Which energy is needed in order to see the various objects around us? On a dark night, would it be possible to see the things in a room without lighting a lamp or a torch? Which source of light is shown in the figure? Besides those shown in the figure, what other sources of light might there be?
During the daytime, with the help of the light that comes from the Sun, we can see the objects around us. At night, in order to see the objects around us we need a lamp, a candle or a torch. When light that has come from an object, or that the object has returned, arrives at the retina of our eye, it forms an image of the object and we are able to see the objects around us. In this lesson we shall discuss the process by which light strikes a plane surface and comes back.
That sentence about the retina is easy to read past, so slow down on it, because it changes what seeing means. Nothing about a book or a wall or a friend ever reaches your eye. What reaches your eye is light, and only light. Your eye is not a window that looks out at the world; it is a collector that catches whatever light happens to arrive, and everything you think you are looking at is really a picture assembled from that light. This is exactly why a dark room defeats you. The objects are all still there, unchanged, at arm's length. Your eyes work perfectly. But no light is bouncing off those objects and coming to you, so there is nothing to collect and nothing to see. Once you have accepted this, the rest of the topic stops being a list of rules and becomes one question asked over and over: where did that light come from, and which way did it go after it hit something?
From the activity at the window it can be seen that objects with a smooth plane surface, such as a mirror, the dial of a watch and the screen of a mobile, return light well. When light falls on a mirror, the light strikes the surface of the mirror and comes back. In the same way, light comes back when it strikes the surface of any opaque object at all.
The process in which light strikes the surface of an object and comes back is called the reflection of light. The surface of any opaque object, whether smooth or rough, reflects light.
Regular reflection
The process in which parallel rays of light strike a plane surface and come back still parallel to one another is called regular reflection. Regular reflection takes place from a mirror, the screen of a mobile, a sheet of metal and so on. It is because of regular reflection that an image of an object is formed. We can see our own image in a mirror, or in the still water of a pond.
Irregular reflection
The process in which parallel rays of light strike a rough or uneven surface and scatter away in different directions is called irregular reflection. Irregular reflection takes place from a piece of wood, the wall of a house, the leaves of plants and so on. When irregular reflection happens, the reflected rays scatter in different directions, so the reflected rays are not seen as they are in a mirror. It is precisely because of the irregular reflection of light that the objects around us can be seen. The rays that strike those objects and come back fall on our eyes, and we see the object.
Irregular sounds like a fault, as though the light has gone wrong somehow and the tidy version is the proper one. It is the other way round. Almost everything you have ever seen, you saw by irregular reflection. This page, the desk, the wall, the trees outside, the face of the person next to you, all of them are rough at a small enough scale, and all of them scatter light in every direction. That scattering is precisely why they are visible from anywhere in the room. Regular reflection is the rare special case, and notice that it does not actually show you the surface at all. When you look at a mirror you do not see the mirror; you see whatever the mirror is pointing at. A perfectly smooth surface sends the whole beam off in one single direction, so unless your eye happens to be standing in that one direction you see nothing from it. So irregular reflection is what makes the world visible, and regular reflection is what makes images.
Some terms related to the reflection of light
| Term | What it means | In figure 7.25 |
|---|---|---|
| Incident ray | The ray that comes from a source of light and strikes the surface of an object | IO, and the arrow shows it going from I towards O |
| Reflected ray | The ray of light that strikes the surface of an object and comes back | OR, and the arrow shows it going from O towards R |
| Normal | The line drawn at 90 degrees to the plane surface from the point where the incident and reflected rays meet it | NO, drawn perpendicular to the mirror surface XY |
| Angle of incidence | The angle made by the incident ray with the normal | ∠ION, written as ∠i |
| Angle of reflection | The angle made by the reflected ray with the normal | ∠RON, written as ∠r |
This is the single most common mistake in the whole topic, and examiners know it. Look again at the definitions above and notice that the word normal appears in both of them. The angle of incidence is between the incident ray and the normal, not between the incident ray and the mirror. Those two angles are different, and together they always add to 90 degrees. So if a question tells you that a ray strikes a mirror at 30 degrees to the mirror surface, the angle of incidence is 90 minus 30, which is 60 degrees, and writing 30 loses the mark. Why measure from the normal at all, when the surface is right there and easier to see? Because a surface can be curved or tilted, while the normal at any point is defined the same way everywhere: it is the upright line at exactly that spot. Measuring from the normal is the one convention that keeps working for curved mirrors and for lenses later on, so it is worth the small trouble now.
