Ray optics (Geometrical optics) - properties of light

Ray optics or Geometrical optics:- 

          Ray optics or Geometrical optics is a classification of properties of light that describes the following characteristics of light.

(a) Rectilinear Propagation of Light
(b) The speed of light is finite
(c) Reflection of Light
(d) Refraction of Light
(e) Dispersion of Light

        And now we are gonna study all these terms in detail.

1. Rectilinear Propagation:-

                                                     The word Rectilinear propagation is just a scientific term to say that light propagates / moves in a straight line. A perfect example for explaining the rectilinear propagation of light is the sharp shadow cast by an object when light falls on it. Let's assume that light doesn't move in a straight line so what would happen?

(a) shadow would never be sharp as it is.

           And if you are thinking that shadows aren't always sharp they are also blurred like this one.

          So, the answer to that question is the secondary sources and reflected light. Yes, if you are looking at any blur shadow it is definitely due to another source that is interfering the effect of the primary source or it is due to the light emitted by the primary source but getting reflected by the other objects around and causing the blurred shadow.

(b) You'd be able to see behind a wall:

            Yes, it sounds crazy but that's true. As you may know that whatever we see is due to the light reflected by the object we are looking at. So if the light doesn't travel in a straight line it would bend and you'll be able to see the things behind a wall. As I've tried to demonstrate below;

You could expect this if light traveled with twists and turns.

          But the reality is something different. Yes, it's like this one:

But we all know that this doesn't happen and I think these examples are sufficient to prove the rectilinear propagation of light.

2. The speed of light:-

                                         The earliest attempt to measure the speed of light was done by one of the most admired people in the history of optics "Galileo". It is said that around the year 1600 Galileo with his assistant tried to measure the speed of light. Galileo stood on a hilltop with a lamp and his assistant on another distant hilltop with another lamp. Their plan was to uncover Galileo's lamp at an agreed signal, to send a flash of light to his assistant. On seeing the light the assistant was to uncover his lamp, sending a flash, back to Galileo. Galileo was to observe the total elapsed time. Many repetitions of this experiment were performed at greater and greater distances between the two observers but in vain. Galileo convinced that light travels at infinite speed.

             But in the year 1849, a French Physicist "Fizeau" became the first man to measure the speed of light. His experimental setup is described as follows:


          An intense beam of light from a source S is made to fall on a half-silvered mirror G. The light gets reflected by the mirror G and after passing through the lens L1 it gets focused at point O. The diverging beam from O falls on L2 which converts it into a parallel beam of light. After traveling a distance of about 8.67 Km (5.387288 Miles) it passes through a lens L3 which converges it to a mirror M. Now the mirror reflects the light back towards lens L3 and the light retraces its path towards the source. Half of the light falling on the mirror G passes through it (as it is half silvered) and enters the observer's eye at E. 

          The function of Gear or you can say toothed wheel A is to cut the light beam into short pulses. The strategy was to calculate the time required for these pulses to travel to the mirror M and back. When the wheel is at rest the light beam is allowed to pass through one of its openings O. In this situation the observer is able to see the source of light through the mirror G at point E. 

          The wheel is set to be rotating with slowly increasing acceleration. At some point, the light passing through O will return before it gets stopped by the first tooth of that wheel. With this same speed, the light will pass through the first opening and return before getting stopped by the next tooth. Under these conditions, light is completely eclipsed from the observer.  At twice the light will reappear and with maximum intensity. This condition occurs when light passes through the openings 1,2,3,4,...., returning in time to get observed by the observer through the openings 2,3,4,5,...., respectively. The wheel that Fizeau used consisted of 720 teeth, he observed the maximum intensity when the wheel was at 25 revolutions per second (25 RPS). The time required for each light pulse to travel up to M and back can be calculated as (1/720)(1/25)=1/18000 Sec. From the measured distance over and back of 17.34 Km, this gave a speed
Ray optics (Geometrical optics) - properties of light Ray optics (Geometrical optics) - properties of light Reviewed by Sourav Mishra on February 04, 2019 Rating: 5

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