Nature of LightPlato thought that light consisted of streamers emitted by the eye. It gained support from Euclid, but was contradicted by the Pythagoreans as the latter believed that light originates from luminous bodies in the form of very fine particles. Empedocles, a forerunner of Plato, believed that light is composed of high speed waves of some sort. Until the beginning of the twentieth century, the real nature of light was a considerable subject for discussion among scientists and physicists all over the world.There were two theories on the basic nature of light. The first theory was the Wave or the Undulatory Theory which explains that light has a wave motion which starts from a vibrating body and is transmitted at high speed. One of the proponents of the Wave Theory was Christian Huygens (1629 1695), who explained the reflection of light using wave motion. He also proposed that light consists of series of waves with their wave fronts at right angles to the path of the rays. According to the Huygens Principle, different points of a wave front of light set a series of secondary waves. Since light can pass through a vacuum, he explained that light may travel through a medium known as ether, a mysterious substance which is not air.In 1704, Sir Isaac Newton contradicted the Wave Theory, as he described light as a stream of particles or corpuscles. Based on this theory known as the Corpuscular or Emission Theory, light consists of tiny particles of matter emitted by a source that travel only in straight lines called rays.Thomas Young (1773 1829), in his experiment in 1801, was able to study the interference and diffraction of light. This established the Wave Theory, especially after James Clerk Maxwell constructed an oscillating electrical circuit which showed that changing electric and magnetic fields could produce electromagnetic radiation that could travel through a vacuum. Light, eventually, was proved to be electromagnetic as Heinrich Hertz demonstrated, in 1880 the existence of electromagnetic waves (within radio frequency) that exhibit the same properties as the light.In 1990, the Wave Theory was challenged when Max Planck (1858 1947) hypothesized that the vibrating electrons in incandescent lights could only have energies restricted to certain values. He introduced the phenomenon known as blackbody radiation which, according to him, was emitted in discrete bundles of energy called quanta (plural for quantum). This gave birth to the Quantum Theory of Light. In 1905, Albert Einstein published a Nobel Prize winning paper which states that light is composed of bundles of wave energy called photons. This finding supported Plancks Theory.In the later part of the nineteenth century, several scientists have observed that light was capable of ejecting electrons from various metal surfaces. If light falls on a clean surface of metals, such as potassium or sodium, electrons are emitted by the surface. This is called Photoelectric Effect. In 1923, Arthur Comptons (1892 1962) study of the scattering of X rays by electrons all required the assumption of a particular nature for electromagnetic radiation without in any way further established in 1924 when Louis Victor de Broglie proposed that every particle of matter is somehow endowed with a wave to guide it as it travels. Hence, the particle wave duality of wave was credited.Sources of Light and its PropagationOptics is the branch of Physics which deals with the behavior and properties of light including its interactions with matter and the construction of instruments that use or detect it.Light is a combination of both electric and magnetic energy which can travel at varying velocities in various media. All light must come from a source. Luminous objects are the objects that emit or send off their own light. They tend to radiate heat as an effect of being luminous and can store energy. The sun, stars, light bulbs, lamps, lasers, campfires are the luminous objects that are found in our everyday life and all of them are considered sources of light. Nonluminous objects, on the other hand, are those that cannot emit their own light. In order for us to see them, a light from a luminous object must be reflected. The moon, cars, buildings, and most of the objects that we see are nonluminous objects and are also termed as illuminated objects.Light can be produced through incandescence and luminescence. When an object is heated at very high temperature, say at around 470oC, it starts to glow and become dull red in color. We call this incandescence. The high temperature causes the atoms to vibrate and give off energy in the form of electromagnetic radiation. The sun gives off both heat and light as a result of nuclear reactions in its core. An incandescent light bulb gives off light when a wire filament inside the bulb is heated to white heat. One can read by the light of a candle flame because burning wax gives off both and light.Luminescence is used to describe a process by which light is produced other than by heating. Two forms of luminescence can be identified, depending on the amount of time emitted light continues to glow. They are fluorescence and phosphorescence.Fluorescence refers to the release of light that lasts no more than about 10 nanoseconds (10 billionth of a second) after it begin. Most familiar form of the former is a fluorescent light bulb. Light is produced when an electric current passes through a mercury vapor in the light bulb. As the electrons produced from the mercury vapor collide with the chemical painted inside the bulb, fluorescence occurs. When the bulb is turn off, the chemical stops glowing and no lights is produced anymore.Phosphorescence refers to the release of light that lasts longer than 10 nanoseconds. This type is temperature dependent since thermal energy is required for the electrons to become de excited. Phosphorescent materials are used in glow in the dark toys that will emit light after exposure to some form of radiant energy.The hands on certain watches and alarm clocks are coated with a phosphorescent material which will emit light for many hours after a light source is removed. The most common form of phosphorous exhibits this phenomenon, phosphorescence.Objects can be classified in terms of the way they allow light to pass through them. Transparent materials such as air, glass, water, and clear plastic allow the passage of light while those that block light are referred to as opaque. Examples of opaque materials are woods, concretes, metals, and flesh of some animals. There are also objects that allow only some amount of light to pass through and are called translucent materials. These materials have both the characteristics of opaque and transparent materials.Brightness of lightPhotometry is a branch of optics which deals with the measurement of brightness of light sources and the illumination. It is concerned with the flow of light energy from a source. Just like sound, the physical property of a light source is also considered. Luminous intensity refers to the brightness of a light source. It is expressed in the unit candela. The brighter the light source is, the greater is its luminous intensity. The luminous energy emitted from a light source is called luminous flux and is expressed in lumens.Properties of lightLight is an electromagnetic radiation that has properties of waves and particle. Until mid 1800s, the generally accepted theory of light was the particle picture. In this viewpoint, advocated by Newton, light was considered to be a stream of tiny particles. However, in the late 1800s, the particle picture was replaced by the Wave Theory of Light. This was because certain phenomena associated with light could only be explained using the wave picture.

