

And the third section covers the idea that actually it's not just light that can reflect, all waves can reflect.

So let's get going with the first section.

So let's start by looking at this torch, which is emitting light towards this sheet of glass.

Now glass is transparent.

That means most of the light or a lot of the light gets transmitted through the glass.

And the light that's transmitted through the glass, in this case, hits the wall and reflects off the wall in all different directions.

And when the light from the torch reflects off the wall into our eyes, then we see that as the bright spot of light on the wall.

So that shows that a lot of the light from the torch is transmitted through the glass and hits the wall, but some of the light reflects from the surface of the glass.

And if we follow that ray of light back reflecting from the glass, we can see behind it is where it looks like that light has come from, which is from the image of the torch, which is in the sheet of glass.

Of course, the light hasn't really come from there.

The light really come from the actual torch, but our eye can't tell that the light has reflected.

And that's why there appears an image of the torch in the glass.

And of course, glass may also absorb some light, and that would make the glass warm up a little bit.

So there are actually different kinds of glass that can transmit, reflect, and absorb different amounts of light, depending on what might be needed.

For example, for the side of a building that gets a lot of sunlight during the day, you might get very reflective glass that reflects a lot of light away instead of transmitting it through to help keep the building cool.

Let's do a quick check.

What are the three things that can happen when light hits the surface of an object? There are three things that can happen when light hits the surface of an object.

What are they? The light can be, and then three things.

See if you can remember all three.

Off you go.

Let's see if you could remember them.

When light hits the surface of an object, the light can be reflected, transmitted, or absorbed.

And that could be completely reflected, completely transmitted, or completely absorbed, or it could be a mix of all three.

So partially reflected, partially transmitted, and partially absorbed.

And that's more likely in a lot of cases.

So let's look at reflection now.

Reflection is the process of light hitting a surface and bouncing off in a different direction.

And different kinds of surfaces can reflect light in different ways.

Curved surfaces will reflect light.

Still water can reflect light and produce lovely clear reflections.

And of course, a mirror can reflect light as well.

So when light reflects from shiny, smooth, and still surfaces, they can form a clear image in the surface, a bit like that image of the torch.

We could see the reflection of the torch in the glass that we saw earlier.

And obviously, that is also called a reflection.

So reflections that you can see in a mirror, they're also caused by the process of reflection.

So all surfaces reflect light, not just shiny ones.

And that's how we see.

Non-shiny objects reflect light in many different directions.

They scatter light in all directions around it.

Whereas reflection happens differently to this for mirror-like surfaces, reflection's much more regular there.

And that's the reason why you can see an object normally, a non-shiny object from any direction.

If you walk around it, it's always visible.

Of course it is.

Whereas if you want to see the reflection of a particular object in a mirror, that's only visible at the right angle because reflection in a mirror is much more regular in a set direction, whereas reflection from non-shiny objects scatters the light in all directions.

Okay, a check for understanding now in two parts.

First part, which of these can reflect light? A, B, C, or D, which of those do you think can reflect light? Five seconds to decide.

Let's see how you got on.

Well, the answer is all of them.

All objects can reflect light.

But in which of these might a reflection, i.

e.

an image, be seen? Choose which ones you think.

Okay, well, I'm sure you would've chosen A 'cause you can see a reflection in a mirror and there might be a reflection in a coin 'cause a coin can be a bit shiny, but the reflection in a coin might be distorted due to the texture of the coin surface.

And obviously if the coin's quite dull, if it's a bit older, if it's a bit rusty or discoloured on the surface, then that might prevent you seeing a reflection as well.

So definitely A, and you might have said C as well, but not B or D.

You can't see a reflection in a brick or in a sponge.

So all of the objects reflect light, but you can only see a reflection in some objects.

And that's because of the differences in the way light reflects from different objects.

So let's look at that.

So reflection from a mirror-like surface is called specular reflection, and that's shown in this animation.

So when the laser turns on, you can see the beam of light hits a mirror at the bottom surface and reflects.

And you can see the different angles that the light is reflected when the light comes in at different angles.

So specular reflection like this occurs in a regular way.

In fact, there's a law that describes the angles made by the light rays, and we'll look at that in just a moment.

So in diagrams, mirrors are often drawn like this.

Well, not like this right now, but like this.

That's the symbol for a mirror in a diagram.

And a ray of light is a straight line with an arrow on it, showing the direction the light travels.

So the incoming ray of light is called the incident ray because it's going in, it's incident on the mirror, coming into the mirror, incident.

And we measure the angle of this ray from an imaginary line drawn at 90 degrees to the surface that the ray's gonna hit.

