The visible light

The example of the photographic camera demonstrates that the light emitted or reflected by one object is able to change the physical state of a body (the film). Hence light carries energy. What can light therefore be? 
How many ways do you know to carry energy from one place to another? The most common way is to "send" an intermediate body. For example you can throw a ball with your hand. The energy that you spend in moving your hand is transferred to the ball, which starts moving. If it hits another ball, then this one too starts moving. So you have transferred energy from your body to a far away body carrying it with a third body.
The other possibility to send energy away is to use a wave. If you throw a stone into water, then circular waves are produced which move away from the point where the stone sank. Dropping down, the stone delivered some energy to the water, and the waves that form are able to carry part of this energy away from the initial point. What moves away is not the water. It is its movement that transmits to the adjacent parts. If you throw a cork on the waves, you can see that the cork goes up and down, but it does not move away. 

The sea waves are irregular. In order to describe the characteristics of a wave, then look at the figure. A "perfect" wave is represented. 


If you want to describe this wave to another person, you just need to tell what is the distance between the crests, and how high they are. They are called wave length and amplitude. It is also important to know at what speed it moves. 

In order to know the speed of a wave, you just need to stand in one point and count how many crests you see, passing within a given time span.  

For example in this case suppose that you start the chronometer when a crest passes in front of you. Suppose that, after 1 minute, 3 crests passed. The number of crests that pass in a given amount of time is called frequency of the wave. Then in 1 minute the wave moved by a distance equal to 3 crests. If the wave length is 2 meters, then the wave travels at a speed of 6 meters per minute. The speed of the wave is then found by multiplying the wavelength L by the frequency f.  

V = f × L 

The speed of light is constant within a certain medium. It just varies when the light goes from one medium to another. This fact is very important, and we will see it better later. Since V is constant, the frequency f and the wavelength L of the wave are inversely proportional: when the frequency increases, the wavelength decreases, and vice versa.  

In order to better understand this idea, think about a train. In order to compute the speed of the train, you can count how many wagons pass in front of you in a certain time interval, then the speed is found when the length of a wagon is known.  

Now watch the following animation. A person is holding the end of a rope, and it makes it oscillate periodically. 

In order to move the arm, the person spends energy. The energy is transmitted to the rope and creates the waves that move away. Even here you see that it is not the rope that goes away, just the motion of the waves. If a second person would hold the opposite end of the rope, then its arm would be set in motion by the movement of the rope. 
 

In order to transfer energy between two bodies, a mediator is then required. This medium can be a body different from the first two, or an extended body which sets them in contact (in the case of the wave). In the first case the body carries the energy from the first to the second one, while in the second case it transmits the same energy. One then says that the extended body is the medium inside which the wave propagates.  

If the light carries energy in the ways that we know, then it could be made by particles so small that we cannot see them, or else it can be a wave that propagates in a medium which must be invisible.  

Which of the two models is working? Both of them! In other words, there are phenomena which can be explained just considering light as a wave, while in other cases we must think that the light is made by particles. There is a physical theory which is able to explain this behavior. After elaborating the theory of light as a wave, the physicists tried to reveal the medium where it should propagate. They used to call that medium "ether". But at the beginning of 1900 it has been demonstrated that this medium does not exist. Light travels in vacuum.  

The wave model of light is the most suitable to describe the characteristics of the light of the stars which are useful at the moment. The most important thing is to discover how a wave changes when the amount of energy that it carries is changed. In the case of the ball, if you throw it more strongly, then it moves faster. In the case of the rope, if the person increases his/her effort what happens to the wave?  

In the initial situation the rope is moving like in this animation.  

When swinging a wave, you can increase the required work in two ways. In the first way you can impart a larger number of hits per minute. What happens can be watched here.  

Or you can make larger movements with the arm. In that case this is what  happens.  

As you can see, if you want to move away some energy, you can do it in a more effective manner by increasing its frequency or its amplitude.  

What does this have to do with real objects? Discover it in the next page! If instead you want to restart reading later, the password is "lira" .