Reflection of LightIn Figure 17 -1 a mirror has been placed on the table in front of a protractor. A laser beam strikes the mirror and is reflected from it. What can you say about the angles the beams make with the mirror? If you look carefully, you can see that each makes an angle of 60o relative to a line perpendicular to the mirror. This line is called the normal to the surface, or normal. The angle that the incoming beam makes with the normal, the angle of incidence, is equal to the angle the outgoing beam makes, the angle of reflection.

As echo is a reflected sound wave. Light waves also bounce off from a reflecting surface and this property is called Reflection. As stated in the Law of Reflection, the angle of incidence is equal to the angle of reflection as measured from the normal. It has to be noted that the incident ray, the reflected ray, and the normal to the reflecting surface all lie on the same plane. When the light exists from a medium, total internal reflection can occur if the angle of incidence is greater than the critical angle.As mentioned earlier, there are two kinds of reflection: Specular and Diffuse Reflection. Specular Reflection occurs when the reflective surface is very smooth such as a mirror or a surface of calm water. Diffuse Reflection, on the other hand, is observed when a light hits a rough surface resulting to the bouncing back of light waves in different direction.The smoothness or roughness of a surface depends on the wavelength of the Electromagnetic Wave bouncing off from it. For instance, a smooth reflecting surface that produces specular reflection of red rays may produce diffuse reflection when incident upon by a violet rays. This dispersal of reflected light called diffusion. Although diffused light gives us irregular images, this phenomenon is also beneficial as it allow us to regulate the amount of light, thus, reducing glare to protect our eyes. Reflection of light may also exhibit absorption and scattering. Absorption is the transfer of energy carried by the light waves to the particles of matter while scattering is the reflection of light by particles. When you use the penlight in the air, some of the energy from the light is absorbed by the air particle, thus, a dimming effect in the room. When the light is released, light is scattered in all direction which allow you to see the illuminated objects.