So in this case, 90 degrees to the mirror.

And we draw that line at the point where the incident ray hits the surface at 90 degrees to that surface.

And the normal is usually drawn as a dashed line, like we've done there.

This line is called, as I just said, it's called the normal.

And in maths, the word normal means at 90 degrees to.

That might sound a bit strange to you at first, but when you get used to the idea that the word normal means at 90 degrees to, then it starts to become more familiar.

So the normal line is just a line which is at 90 degrees to, it's normal to the surface, at 90 to the surface.

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

Which of these can mean at an angle of 90 degrees to? At a right angle to, perpendicular to, parallel to, or normal to? Take as many as you think which of these can mean at an angle of 90 degrees to.

Off you go, five seconds.

Let's go through the answers now.

Which of these can mean at an angle of 90 degrees to? It was A, B, and D.

Perpendicular is a word you've probably heard from math lessons.

Perpendicular just means at right angles and so does normal.

If something is normal to a surface, then it's at right angles to a surface, like the normal line that we draw in reflection diagrams. So we said we measured angles of light rays to the normal line, the line at 90 degrees to the surface.

So we've got an angle for this light ray labelled now on the diagram, the angle that's labelled i.

It's labelled i because it stands for the angle of incidence, because it's the angle between the incident ray and the normal.

So it's the angle of incidence for the incident ray, the ray coming in.

Now you might be thinking, "Well, why don't we just measure the angle of the ray to the surface?" Well that wouldn't work for curved surfaces.

If we measure the angle to the normal, then we can apply our law of reflection, which is just coming up, to curved mirrors too if we measure the angle of incidence from the ray to the normal, 'cause you can't measure the angle between the incident ray and the mirror accurately with the curved mirror because the mirror is curved.

So there's no angle there.

But there is an angle between the incident ray and the normal, even for a curved mirror.

So that's why we measure angles to the normal.

It's because it makes the law more general.

So it applies in more situations.

Flat mirrors or plain mirrors, as well as curved mirrors.

So now, let's look at what that law actually is.

So the law is about the angle of the reflected ray, which is called the angle of reflection, and it's labelled r in this diagram.

And that angle's also measured to the normal, the line at 90 degrees to the surface.

And here's the law of reflection, which you might remember from earlier studies.

The law of reflection is that the angle of incidence is always equal to the angle of reflection.

So this is for mirror-like surfaces.

That angle of incidence is always equal to the angle of reflection.

That's called the law of reflection.

And this animation shows that the angles are equal.

Look at the normal line, the mirror surface is on the bottom surface of the animation.

The normal line is 90 degrees that surface.

And you can see the reflected ray, the angle of reflection is always equal to the angle of incidence when the angle of incidence changes.

Let's do a check on that.

Which of the following correctly shows the light ray reflecting from the mirror? Diagram A, diagram B, diagram C, diagram D? Remember, you are looking for a ray that obeys the law of reflection here.

Five seconds to decide.

I'm sure you will have said answer A.

Well done if you got that right.

That's the only one way.

If you imagine a normal line being drawn in, whether ray hits the mirror, the angle of incidence would be equal to the angle of reflection.

Well done.

In option C, option C shows a bit of scattering from the surface, and that's not what happens from a mirror.

Mirrors reflect light in a regular way.

One ray comes in, one ray comes out.

Here's another check.

Which of the following correctly shows the angle of reflection? Okay, so not the angle of incidence.

Which of the following shows the angle of reflection in red? Is it A, B, C, or D? Five seconds to decide.

Okay, I'll give you some feedback now.

Now in A, that's the angle of incidence 'cause it's the angle between the incident ray coming into the mirror and the normal.

So it's not A, it's B.

B shows the angle of reflection, because it's the angle between the reflected ray and the normal.

C is a mistake that students often make because it's the angle between the reflected ray and the mirror.

But the angle of reflection is between the reflected ray and the normal in B, not C.

And D is effectively double the angle of incidence or double the angle of reflection because the two angles should be equal, but it's showing them both as one angle, which isn't what the question is asking for.

So very well done if you said B, the angle of reflection.

So let's go into more detail now about the difference between reflection from mirror-like surfaces, which is called specular reflection, and reflection from other surfaces that scatter light.

So specular reflection from mirrors and very smooth surfaces, that's reflection from a smooth surface in a single direction.

So if you look at each ray of light, it's reflected kind of parallel rays of light are all reflected in the same direction 'cause it's reflection from a smooth flat surface, obeying the law of reflection.

Whereas reflection from a rough surface is called as scattering.

