Difference between light and sound waves

We are now in a position to distinguish fully between light and sound waves. The physical difference, of course, is that light waves are due to varying electric and magnetic forces, while sound waves are due to vibrating layers or particles of the medium concerned. Light can travel trough a vacuum, but sound cannot travel trough a vacuum. Another very important difference is that the vibrations of the particles in sound waves are in the same direction as that along which the sound travels, whereas the vibrations in light waves are perpendicular to the direction along which the light travels. Sound waves are therefore longitudinal waves, whereas light waves are transverse waves. As we have seen, sound waves can be reflected and refracted, and can give rise to interference phenomena; but no polarization phenomena can be obtained with sound waves since they are longitudinal waves, unlike the case of light waves.

Reflection and refraction of light

Bodies like the sun and other sources of light with which we are familiar are said to be “self-luminous” since they emit light of their own accord. On the other hand, the common objects around us are not self-luminous. Nevertheless, these bodies are vibrating as they reflect in all directions the light which falls on them from the sun. Mirrors and highly polished surfaces reflect light strongly in a particular direction.

When a beam of light, traveling in a homogeneous medium, comes to a second medium, some of the light is reflected. At a polished or silvered surface, nearly all the light is reflected. At the surface of clear glass, only a small part of it is reflected. The greater part of it enters the glass and passes through.

Laws of reflection

1. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. 2. The angle of incidence is equal to the angle of reflection.

Refraction. Experiments have shown that light travels with the great speed in a vacuum and that it travels with different speeds in different media. When it passes obliquely from one medium to another in which it has a different velocity, there occurs a change in the direction of propagation of the light. This bending of the ray of light when passing from one medium to another is known as refraction.

Ex. 1. Answer these questions.

May the light be absorbed by the material? Do any things glow because of their high temperatures? Can any objects emit light without the requirement of a high temperature? Did Isaac Newton think that light energy was transmitted as a stream of particles or corpuscles? Was it realized that light is a form of energy? How does light travel? How do mirrors and highly polished surfaces reflect light? When is nearly all the light reflected? How can you observe the laws governing reflection? What bodies are self-luminous? What bodies are not self-luminous? What is known as refraction?

Ex.2. Learn the dialogue by heart

- Will you kindly tell me details about light? What is a light ray?

- It is the direction along which light propagates. Do you know what the angle of incidence is?

- Yes, I do. The angle of incidence is the angle formed by the incident way and the perpendicular. But what is the incident ray?

- It is the ray that falls on to the reflecting surface. What angle is called the reflected one?

- The angle formed by the same perpendicular and the reflected ray. What is the reflected ray?

- It is the ray which is reflected by the reflecting surface.

Heat

Heat is a branch of physics. Heat was thought to be a weightless fluid which escaped from materials when they burned. They invented the word caloric for this imaginary fluid. Experiments by Benjamin Thompson and Sir Humphry Davy showed that mechanical energy can be converted into definite quantities of heat. Benjamin Thompson measured the amount of heat generated in boring the barrel of brass cannon. Since the boring tool was turned by horses, who traveled around in a circle, he knew how much work was put into the operation. Davy showed that blocks of ice, even at a temperature below freezing, can be melted by rubbing them together. As a result of these experiments, scientists cast aside the erroneous caloric theory. Today we conclude that heat is a form of kinetic energy. It is manifested by the vibrations and oscillations of the molecules that make up substances.

We are particularly interested in the effects which heat produces. Solids, liquids, and gases generally increase in volume when heated. Conversely, they usually contract in size as they cool. We measure the intensity of heat in a body by its temperature. Experiments have shown that it requires a much larger quantity of heat to change a pound of water at a given number of degrees in temperature than it does to produce the same temperature change on a pound of iron. Such experiments introduce us to the subject of specific heat. Heating many substances changes them from solids to liquids. Further heating converts the liquid into a gas or vapour. The heat that is absorbed when such a change of state occurs is very considerable. Reversing the process, vapours changing to liquids, and liquids changing to solids set free the same amount of heat that was absorbed by the change of state.

Heat energy enables us to do work with suitable machines. Fuel is burnt to produce steam. The steam is used to operate steam engines or steam turbines. Automobiles, diesel trains, and airplanes burn liquid fuels in the cylinders of their engines, producing heat which is converted into kinetic energy of motion. Friction produces heat, but the heat produced by friction is usually wasted. The digestion of fats in our diet produces heat which keeps our bodies at a nearly constant temperature, even though the temperature of the air may fluctuate widely. Electrical energy can be changed to heat in an electric appliances are very convenient because they produce the heat just where it is needed, and they are also capable of accurate regulation.

Optics

Have you ever heard the old saying “seeing is believing”? Perhaps you think that it is not always true. Nevertheless, “seeing” does play a most important part in helping us to learn. Scientists have found that we learn more through our eyes than through our four other senses combined. In fact, almost all that we learn comes to us through vision.

We cannot see unless there is light. It is the light that is reflected from the surface of an object to our eyes that makes the substance visible.

The invention of the telescope in 1609 enables man to increase the range of his natural eyesight. Galileo, the great Italian scientist was one of the first to construct such an instrument. With his crude telescope, he made many discoveries concerning the heavenly bodies, and became convinced that Copernicus was right when he advanced the revolutionary idea that the sun is the centre of our solar system. What progress has been made since the time of Galileo in the construction of optical instruments! Today the eyes of the world are turned to the greatest optical instrument, the telescope. What distant stars will be brought within the range of our vision with this remarkable instrument!

Not only the distant, but also the near at hand can be seen more clearly by combinations of lenses. Microscopes reveal to us the arrangement of the particles in a piece of steel, the structure of plant and animal cells, and the existence of minute forms of life such as protozoa and bacteria.

In our daily life we make practical use of simple optical instruments. We use mirror when combing our hair. Many of us are dependent upon eyeglasses to improve our vision. A simple convex lens is used as a reading glass to make fine details on a map more legible, etc.