Ray Optics-Introduction

Light plays a very important role in our life. Some experiments indicate that light is a wave phenomenon and some other indicate that light is made up of small particles which travel in a straight line. Finally it was accepted that light has dual character, which means, depending on the situation light may behave as a wave or particle. In fact later on it was established that materials also has character of waves along with their existance as a particle. These concepts of light as a wave is covered in detail in another article of mine named as "wave optics" To go into the further details of light as a wave you need to look at that article. This optics series is basically ray optics in which we cover in detail all the aspects of light related to reflection and refraction.

When ever there is a change of medium encountered by rays of light then some portion of light is reflected back and some part is transmitted into the medium, and some part is absorbed or lost. In this series of optics we will be covering reflection and refraction in details. So let us start reflection phenomenon.

It is the reflection of light due to which we can see objects. When light falls on an object then it is reflected by the object and when this reflected light enters into the eyes of a person then an image of the object is formed on the retina and our mind interprets this image and then we get to see the object. have you ever noticed that you are looking at something but your mind is busy thinking about something else with great concentration and you really dont see that object. Your eyes are forming the image on the ratina but you are not aware of that thing due to your mind is highly engrossed into something else. Any way we are currently talking about reflection of light.

Suppose a ray of light is incident on some object at a particular point on it. We can geometrically draw a normal line to the surface at that point. Angle between the incident ray and this normal is called angle of incidence. In nature light when gets reflected from the surface then this reflected ray makes the same angle with the normal, as the angle of incidence. Angle made by reflected ray with the normal is called angle of reflection. Hence the law of reflection of light can be written as follows along with some other details.

According to law of reflection of light, the angle of incidence and angle of reflection are same and incident ray, reflected ray and normal always lie in the same plane. It is valid for all rays when several rays are incident in the form of beam of light.

Nature of the surface on which this light is incident plays a very important role here. Many objects we see nearby in daily life dont have such a precise surface that all normals drawn on some portion of surface remain parallel to each other, but normally surfaces on microscopic level has a surface with many hills and valleys, although we see them as a flat surface using our eyes. So normals in a portion of surface are randomly oriented so light beams is dispersed in all random directions. Such reflection of light is called "diffused reflection".

On the other hand we have reflecting surfaces like plane mirrors in which almost all the normals in a portion of surface are parallel to each other and so all light rays gets reflected in the same way. Such reflection is called "specular reflection".

So here onwards we shall be assuming surfaces perfectly smooth so that we may get specular reflection. In a short while, after reading few paragraphs ahead, you would be able to understand why we can see our image in mirror or mirror like smooth surface, but not in the case of other objects like wall of your room, or any paper sheet like that of newspaper etc. If you guess, it is due to diffused reflection from normal objects, and specular reflection from mirror then you are absolutely right.

In ray optics we make ray diagrams very often. So what is "ray diagram"? Basically we represent light rays by lines and usually put small arrow on this line to indicate the direction in which light is travelling. Light rays follow a straight line path and deviates from its path, when there is a change of medium or it gets reflected from some surface. In ray diagram we trace path of light using laws of optics.

Now we shall start the discussion about Ray Optics. Please move to the next article.