With help of our Eyes, we can see the beautiful world around us. This happens due to the light rays. The light rays fall on any object and get reflected by it. These reflected rays fall on our eyes. The eyes have many parts like cornea, aqueous humor, and crystalline lens. The light from here in the form of electric signals moves towards the brain through the optical nerve.
But there are certain objects which we can’t see clearly with our naked eyes. We find it difficult to see distant planets and stars clearly with our eyes. Also, small objects like the cell or the virus, or the atoms can’t be seen with the naked eyes. Hence to see these objects we take the help of some instruments. These are known as optical instruments. A hand lens or magnifier is a common example of such an instrument. We can divide these into two categories - one we use to see the long-distance objects, and one we use to see the small objects.
The device which is used for looking at small objects is called a Microscope. Although there are many kinds of microscopes, In the next sections we will learn about the light microscope and electron microscope.
It is a microscope in which the visible light waves are used. This means light waves get reflected from the object which we want to see, and then it falls on our eyes. And finally, we can see the magnified image of the object. It is also known as the optical microscope. In this kind of microscope, we use visible light, the wavelength varies between 400-700 nm.
We can use lenses in this kind of telescope for magnification. A microscope with a single lens is called a simple microscope. Compound microscopes have two lenses.
Simple and compound microscopes are the two different kinds of microscopes.
Simple Microscope − It consists of one convex lens between the object and the eye. The convex magnifies the object and we see a bigger image. This is the microscope used in magnifying glasses and eyepieces.
Compound Microscope − In this kind of microscope, two convex lenses are used. The lens near the object is called the objective lens. It creates a real image of the object. This image forms inside of microscope (Image 1). The lens close to the eyes creates an enlarged image of this real image (Image 2). This lens is called the eye-piece lens.
| Advantages | Disadvantages |
|---|---|
| Very easy to use, lighter to carry | Low resolution |
| Unaffected by Electromagnetic radiation | Can’t provide 3D images |
| Allows to use on live organisms | Magnification is not very large |
| Maintenance cost is very low | Can’t operate in Dark |
In the electron microscope, instead of visible light, we use a beam of electrons as illumination. If we use the electron beams then their wavelength is approximately 104 times less than the wavelength of visible light. Hence it gives more resolution than the light microscope. Not only does it give a picture of the object, but it also tells about the structure of the object. Some main types of electron microscopes are the following
Transmission electron microscope
Scanning electron microscope
Reflection electron microscope
Scanning tunnelling microscope
The main parts of the electron microscope are the followings -
Electron Gun - A tungsten filament is used here and it emits electrons when heated.
Electromagnetic lens - Condenser lens, objective lens, and projector lens are used.
Holder - It is made of a very thin sheet of carbon.
Imaging system - The camera is used below the fluorescent screen
An electron gun is used as a source of electrons. The first condenser lens focuses the beam on the object, and then the second condenser lens is used to form a thin beam of electrons. The electrons coming out from the object after the reflection are passed through a magnetic coil called the objective lens. Then the final image is obtained from the Projector lens. The image can be seen on the fluorescent screen.
| Advantages | Disadvantages |
|---|---|
| Give very high magnification | The electron beam can kill living organisms |
| Can create 3D images also | Not very easy to use |
| High-resolution | Presence of Electromagnetic field can affect the image. |
Light microscope and electron microscope both are used for magnifying tiny objects. However, there are certain differences between them. In a light microscope, we use visible light and in an electron microscope, the beam of electrons is used. The magnification and resolution of the electron microscope are higher than the light microscope.
Q1. Magnification power is the ratio of the height of an object seen by the naked to the height seen after using the lenses.
Ans. $$\mathrm{m=\frac{h^{'}}{h}}$$
For a simple microscope the magnification is given by -
$$\mathrm{m=1+\frac{D}{f}}$$
Here D = least distance for clear vision.
f= focal length of lens
Q2. What are the applications of the Electron Microscope?
Ans. Electron microscope is widely in the following areas
In the Medical field, in biopsy samples and study of cells, etc.
In industries, it is used for quality control
Crystallographers use it to study the internal structure of crystals.
In biology, it is used for the study of micro-organisms such as viruses, bacteria, etc.
Q3. Write the main differences between the light microscope and electron microscope.
Ans.
| Light Microscope | Electron Microscope |
|---|---|
| Uses light as illumination | Uses electron beam as illumination |
| Live or dead both kinds of objects can be seen | Mostly used for dead and dried sample |
| Its magnification is 500 to 1500 times the original size | Its magnification is 100000 to 300000 times the original size |
| Image of the object is colored | Image of the object is black and white |
| The sample to be studied can be prepared in minutes | The preparation time for the sample to be studied can be in days. |
Q4. What is the difference between the Scanning electron microscope and tunneling microscope?
Ans.
| Scanning Microscope | Tunnelling Microscope |
|---|---|
| It is used for thin samples | It has a large depth |
| Projecting Image of an object is formed by transmitted electrons through the samples. | The image formed by the secondary electrons generated by the sample. |
| Analogous to conventional Compound microscope | Analogous to stereo light microscope |
| Easy to use | Need training |
| Easy sample preparation | Laborious sample preparation |
Q5. Can we Change the magnifying power of the Compound microscope? How?
Ans. Magnification power of the compound microscope is given by -
$$\mathrm{m=-\frac{L}{f_{0}}\frac{D}{f_{e}}}$$
Where L = tube length or distance between the eyepiece and second lens
D = the least distance for clear vision
$\mathrm{f_{0}}$ = Focal length of objective lense
$\mathrm{f_{e}}$ = focal length of objective lens
We can increase the magnification of the compound microscope if we increase the tube length. Since the magnification is directly proportional to the tube length. We can also see that taking lenses of short focal length will also increase the magnification of the microscope.
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