20.TELEPHONES

The telephone consists of coils of fine insulated wire that is wound around a permanent horse shoe magnet.A soft iron disc diaphragm is held near the end of the magnet.The magnet lines of force gather in this disc.When the disc is thrown into vibration by a human voice,the number of lines of force passing through the coil changes and a flutuating current is induced.At the receiving end the terminals over the coil wound over the oles of another horse shoe magnet produces the similar vibrations that are produced at the transmitting end and thus helps in producing the sound.

--21.CAMERA
Equipment for taking photographs which usually consists of a lightproof box with a lens at one end and light-sensitive film at the other.

Photography is undoubtedly one of the most important inventions in history -- it has truly transformed how people conceive of the world. Now we can "see" all sorts of things that are actually many miles -- and years -- away from us. Photography lets us capture moments in time and preserve them for years to come.

The basic technology that makes all of this possible is fairly simple. A still film camera is made of three basic elements: an optical element (the lens), a chemical element (the film) and a mechanical element (the camera body itself). As we'll see, the only trick to photography is calibrating and combining these elements in such a way that they record a crisp, recognizable image.

EVERY camera has these basic parts.
This first and main part is called the body.

The second part is the shutter which might be located in the lens (leaf shutter) or it might be located right in front of the film (focal plane shutter).

The shutter controls WHEN the light enters the camera and for how long it enters. The shutter in the lens is often faster and quieter, but makes changing the lens difficult. The shutter in front of the film allows for easy lens removal, but is often loud and slow. A good camera will have some way of adjusting the time the shutter is open plus there has to be some type of release for the shutter.

The lens lets in light. The larger the lens the more light. The lens also effects how large the image appears based on the focal length of the lens. The aperture is located in the lens and is a set of leaf like piece of metal that can change the size of the hole that lets in light. We consider the lens to be part of the shutter as we do not actually need a lens to focus an image if we have a small enough hole to let in the light.

Finally, the third part is film holder inside the camera. This must have some attachment that allows for the film to be moved which can either be a lever or a motor.

--22.LASER:

Laser light has several features that are significantly different from white light. To begin with, light from most sources spreads out as it travels, so that much less light hits a given area as the distance from the light source increases. Laser light travels as a parallel beam and spreads very little.

Furthermore, laser light is monochromatic and coherent. White light is a jumble of colored light waves. Each color has a different wavelength. If all the wavelengths but one are filtered out, the remaining light is monochromatic. If these waves are all parallel to one another, they are also coherent: the waves travel in a definite phase relationship with one another. In the case of laser light, the wave crests coincide and the troughs coincide. The waves all reinforce one another. It is the monochromaticity and coherency of laser light that makes it ideal for recording data on optical media such as a CD as well as use as a light source for long haul fiber-optic communications.

The laser uses a process called stimulated emission to amplify light waves. (One method of amplification of an electromagnetic beam is to produce additional waves that travel in step with that beam.) A substance normally gives off light by spontaneous emission. One of the electrons of an atom absorbs energy. While it possesses this energy, the atom is in an excited state. If the electron gives off this excess energy (in the form of electromagnetic radiation such as light) with no outside impetus, spontaneous emission has occurred.

If a wave emitted by one excited atom strikes another, it stimulates the second atom to emit energy in the form of a second wave that travels parallel to and in step with the first wave. This stimulated emission results in amplification of the first wave. If the two waves strike other excited atoms, a large coherent beam builds up. But if they strike unexcited atoms, they are simply absorbed, and the amplification is then lost. In the case of normal matter on Earth, the great majority of atoms are not excited. As more than the usual number of atoms become excited, the probability increases that stimulated emission rather than absorption will take place.

Physicist Gordon Gould invented the laser in 1958. The first working model was built in 1960 by T.H. Maiman. It contained a synthetic, cylindrical ruby with a completely reflecting silver layer on one end and a partially reflecting silver layer on the other. Ruby is composed of aluminum oxide with chromium impurities. The chromium atoms absorb blue light and become excited; they then drop first to a metastable level and finally to the ground (unexcited) state, giving off red light. Light from a flash lamp enters the ruby and excites most of the chromium atoms, many of which fall quickly to the metastable level. Some atoms then emit red light and return to the ground state. The light waves strike other excited chromium atoms, stimulating them to emit more red light. The beam bounces back and forth between the silvered ends until it gains enough energy to burst through the partially silvered end as laser light. When most of the chromium atoms are back in the ground state, they absorb light, and the lasing action stops. In continuous-wave lasers, such as the helium-neon laser, electrons emit light by jumping to a lower excited state, forming a new atomic population that does not absorb laser light, rather than to the ground state.

--23.MICROSCOPE

Microscopes give us a large image of a tiny object. The microscopes we use in school and at home trace their history back almost 400 years.

The first useful microscope was developed in the Netherlands between 1590 and 1608. There is almost as much confusion about the inventor as about the dates. Three different eyeglass makers have been given credit for the invention. The possible inventors are Hans Lippershey (who also developed the first real telescope), Hans Janssen, and his son, Zacharias.
Lens quality in early microscopes was often poor so the images were not very clear. But even these rather crude microscopes were a great help in learning more about animals and plants.

The microscope works a lot like a refracting telescope except that the object is very close to the objective lens.The clips on the microscope's flat stage hold the slide in place.A mirror at the bottom of the microscope reflects light rays up to the daphnia through a hole in the stage. Objective lenses magnify the image which is made even larger when we see it through the eyepiece lenses.

The objective lens is usually a compound lens, a combination of two lenses made from different kinds of glass. When only one lens is used, we often get distortion. This distortion (chromatic aberration) is caused because the colours making up light are not refracted (bent) the same amount when passing through a glass lens. When we use a compound lens, any distortion from the first lens is corrected by the second lens.

Different types of microscopes have been used to look at human cells, identify minerals, solve crimes.
Microscopes are an essential tool in medicine too. They have been used to identify the causes of many deadly diseases like malaria and tuberculosis. Microscopes can also help to find out why a person or animal died.

Scientists can even use a microscope to figure out where illegal drugs come from. For example, looking at opium crystals through a microscope reveals different shapes depending on where the poppies they came from were grown. This information can help pinpoint the source of illegal drugs.