• The Light
• Dispersion
• Absorption
• Applications of Reflection
• Applications of Total Internal Reflection
• The Photoelectric Effect

The Light

Light is a form of energy radiated by moving charged particles (atoms). There were different theories regarding the nature of light. One group of scientists belived that light consists of particles, and another group belived that light is of wave nature. Experiments, however, have proved both theories right, thus the light possesses dual nature. The dual nature of light is explained by the quantum theory, i. e. the electromagnetic theory. In this theory, the processes of emission and absorption are explained by the particle theory, and propagation of light - by the wave theory.

The basic phenomena related to light are: 

reflection, refraction- is the phenomenon in which a ray of light changes the direction of travel when crossing the boundary between two media,

For example, if we are looking at a strong light source directly, we will often see blured spots. They appear as a consequence of light diffraction.

1. Monochromatic light 2. Screen with slits 3. Interference fringes

• polarization is the phenomenon of directing the oscillations produced by reflecting or refracting a ray of light.

Rainbow light is polarized, due to reflection and refraction of light at raindrops.

A linearly polarized ray of light is the one in which components vibrate in only one plane. The plane perpendicular to this plane and the direction of the ray of light is called the plane of polarization.

The Light Spectrum

Ordinary light is known as white light or sunlight. White light disperses into waves with different wavelengths which we see as red, orange yellow, green, blue and violet colour. These waves form a beam of parallel strips otherwise known as a spectrum. The colours of the rainbow are the colours of the light spectrum.

The rainbow is formed by dispersion of sunlight. As a beam of sunlight passes through a raindrop, it is dispersed in the same way as if it was striking as a glass prism. A rainbow is only visible if the sun and rain are present at the same time, and then only when the viewer is standing between the two occurrences so that the sun is behind him.

Absorption of Light

Light carries the energy which can be absorbed by things. Sunlight striking an object will usually have some of its wave lengths absorbed and other wave lengths reflected. If the object looks green, it’s because all the visible radiaton has been absorbed except green light. White cloth reflects light and heat, whereas black cloth absorbs them. That is why we feel cooler in white cloth. Bodies which absorb very small amounts of visible light are transparent.

Differently colored pieces of cloth in the snow which are exposed to sunlight will reach different depths, depending on their colors.

Light filter is a device that uses absorption to change the intesity of light. An example of a light filter are sunglasses.

Polaroids reduce the brightness of sunlight. .

Photosensitive glasses become dark when exposed to strong sunlight, and transparent when exposed to weak sunlight. This is explained by silverbromide crystals cointained in the glasses, which react to light.

Applications of Reflection

Mirrors

Reflection occurs when light hits a surface. The objects around us are visible due to their visible properties.A mirror is a smooth planar surface. That can be a plained metal or a metal-plated glass plane. A mirror can be either plane or curved (concave or convex). Concave mirrors are used in telescopes.

Periscope

A perisope is an optical instrument which enables us to see things without direct eye contact. Its main parts are the ocular optics, two mirors (or prisms) and two lenses. One of the lenses reduces the size of the picture and inverts it, and the other one inverts the picure again and enlarges it. In war the enemy can be watched through the telescope from trenches, wihout being exposed to enemy fire. The artillery periscope can be used as an additional aiming device. It can include an infrared device for night vision. The most complicated type of periscope is the one used in submarines. A submarine periscope must contain binoculars, and it can also have a distance meter, altimeter, radar or an infrared device.

Applications of Total Internal Reflection

Optical fibers

On the principle of total internal reflection light can be transmitted through fibers or thin rods of glass, or some other transparent material with very low loss, even if the fiber is curved.

Laser

A laser is a special kind of light source that produces very regular waves that permit the light to be very tightly focused. Each radiating charge in a non-lever light source produce a light wave that may be a little different from the waves produced by the other charges. Laser sources have atoms whose electrons radiate all in step, or synchronously. As a result, the electrons produce light that is polarized. Monohromatic and coherent lasers have many applications in medicine, scientific research, telecommunications, radioastronomy, holography, military technology. They provide a very focused powerful and controllable energy source that can be used to perform delicate tasks. Laser beams can be used in military operations to determine distances from enemy ships or planes. Besides this, lasers can be used for bomb or missile guidance.

Hologram

Hologram is a three-dimensional photographic image, made by a beam of coherent light, produced by laser. The image of the object is recordered on a special film illuminated by the laser, too.

1. Laser 2. Mirror 3. Object 4. Photographic film

The Fata Morgana

The Fata Morgana, which is a double mirage of the looming effect produces exaggerated images of routine objects. Mirage, realistic image of an object that is either totally imaginary or that appears to be in a location other than the true one. The phenomenon that causes objects to appear out of place, usually in desert or at sea, is the result of atmospheric conditions. When a heat radiates from a hot earth surface, it causes diminution of the density of the air just above the surface and forces a denser layer of air to remain above the hot, rarified air instead of, as is usually the case, bellow it. The boundary between the two layers produces a lenslike effect and refracts or bends rays of light from a distant object. This phenomenon got the name after enchantress Morgana , which was believed that she made this appearance.

Fata Morgana in the desert

Fata Morgana on the sea

Photoelectric effect

Photons may be described as packets of light energy and they are not limited to a specific volume in space or time. German scientist Max Planck discovered that light energy E = h * f is carried by photons, where h is Planck’s constant and f is frequency of its electromagnetic wave. The higher frequency is, thus the greater is the energy given out. The photoelectric effect is a process in which an atom absorbs a photon that has so much energy that the photons sets one of the atom`s electrons free to move outside the atom. When an atom absorbs a photon, it is put in an excited state. When returning to its normal, unexcited state, the atom emits a photon. Lasers are based on this principle.