So reflection from a rough surface is called diffuse reflection.

So where you've got parallel rays of light coming in, in specular reflection, they were all reflected in the same direction, in diffuse reflection, they're all gonna be reflected in different directions because of the rougher surface.

So that second row of light is actually gonna be reflected over there, obeying the law of reflection at that point on the surface, but this ray of light is reflected over there.

So that all the rays of light, even though they start parallel, they don't end up parallel because of the diffuse reflection from the rougher surface.

So specular reflection is reflection in a regular way from a smooth surface in a single direction, whereas diffuse reflection, reflection from a rough surface causing scattering.

So this is why cleaning, dusting, or smoothing a surface can make it shinier, even if you don't use any cleaning products or polish.

It's because if you clean dust or smoother surface, then you make the surface smoother and you'll end up with more specular reflection and less diffuse reflection if the surface is smoother.

So let's do a check about what we just went through.

Use the words specular and diffuse in the right places to fill the gaps.

So have a look at the picture.

The wall around the door is painted in a white matte paint.

Matte is the word for that kind of finish of paint.

And the reflection is almost all, so it's that specular or diffuse.

Whereas then look at the door and door frame, which are painted in a white gloss paint.

And the gloss paint gives more of which kind of reflection, specular or diffuse? So five seconds to make sure you've got specular and diffuse the right way around in those gaps.

I'm sure you will have got this right.

Let's go through it.

So for the wall paint, the reflection is almost all diffuse, whereas for the gloss paint, you're getting more specular reflection because the light is reflecting more in a single direction, causing that patch of lights on the door because all of the light being reflected in one direction from that point into our eyes, causing that bright patch on the door.

So shinier things reflect light with more specular reflection, and dull or matte surfaces reflect light with more diffuse reflection.

So surface roughness is actually, it's not the only cause of diffuse reflection.

So a piece of white marble for example, like in the picture, you could make it extremely smooth, and that would increase the amount of specular reflection and make it shinier, but it would could never turn into a mirror and it would always be white, not kind of just like a mirror, taking the colour of whatever light it's reflecting.

And that's because single rays of light from a surface like marble, they're actually reflected at many different points just under the surface, and that leads to reflection in multiple directions, which is diffuse reflection.

Whereas specular reflection is reflection from a smooth surface in a single direction.

Okay, polishing that piece of marble could make it smoother.

You'll get more specular reflection, but you can never turn it fully into a mirror because of the way light is reflected from the nature of that surface.

So what that means is if you want to make a mirror, you can't start with any material you like and just make it as smooth as possible.

You have to start with the right kind of material that's actually gives you proper specular reflection when you do make it as smooth as possible, like a metal surface, which is polished metal is the reflecting surface of a mirror.

Okay, let's do a last check on specular and diffuse reflection.

Fill the gaps to complete these descriptions for specular and diffuse reflection.

Off you go.

Pause the video if you need to on this one.

Okay, here are the answers.

Specular reflection is reflection from smooth surfaces in one direction, whereas diffuse reflection is reflection from rougher surfaces in more than one direction.

Well done if you've got those.

Okay, let's do a task now about our work on reflection so far.

So part one of the task, complete and label the diagram to show the law of reflection.

And by the side of the diagram, you should also include a statement of the law of reflection.

So complete the diagram to show a reflected ray, add all the labels that you think you should add to properly show the law of reflection, and then write the law next to your diagram.

Part two of the task, write an explanation this time.

I want you to write an explanation of why you can see a clear image of the torch bulb in the glass, but why can you not see a clear image of the torch bulb on the wall? So pause the video now and have a good go at both parts of that task with your best effort.

Off you go.

Right, I'm gonna give you some feedback now on the task.

Part one, complete and label the diagram to show the law of reflection.

You should have added, labelled the incident ray drawn in a normal, and then you can label the angle of incidence, which is I've used the letter i for that.

Then you can draw the reflected ray.

And a key skill in this task, in this part of the task was to make sure that you drew the reflected ray so that that angle of reflection that I've labelled r looks about the same, it should be equal to the angle of incidence.

Those two angles have to be the same.

And of course, that is the law of reflection, which I asked you to write next to your diagram.

The angle of incidence should be equal to the angle of reflection.

So the angles you've drawn in the diagram should look about equal as well.

So well done if you've done that.

And then part two of the task, explain why you can see a clear image of the torch bulb in the glass but not on the wall.

Well, the glass is a smooth, so specular reflection occurs and the law of reflection is a obeyed, producing a reflection.

And that means an image, producing an image in the shiny surface.

