Definition
Magnification refers to the process of making an object larger or smaller in size. It can be said basically to be the altering of the size the object. The magnified object is referred to as an image. Therefore the magnified object is shall only appear larger or smaller in size with the physical size maintained. This change is calculated and given in a numerical number referred to as the magnification.
Areas where magnification is used
Magnification is used in many fields of study and real life situations. In biology, small objects are magnified so as to be viewed properly. This involves the use of microscopes. The same is used in hospital to help diagnose several types of illness of identification of pathogens present in specimens. Magnification is also used in actual rectification of problems like eye defects using specially made glasses.
Magnification is used in security devices like the CCTV cameras which are located in specific regions to help see them remotely. In photography, magnification is used to alter the size of image to be captured. It is a basic aspect he whole art of photography. In terrestrial sciences and astronomy, magnification helps the scientist to view objects that are very far for easy and detailed viewing using the telescope. The application of magnification is quite diverse.
Types of magnification
There are two basic types of magnification. Typically, this is just called enlargement. An object can be made to appear small i.e. reduced in size. In such a scenario the magnification is always less than one. At times we call this ‘minification’ or de-magnification. When the magnification value is more than one, then the object is said to have been enlarged.
Instruments used in magnification
Magnification uses several instruments specifically made for particular type of magnification. These include
- Microscope which is used for making minute objects appear much larger and easier for viewing at a particular comfortable distant.
- Telescope which is used create a larger images of distant objects for close examination.
- Slide projector which is used to project large images of small slides on a screen
- Magnifying glass. This uses lenses to make things appear large or small.
Calculation of magnification
In general optical magnification is the ratio of the apparent size of an object i.e. the image and the actual size of the object. Therefore this number so obtained has no dimension. However a sign attached to the number indicates whether the image is inverted or upright. In general when the sign is negative the image is inverted while if the sign is positive, then the image is upright.
Calculation of optical magnification
In this, single lens magnification is considered. The formula is as follows. Considering the diagram below, the magnification can be calculated as shown
Where M is the magnification, f is the focal length; d0 is the distance from the lens to the object. Example Consider an object whose focal length (f) is 20 cm, the distant from the lens to the object (d0) is 30 cms and the image distance is 60 cm.
M= 2020-30
Therefore the M=-2. This indicates that the image is inverted. When f is positive, then images height, distance and the magnification are real. If f is negative, then the magnification, height and distance are virtual. It is easiest to observe real images when projected on an opaque screen (although a screen is not necessary for the image to form, in other words, it can be obtained on a screen) Knight, Randall D. (2002 pp. 276–277).
Alternatively, the magnification can be calculated as
Where di is the image distance from the lens hi is the image height and d0 is the object distance from the lens and h0 is the object height. Negative value indicates also an inverted image.
The calculation of the magnification depends on the aspect of object been considered. Basing on this we have linear/transverse and angular magnifications. Linear magnification the size of the object is taken as the actual dimensions of the object measured in millimeters, centimeters, inches etc. angular magnification is when the size refers to the angle subtended to the focal point referred to as the angular size. Therefore the tangent to the angle is taken. The formula for angular magnification shall therefore be
Where ε0 is the angle subtended at the focal point by the object and ε is the angle subtended at the image real focal point of the eye pieces by the image.
Magnification in photography
The art of photography is basically based on magnification. Most cameras use lens which capture real object and can magnify then as required. This is where the image recorded on the film is a real image which is usually inverted. The magnification can also be negative, however in this “real is positive, virtual is negative” ( Ray and sidney , 2002). The formula therefore is
Where M is the magnification, F is the focal length di is the image distance from the lens, hi is the image height and d0 is the object distance from the lens and h0 is the object height.
Angular magnification of a telescope
The first working telescope was used in 1608 and was invented by Hans Lippershey. However there have been others who claimed to have invented the telescope like Zacharias Janssen and Jacob Metius. It mainly comprised of a convex objective lens with a concave eyepiece.
The formula used in magnification is as follows.
Where f0 is the focal length of the objective lens while fe is eyepiece focal length.
The measurement of the actual angular telescopic magnification is difficult. However it is possible to use the reciprocal relationship that exists between the linear and the angular magnification. Note that the linear magnification is constant for any object. If a telescope is focused correctly to view objects at a distance using the angular determination, then the object glass is taken as the image which is called the exit pupil. The diameter of this can be determined using the Ramsden Dynameter that consists of a Ramsden eyepiece with micrometer hairs at the back focal plane. This gadget is fixed at the front of the telescope eyepiece and is used to determine the exit diameter. Since this is smaller than the object glass diameter which gives linear magnification which is a reduction. Then the magnification shall be calculated as follows
Magnifying glass
This is a transparent optical gadget that has one or two spherical surfaces. A magnifying lens can either be concave or convex. The lenses are usually mounted on a frame with a handle and usually the lenses are thick and the centre than on the edges. Its invention can be dated back to the 11th century when Abu ali al-Hasan Ibn al-Haytham who was an Arab scientist published the ‘Book of Optics’ which described the utility of the convex lens that could be used to develop a magnifying glass.
The formula for the magnifying glass is; when the object is infinitely located,
Where f is the focal length of the lens given in centimeters. The constant 25 cm is the near point that a healthy naked eye can focus. When the object is placed at the near focus point then the formula is
Magnification using a microscope
A microscope is mainly used to see small objects. It comes in different types depending on mode of operation. There is the optical microscope which uses light to image an object. There is also the electron microscope which can be either transmission electron microscope or the scanning electron microscope. The inventor is highly disputed.
Magnification is given in terms of the screen size divided by the subject size (Nelson 2011 pp 2).
M0 been the objective lens magnification while Me is the eyepiece magnification.
Both microscope and astronomical telescope give inverted images. Therefore there equation in most cases shall have a negative sign.
The slide projectors
This became common in the 1950s as a form of entertainment. It is an opt-mechanical device used to view photographic slides. The inventor is Louis Misuraca. The projector has four main parts; the electric incandescent light bulb or any other source of light, a reflector and condensing lens to direct the light, slide holder and a focusing lens.
The micron Bar
Magnified figures can be misleading after printing. Most of the time editors of journals and magazines resize images to fit in the pages or in much smaller sections in their work. This makes the magnification wrong. The micron bar or scale bar is a bar that is of known length displayed on the picture and it can be used to take measurements on the pictures. Resizing the bar gives a corresponding resizing of the picture. Any picture with a bar displayed can be resized and the magnification easily calculated. In an ideal case all picture set for printing and publication should have the micron bar so that any resizing gives the right magnification.
Works Cited
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/simmag.html
http://www.physicsclassroom.com/class/refrn/u14l5f.cfm
Knight, Randall D. Five Easy Lessons: Strategies for successful physics teaching. Addison Wesley, 2002. pp. 276–277.
Ray, Sidney F. Applied Photographic Optics: Lenses and Optical Systems for Photography, Film, Video, Electronic and Digital Imaging. Focal Press. 2002. p. 40..
Richard J. Nelson. Microscope Magnification (2011). Desert Trail Mesa, 2011.