Laws of reflection of light
When light is reflected regularly it follows certain laws. The laws of the reflection of light are as follows.
| 1 | The value of the angle of incidence and the value of the angle of reflection are always equal. That is, ∠i = ∠r. |
| 2 | The incident ray, the reflected ray and the normal always fall at the same point on the same surface. |
Here is something the book leaves for you to notice, and it ties the whole topic together. Regular and irregular reflection are not two different behaviours of light. Every single ray, on every surface in the world, obeys the same law: the angle of incidence equals the angle of reflection. Nothing ever disobeys it, not even light bouncing off a rough mud wall. So why do the two look so different? Because of the surface, not the law. On a mirror, every point of the surface faces the same way, so every normal points the same way, so a set of parallel rays all turn through the same angle and come away parallel. On a rough wall, the surface tilts this way and that from point to point, so each ray meets a normal pointing somewhere slightly different, and each ray obediently reflects at an equal angle to its own normal, and the beam ends up scattered. Same law, applied to a surface that is not flat. That is the entire difference between seeing your face and seeing a wall.
Worked example: finding an angle from a ray diagram
In the figure, AO is the incident ray, OB is the reflected ray, and ON is the normal drawn at 90 degrees to the mirror surface MN. The angle between the incident ray and the mirror is 30 degrees. Find the angle of incidence and the value of angle BON.
| Step | Working |
|---|---|
| The angle given is with the mirror, not the normal | Angle between AO and mirror = 30° |
| The normal is at 90° to the mirror | Angle of incidence ∠AON = 90° - 30° = 60° |
| Apply the first law | ∠i = ∠r, so ∠BON = 60° |
So the angle of incidence is 60 degrees and angle BON, which is the angle of reflection, is also 60 degrees. Notice a shortcut that these questions often use. The total angle between the incident ray and the reflected ray is ∠i plus ∠r, which here is 120 degrees. So if a question instead tells you that the angle between the incident and reflected rays is 80 degrees, then since the two angles are equal each of them must be 40 degrees.
The image formed in a plane mirror
The figure shows rays of light that have come from an object being reflected in a plane mirror and forming an image. The way of showing this process of image formation in a drawing is called a ray diagram. The image formed in a plane mirror is virtual, that is to say unreal, because the reflected rays have not actually reached as far as the image. A virtual image cannot be caught on a screen. This image is upright and the same size as the object. The image we see when we look in a mirror is also a virtual image.
Look at the dotted lines in figure 7.28. They go behind the mirror, where no light has ever been, and they meet at the point marked as the image. Why draw lines through a place light never went? Because that is exactly what your brain does. Your brain has one deeply built in assumption about light, which is that light travels in straight lines. So when rays arrive at your eye spreading out as though they came from a point somewhere behind the mirror, your brain runs them straight backwards and concludes that there must be an object sitting back there. There is no object back there and no light back there. There is only the wall behind the mirror. The image is not a thing at all; it is the place your brain believes the light came from, and that is precisely what the word virtual means. Now you can see why a virtual image cannot be caught on a screen. Put a sheet of paper behind the mirror and nothing appears on it, because no light ever arrives there to make a picture. This is the real difference between virtual and real, and it is worth more than the definition.
Some applications of the reflection of light
The mirror in a microscope reflects light to one single point. Because of this, tiny living things can be seen with the help of the microscope. Because of the reflection in the mirror used in a vehicle, the scene behind the vehicle can be seen, and this mirror helps in seeing a wide area. Mirrors are also used for shaving a beard and for doing makeup, and such mirrors reflect light in a way that makes the image larger.
Periscope
A periscope is an instrument made on the basis of the principle of the reflection of light. This instrument is used in order to see easily, from down below, an object that is at a great height. You too can make a model of this instrument at home by collecting the materials needed.
| Where a periscope is used | Why it helps |
|---|---|
| In a submarine | Objects above can be seen easily even from inside the water |
| In battle | Soldiers can see an opponent hiding at a height without exposing themselves |
There is nothing new inside a periscope, which is what makes it satisfying. Light coming in horizontally at the top strikes a mirror tilted at 45 degrees. The normal to that mirror is also tilted at 45 degrees, so the angle of incidence is 45 degrees, and by the first law the angle of reflection must be 45 degrees too. Adding those together turns the light through a right angle, and it now travels straight down the tube. At the bottom the second mirror does exactly the same thing again, and turns the light through another right angle into your eye. Two right angle turns bring the light back to the direction it started in, which is why what you see through a periscope looks the right way up and not tilted. Everything depends on the 45 degrees. If a mirror slips even a little, the light no longer turns through a full right angle and it misses the second mirror altogether, which is the commonest reason a home made periscope shows nothing at all.
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