Introduction to Light
Have you ever thought that what it would feel like to live without the presence of light? But we have rarely experienced a total darkness. Without the presence of light, humans can’t see anything. But what is light?
To understand what light is one must know that what we call light is actually what is visible to us or what we see. The light that human beings can see is called as visible light. Scientists discovered that other animals can see different types of light that are not visible to humans. So what is light? In order to answer this question, we have to go to the deepest part of what science really is.
When we were young we were told that light is a ray and it travels in a straight line and bounces off surfaces. But later we learned that light is a wave. It behaves exactly like the waves formed on the surface of the water when we throw a stone. But when we were older enough we were told that light is a stream of particles. And the particles are called as photons. Now to top that, in the end, we were told that light is both wave and particle, or it is neither wave nor particle. So the real answer is that we don’t know what light is. Nobody knows what light really is, but we are building up a map of what light could be and if we imagine the light that way, then we can make predictions in a better manner.
Visual light can be defined as an electromagnetic radiation with wavelengths between 380 to 750 nm which is visible to the human eye. We can imagine light as a ray, a wave or a particle as long as it useful for us. For example, if we want to understand phenomena like photoelectric effect, we can imagine light as a stream of particles called photons.
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Today, I am going to light you; and I am going to light you in a way that it is going to be challenging for you to find out which of them lights and which of them are not lights. So the astute ones amongst will begin to understand what exactly I am saying when I am saying about light. Lets begin. When I was in seventh standard, they told me that light is a ray. It travels in beautiful straight lines, bounces off surfaces, following very beautiful geometrical law- angle of incidence is equal to angles of reflexes and bounces off comes here comes there and it definitely goes well until I went to eleventh standard and all of a sudden, which you are going to very soon- suddenly they told me that light is a wave! It behaves as if you throw rocks into water, there are ripples and light is very similar to that and I said ok, fine, I can live with that. And when I was at the twelfth standard, they told me light as a stream of particles its like a machine gun or torch light when you open it, its like you have a machine gun and start shooting out little bullets of light. And the little bullets that call photons. And I said ok, now I am beginning to get confused! To top it all, at the end of twelfth, they told me now we will tell you what light it. Light is both wave and particle, or its neither wave nor particle. Which of it is? They also told me that it depends upon what experiment you perform. You know sometimes they will behave like waves and sometimes like particles and by now if you are not confused you are not really listening. Isn’t it? Because I was really confused. And I began to ask the question. What really is light. In order to answer this, you have to go into the deepest parts of what really science is. And if science trying to say something about reality, are we going to find out what light is. Nobody can say. The best analogy to take would be to answer this question- have you seen the political map of India? We will see that with states which have capitals. Very simple, clean. Have you seen the physical map of India? There are mountains and rivers all that. And there are road map which shows you if you take this road, you can reach somewhere. Now, which of them is India? Is that a bad question or is it a difficult question to answer? Because which of them is India? Are all of them India, or none of them India? Because if you try to build a map that has everything in it what India has you have to build another India-so that’s not the point. You have to build something that’s not really the question where its right or wrong but we are trying to build something that is useful. Because what’s the point of a road map? It will tell you where to go. So if you have to find out where to go which map will you go to? The road map! If you want to find out how the cities are divided and where the capitals are you go to the political map. If you want to find out where the mountains are where the rivers are and predict the weather, you go to the physical map. Why am I saying this? Because, the real question is we don’t know what light is. Nobody knows what light really is, but we are building up a map of what light could be and if we imagine light that way, then we can make predictions in a better manner. That’s what science tries to do. If I imagine light as a wave, will that useful? If I imagine light as a ray, will that be useful? So the question- is light a ray or is light a wave- is a really a pretty bad one because- we don’t know! so when is light as a ray is useful? If you want to build and find images if you want to build a telescope if you want to build a microscope, imagining light to be a ray works really well. If you want to find out or answer questions like that why light something interferes, there is something called interference of light, that is something called Diffraction, which we will learn later and if you want to understand why that happens it is better to imagine its better to understand light as a wave. And if you want understand other phenomena like photoelectric effect, we imagine light as a particles- a stream of particles called photons and its pretty surprising that we don’t know what really light is but depending on what situation is it, we imagine light to be one particular thing.. its what science tries to do. It builds a map of reality and these maps are never really a description completely of reality. As long as they are close enough, they can start predicting what happens. Because Rutherford’s model of the atom was wrong- its wrong- you know that. But using Rutherford’s model, we build plastic chairs. And the moment we realize Rutherford’s model was wrong, did the plastic chair vanish. They didn’t. they are here! Which means even a wrong model can predict good things and you can make things out of it, in other words- the real question to ask is- is the model useful? So we will finish this off by making one statement- the map is not the territory. You have a map or something, let’s find out how useful it is. So in this particular episode, we will treat light in the simplest form it can be treated- as a ray! So we will imagine light as a ray that goes from one place to another place, gets bounced off you know the most beautiful way to think about it the simplest way rather. So, lets make some predictions looking at light as a ray and how we can play with it. And then lets see what happens.
How do we see?
We see something around us that how live, we watch everything around us- how exactly we see. Long long ago, there was an old man who predicted theoretically that light leaves from our eyes goes in all possible directions and when it hits something that comes back to us and when it comes back to us, we see! Isn’t that a problem here? Because somebody switches off the light we cannot see. If the lights go out of our eyes then we must be able to see whether there is light or not. So, he wasn’t the brightest candle on the cake. But now we know that there must be some way we are seeing because we are always need something called the source. Otherwise, we can’t so need a source, there is a receptor called eye. The light comes from the source hits something and comes to our eyes. So all seeing happens using reflection, and we already know that. And typically it’s going to be irregular reflection. Some objects reflect regularly and some objects reflect irregularly, we know that. All the objects we see around us except heavily polished surfaces are reflect irregularly.