The wall, on the other hand, is rougher, so the reflection from the wall is diffuse.

The incident light is scattered in many directions.

And that's why you don't get an image of the torch bulb in the wall.

That takes us to the second section of the lesson.

In this section, we're gonna look at how light is a wave, but how also we can use rays to represent light, which is what we've just been doing actually in our diagrams of reflection, but let's just look into it in a little bit more detail.

So let's talk about waves.

A wave occurs when something, which we call the wave medium, is disturbed or made to oscillate.

A bit like the end of this string.

It's being disturbed, made to oscillate, but then the disturbance and the oscillation travels.

In this case, from left to right.

You can see the waves travelling.

So this is the defining feature of waves.

They transfer energy but without transporting any of the medium to anywhere new, each part of that rope or that string stays in the same location, whereas the waves have moved from left to right.

This dripping tap disturbs the water surface and creates ripples.

So that was a side view.

And now this is a top view, okay? So that's the side view of the water waves, and now here's the top view.

And again, water ripples or water waves share the defining feature of waves.

They transfer energy without transporting any of the medium.

No water is moving from left to right there.

Each bit of water is just oscillating up and down as the wave travels.

And it's the same with sound waves.

This speaker disturbs nearby air particles, making them oscillate, and that's what creates the sound.

So the sound waves is shown as ripples or pulses in the air, that's here, and then oscillating air particles.

So two different views of the same kind of wave idea.

And it's the same with light.

A green LED produces a light wave.

So a light wave is really a ripple or oscillation in invisible electric and magnetic fields that are all around us, but they work in kind of very, very similar way to how the sound waves and the water waves and the wave on a string work.

Something is made to oscillate, in this case, it's invisible electric and magnetic fields that's all around us, and then it's the oscillation that travels.

Yeah, so the LED disturbs nearby electric and magnetic fields, causing ripples that travel outwards, transferring energy.

And this is one way of picturing what light is.

Let's do a quick check now of what we just said about different kinds of waves.

So state what oscillates for each of the following kinds of waves.

A wave on a rope, a water wave, a sound wave, and a light wave.

What oscillates for each of the following waves? Pause the video now.

Off you go.

Okay, I'll go through the answers.

For a wave on a rope, it's the rope that oscillates.

For a water wave, it's the water surface that oscillates up and down when the wave travels along.

For a sound wave, it's particles.

Might have said air particles, but particles of any gas, any liquid, or any solid.

And then for a light wave, this might be the new learning for this bit, light wave, we said it was electric and magnetic fields, which were invisible, but anything that's giving out light, what we can think of that object as doing when it's giving out light is creating ripples in invisible electric and magnetic fields all around us.

Well done if you remembered that from the previous explanation.

So although light is a wave, it's often more useful and convenient to represent light by drawing rays.

So rays are just lines that we draw to represent the direction that the light waves travel.

So they're drawn using straight lines because light almost always travels in straight lines.

And rays should be drawn with an arrowhead in the middle to clearly show the direction the lights travelling.

Beams of light can be seen if there's dust or droplets of liquid in the air for light to reflect from.

And in everyday life, beams of light like this can sometimes be called light rays.

People often talk about the sun's rays.

But what people are talking about here are a different thing to the light rays that we draw.

'Cause remember, light is not made up of rays.

What we can imagine light as being made up of is waves, oscillations in electric and magnetic fields.

So the light rays that we draw just represent the direction of travel of a beam of light or some light waves.

A single light ray that we draw on a diagram, it doesn't necessarily represent a single light wave.

It represents the direction of travel of some light waves.

So diagrams would become very unclear if you tried to draw rays of light to represent all of the directions light travels in a situation.

Glowing objects, for example, they emit light in every direction from every point on their surface.

So from that point, that point, that point, that point, that point, that point, that point, rays of light from every direction on the surface.

And also, light could be hitting and scattering from every point on an object surface.

So from there, from there, from there, from there, from there.

So in diagrams, we often only draw a few useful light rays to represent the direction that some light is travelling.

Time for a check on what we've just said about light rays.

Identify two mistakes that have been made when drawing these rays.

Five seconds to identify two mistakes.

What should the person have done instead? Off you go.

I'll give some feedback on this.

So first mistake is that rays of light should be straight lines because light almost always travels in straight lines.

That's better.

And the second mistake is that the rays of light should always have arrows, one, two, showing the direction of travel of the light.

Well done if you've got both of those.

Okay, time for a task now.

You need to look at each statement A, B, C, or D, and discuss whether each statement about light rays is correct or incorrect.

