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10th Science : Chapter 2 : Optics : Microscope |
Microscope
MICROSCOPE
This is an optical instrument, which helps us to see tiny (very small) objects. It is classified as
1. Simple microscope
2. Compound microscope
Simple Microscope
Simple microscope has a convex lens of short focal length. It is held near the eye to get enlarged image of small objects.
Let an object (AB) is placed at a point within the principal focus (u < f) of the convex lens and the observer’s eye is placed just behind the lens.
As per this position the convex lens produces an erect, virtual and enlarged image (A'B'). The image formed is in the same side of the object and the distance equal to the least distance of distinct vision (D) (For normal human eye D = 25 cm).
Uses of Simple microscope
Simple microscopes are used
- a) by watch repairers and jewellers.
- b) to read small letters clearly.
- c) to observe parts of flower, insects etc.
- d) to observe finger prints in the field of forensic science.
Compound microscope
Compound microscope is also used to see the tiny objects. It has better magnification power than simple microscope.
Magnification power of microscopes can be increased by decreasing the focal length of the lens used. Due to constructional limitations, the focal length of the lens cannot be decreased beyond certain limit. This problem can be solved by using two separate biconvex lenses.
Construction
A compound microscope consists of two convex lenses. The lens with the shorter focal length is placed near the object, and is called as ‘objective lens’ or ‘objective piece’. The lens with larger focal length and larger aperture placed near the observer’s eye is called as ‘eye lens’ or ‘eye piece’. Both the lenses are fixed in a narrow tube with adjustable provision.
Working
The object (AB) is placed at a distance slightly greater than the focal length of objective lens (u > fo) . A real, inverted and magnified image (A'B') is formed at the other side of the objective lens.
This image behaves as the object for the eye lens. The position of the eye lens is adjusted in such a way, that the image (A'B') falls within the principal focus of the eye piece. This eye piece forms a virtual, enlarged and erect image (A"B") on the same side of the object
Compound microscope has 50 to 200 times more magnification power than simple microscope
Travelling Microscope
A travelling microscope is one of the best instrument for measuring very small length with high degree of accuracy at the order of 0.01mm. It works based on the principle of vernier. Its least count is 0.01 mm.
TELESCOPE
Have you seen the recent lunar eclipse? With our naked eye we can’t visualize the phenomena distinctly. Then, how can we see the distant object in clearer manner? It is possible with telescope.
Telescope is an optical instrument to see the distant objects. The first telescope was invented by Johann Lippershey in 1608. Galileo made a telescope to observe distant stars. He got the idea, from a spectacle maker who one day observed that the distant weather cock appeared magnified through his lens system fitted in his shop. Galileo observed the satellites of Jupiter and the rings of Saturn through his telescope. Kepler invented Telescope in which was fundamentally similar to the astronomical telescope.
Types of Telescope
According to optical property, it is classified into two groups:
i) refracting telescope ii) reflecting telescope
In refracting telescope lenses are used. Galilean telescope, Keplerian telescope, Achromatic refractors, are some refracting telescopes.
In reflecting telescope parabolic mirrors are used Gregorian, Newtonian, Cassegrain telescope are some Reflecting telescopes
According to the things which are observed, Astronomical Telescope and Terrestrial Telescopes are the two major types of telescope.
Astronomical Telescope
An astronomical telescope is used to view heavenly bodies like stars, planets galaxies and satellites.
Terrestrial Telescopes
The image in an astronomical telescope is inverted. So, it is not suitable for viewing objects on the surface of the Earth. Therefore, a terrestrial telescope is used. It provides an erect image. The major difference between astronomical and terrestrial telescope is erecting the final image with respect to the object.
Advantages of Telescopes
- Elaborate view of the Galaxies, Planets, stars and other heavenly bodies is possible.
- Camera can be attached for taking photograph for the celestial objects.
- Telescope can be viewed even with the low intensity of light.
Disadvantages
- Frequent maintenances needed.
- It is not easily portable one.