Refraction of LightRefraction is the bending of light as it passes from one medium to another. As light refracts, the velocity of the wave is altered. Its wavelength increases or decreases, but its frequency remains constant. The speed of light varies in different substances. The speed of light in vacuum is usually denoted by c, for constant or the Latin celeritas (meaning swiftness). Based on the International System of Units (SI), the meter is defined as the distance light travels in vacuum in 1/299, 792, 458 of a second. This definition fixes the speed of light in vacuum at exactly 299, 792, 458 m/s. For our convenience, we will use the value 3 x 108 m/s to denote the speed of light in a vacuum. In water, light travels at 2.25 x 108 m/s while its speed in crown glass decreases to 2.0 x 108 m/s. The ratio of velocities of light as it passes from a vacuum into another medium is the index of refraction (n) for that other medium and remains the same whatever angle of the incident ray upon the refracting surface.Refraction can be simply observed by putting a spoon in a glass of water. It appears as if the spoon is broken, but it is just because of the refraction of light. The light from the spoon is refracted as it passes from the water to the glass to air, causing it to be displaced. Since the surface of the glass is curved, the water in the glass also acts as an magnifying glass, slightly enlarging the pencil.When light strikes the top of a block of plastic, as in Figure 17 3a, faint beams of light are reflected from the surface, but a bright beam goes into the block. It doesnt go in as a straight line, however; the beam is bent at the surface. Note that when a light beam goes from air into glass at an angle, it is bent toward the normal, as shown in Figure 17 3b. The beam in the first medium is called the incident ray, and the beam in the second medium is called the refracted ray. In this case, the angle of incidence is larger than the angle of refraction, which is the angle that the refracted ray makes with the normal to the surface. If the angle of refraction is smaller than the angle of incidence, then the new medium is said to be more optically dense. Later in the chapter youll learn that the speed of light is slower in more optically dense materials.FIGURE 17 3 Light is refracted toward the normal as it enters denser medium. Compare the deflection of a set of wheels as it crosses a pavement-mud boundary. The first wheel that enters the mud is slowed, causing the wheels to change direction towards the perpendicular.

FIGURE 17 4 Light is refracted away from the normal as it enters a less-dense medium. Compare the deflection of a set of wheels as it crosses a mud-pavement boundary. The first wheel to leave the mud speeds up, and the direction of the wheels changes away from the perpendicular.

What happens when a light ray passes from glass to air? As you can see in Figure 17 4, the rays are reflected away from the normal. The angle of refraction is larger than the angle of incidence.When light strikes a surface along the perpendicular, the angle of incidence is zero, and the angle of refraction will also be zero. The refracted ray leaves perpendicular to the surface and does not change direction.SNELLS LAWWhen light passes from one medium to another, it may be reflected and refracted. The degree to which it is bent depends on the angle of incidence, and the properties of the medium as shown in Figure 17 5. How does the angle of refraction depend on the angle of incidence? The answer to this question was found by Dutch scientist Willebrord Snell in 1621. Snells law states that the ration of the sine of the angle of incidence to the sine of the angle of incidence to the sine of the angle of refraction is a constant. For light going from the vacuum into another medium, the constant, n, is called the index of refraction. Snells law, is written as when r is the angle of refraction, n the index of refraction, and i is the angle of incidence. In the more general case, the relationship is written asSnells law ni sin i = nr sin r

Here, ni is theindex of refraction of the medium in which the incident ray travels, the first medium, and nr is the index of refraction of the medium in which the refracted ray moves, the second medium Figure 17 6 shows rays of light entering and leaving glass and water from air. Note how i is always used for the angle the incident ray makes with the surface, regardless of the medium. From the angles of refraction, how would you expect the index of refraction of water to compare to that of glass?The index of refraction of a substance often can be measured in the laboratory. To do this, direct a ray of light onto the substances surface. Measure the angle of incidence and the angle of refraction. Then use Snells law to find the index of refraction. Table 17 1 presents indices of refraction for some common materials. Note that the index of refraction for air is only slightly larger than that of a vacuum. For all but the most precise measurements, you can set the index of refraction of air to 1.00.