الآلات البصرية - الضوء .. الفيزياء

Optical Instruments - Light .. Physics

Optical Instruments

The camera basically consists of a set of lenses that collect light on a light-sensitive film in a sealed chamber, which does not enter light except when the shutter is opened, which exposes the film to a portion of the external rays. The amount of incident light is determined by the size of the aperture with a variable diameter, and the exposure time (which is usually a fraction of a second). It is known that choosing the appropriate aperture and exposure time is done by the exposure meter.

A good set of lenses consists of two pairs of lenses, each of which has no color, and each has a fixed focal length, and is at the same distance from the aperture.
It is known that red light is refracted when passing through a convex lens and is focused at a focal point further from the lens than the focus where the blue color itself is focused. This is one of the most important disadvantages of lenses and is called chromatic aberration. To avoid it, two glass lenses with different refractive indices must be attached, the first of which collects light and the second of which disperses it, but to a small extent. This pair can focus two light waves of different lengths into a single focus and thus reduces the importance of chromatic aberration. Most modern astronomical telescopes consist of a light reflector whose mirrors collect the light coming from small and faint stars that are usually difficult to see with the naked eye. This is because the mirrors are free of chromatic aberration and can be polished well and with great precision.

The light is then reflected on a concave mirror with a large opening whose task is to collect the light on a photographic plate that is exposed to the rays for a long period of time, sometimes several hours. Sometimes, a set of additional mirrors are used whose task is to determine the location of the image more clearly. It is necessary to choose a mirror with a large diameter in order to increase the clarity of the final image. The simplest model of a refracting telescope consists of two convex lenses. The first, called the objective lens, collects the incident light (which is supposed to be parallel to itself) at its focus, and the second, the eyepiece lens, gives the final image of the star, which is inverted and illusory at the same time. It is known that the magnification of this type of telescope is equal to the ratio of the length of the objective focus to the length of the eyepiece focus. The binocular prism consists of two refracting telescopes, one for each eye, and the eyepiece of each one is separated from its object by a set of prisms whose job is to invert the image by the process of total internal reflection, which straightens the final image and gives it its natural position. The process of straightening the image is not important in astronomy, unlike terrestrial uses, which require the top of the image to be up and the bottom to be down.

(Lens and Optical Instruments Groups)

As for the microscope, it provides us with an enlarged image of very small objects, and the magnifying lens in it is usually convex in shape and has a short focal length. Then the object we want to see is placed between the lens and its focus, and as a result an imaginary image is formed behind the focus, i.e. on the side of the object itself in relation to the viewing eye, and the resulting magnification is the ratio of the size of the image to the size of the object itself.

An increase in magnification is obtained by means of a compound microscope, where the objective lens composes the image of the object somewhere between the eyepiece and its focus. Then the latter plays the role of the magnifying lens and gives the final image of the object. In this type of microscope, the magnification is equal to the product of the magnification of the eyepiece and the magnification of the objective. In practice, the object is placed on a thin, sliding slide, illuminated from below by a strong light beam, and care is also taken in the two lenses to be free of chromatic aberration and the rest of the defects that occur during the manufacture of lenses. The maximum magnification ranges between 1000 and 3000 times, and is determined by the resolving power of the lens itself, which is the smallest distance between two points that the microscope can distinguish between. Since this distance is proportional to the wavelength itself, the nature of the light is what determines the resolving power of the lens, and it cannot be improved through the technical development of the lens industry. In the optical microscope, it is about 200 nanometers.

It is worth noting that the use of short-wavelength rays allows a huge increase in the resolving power of the microscope. The difference between the electron microscope and the optical microscope is that the former uses electron rays instead of light rays.

It is known that the movement of electrons is accompanied by a wave motion that has the same properties as light waves, and since the magnetic field is able to deflect electrons from their path, it is also able to collect them at a specific point. This is called a magnetic lens. The wavelength of the electrons' movement depends on the voltage used to accelerate them. If the voltage ranges between 30 and 100 kilovolts, the wavelength becomes between 2 and 4 nanometers, which results in the magnification reaching a million or more and the resolution capacity becoming approximately one nanometer.

Prismatic Specimens
Simple Microscope and Compound Microscope
Comparison between Light Microscope and Electron Microscope.