And then tick one column that shows how confident you are, whether you are sure it's correct or just think it's correct, whether you think it's incorrect or whether you are sure the statement is incorrect.

So do that now for each statement.

You should pause the video, have a good discussion, and then tick the box that reflects what you think.

Off you go Right, I'll give some feedback now.

Statement A, a light ray is a long thin piece of light.

That's not true.

Light rays just represent the direction light travels.

B, light is really made up of light rays.

That's not correct.

We can think of light as being oscillations in electric and magnetic fields, and that's one way of picturing what light really is.

Statement C, light ray show the direction light travels.

That's correct.

And statement D, light can be thought of as a wave.

Yes, ripples in electric and magnetic fields that are all around us.

So well done if you've got those right.

It's now time for the final section of our lesson.

We're gonna look at how all different kinds of waves can reflect.

So all waves can reflect and obey the law of reflection when they meet a boundary between materials, or media.

So a wave on a string can reflect from a fixed end.

There it goes.

Water waves can reflect when the hits a wall, the end of the tray, in this case, water waves reflecting.

Sound waves can reflect from hard surfaces, sometimes that can cause an echo when the sound wave hits the wall and then reflects back, for example.

And light waves reflect from the surface of objects as well.

And this animation firstly represents light using rays, but then switches to representing light using wavefronts, which are moving.

You see the wavefronts hit the mirror surface and reflect, obeying the law of reflection.

Here they come again, the wavefronts.

So what's our wavefronts? Wavefronts are lines that represent the positions of the peaks of any wave.

So in the diagram, the top section shows a wave pattern with peaks and troughs.

And then the bottom part of that diagram shows the wavefronts, lines to represent the positions of the peaks of a wave.

And the distance between the wavefronts represents the wavelength of the wave, which, remember, it's given that Greek letter lambda to represent wavelength.

So wavefronts are like the top view of water ripples.

There's the side view of some water ripples, and here's a top view of those water ripples.

And there's the peaks, and you can see the water ripples are actually the peaks of the wave.

And the direction of travel of waves is always 90 degrees to the wavefronts.

Okay, true or false? Wavefronts always 90 degrees to the rays.

Is that true or false? Right, I'll tell you the answer now.

That is true.

But what's the best reason why that's true? Justify your answer.

Is it A or B? Which of those is the best reason why it's true? The wavefronts are always 90 degrees to rays, decide now.

Wavefronts are always 90 degrees to rays because rays show the direction of travel, and the wavefronts always 90 degrees to the direction of travel, so they're always 90 degrees to the rays which show the direction of travel.

Rays are not 90 degrees to the direction of travel.

That's the wavefronts.

Wavefronts are 90 degrees to the direction of travel in B.

That's why the answer is not B.

Statement B is wrong.

Time for a final task to this lesson then.

Add wavefronts to these ray diagrams. Each diagram could represent any kind of wave.

In each diagram, the wavelength of the waves would be constant.

So don't let the wavelength vary.

You could use a ruler to make sure that you draw in wavefronts on each ray, say every centimetre, for example.

Okay, that's all you have to do for this task.

Draw, add wavefronts to each ray, keeping the wavelength constant.

Pause the video now.

Off you go.

Okay, time for some feedback on this task.

Hopefully it was quite quick.

Here are some wavefronts for the first ray, showing a constant wavelength.

Here are some wavefronts for the second ray, showing constant wavelength as the wave reflects.

Remember, the wavefront should be drawn 90 degrees to those rays.

And for the third diagram, here are some wavefronts along that ray, this ray, this ray, this ray, this ray, this ray, and this ray.

And if you drew them all starting from the same position, then what you could actually do is you could connect those wavefronts together to show the wavefronts for rays that aren't shown as well.

So that's actually what the wavefronts could look like.

You could have joined them together, going from wavefronts for individual rays to showing the wavefronts for all rays, even the ones we've not drawn, coming out from the light bulb.

Very well done if you got those on the right lines.

And for diagram three, if you can do so, you could join your wavefronts together to create a diagram like the one that's on screen now, with circular wavefronts.

So well done.

That's the end of this lesson on reflection of light.

Here's a summary.

Specular reflection is reflection from a smooth surface in a single direction, but diffuse reflection is reflection from rougher surfaces in multiple directions.

In specular reflection, the angle of incidence equals the angle of reflection, and that's shown in the diagram.

All waves can reflect, obeying this law, at a boundary between materials, media.

Light can be thought of as a wave, ripples, in electric and magnetic fields.

And like all waves, light can be represented using rays, straight lines that show the direction of travel, or wavefronts, lines that represent wave peaks.