Revise notes
Class 12 Physics
Ray Optics and Optical Instruments
Class 12 Physics
Ray Optics and Optical Instruments
Microscope
- Microscope is an instrument that gives an enlarged image of minute object.
- There are 2 types of microscope:-
- Simple
- Compound
Simple Microscope
- An instrument that gives an enlarged image of a minute object.
- A simple magnifier or microscope is a converging lens of small focal length.
- There are 2 types of Microscopes:-
- Simple Microscope 2. Compound Microscope
Simple Microscope
- The lens is held near the object, one focal length away or less, and the eye is positioned close to the lens on the other side.
- Image which we will get is an erect, magnified and virtual image of the object at a distance so that it can be viewed comfortably, i.e., at 25 cm or more.
To Increase Magnifying Power of Simple Microscope
- If the object is at a distance f, the image is at infinity. However, if the object is at a distance slightly less than the focal length of the lens, the image is virtual and closer than infinity.
- Although the closest comfortable distance for viewing the image is when it is at the near point (distance
D ≅ 25 cm), it causes some strain on the eye.
- Therefore, the image formed at infinity is often considered most suitable for viewing by the relaxed eye.
- Both the cases can be seen in the figures given below:
- The linear magnification m, for the image formed at the near point D, by a simple microscope can be obtained by using the relation:-
- m=(v/u) = v((1/v)-(1/f))
- =(1- (v/f))
- Using the sign conventions, v= (-) ive and same as D.
- Therefore, magnification will be m =(1 +(D/f))
- Since D is about 25 cm, to have a magnification of six, one needs a convex lens of focal length, f = 5 cm.
- Magnification when the image is at infinity.
- Suppose the object has a height h. The maximum angle it can subtend, and be clearly visible (without a lens), is when it is at the near point, i.e., a distance D.
- The angle subtended is then given by:-
- tan θ0 =(h/D)≈ θ0
- To find the angle subtended at the eye by the image when the object is at u.
- Therefore, (h’/h) = m = (v/u)
- Angle subtended by the image will be;-
- tan θ1= (h’/-v) = (h/-v) x (v/u)
- = (h/-u) ≈ θ.
- The angle subtended by the object, when it is at u=-f.
- θi =(h/f).
- The angular magnification is m =( θi/ θ0) =(D/f)
Compound Microscope
- In order to have large magnifications compound microscope is used.
- The lens nearest the object, called the objective, forms a real, inverted, magnified image of the object. This serves as the object for the second lens, the eyepiece, which functions essentially like a simple microscope or magnifier, produces the final image, which is enlarged and virtual.
- The first inverted image is thus near (at or within) the focal plane of the eyepiece, at a distance appropriate for final image formation at infinity, or a little closer for image formation at the near point.
- Clearly, the final image is inverted with respect to the original object.
- Using tanβ = (h/f0) = (h’/L)
- Magnification (mo) due to objective = (h’/h) =(L/f0)
- Where h’ = size of the first image
- h= size of the object
- fo = focal length of the objective lens
- fe= focal length of the eye-piece
- L (tube length) = Distance between focal length of the second objective lens and the first focal length of the eye-piece.
- When the final image is formed at the near point, then the angular magnification will be :-
- me=(1+(D/fe))
- When the final image is formed at infinity, the angular magnification due to the eyepiece is:-
- me = (D/fe)
- Total magnification will be given as:-
- m=(mome) =(L/f0)(D/fe)
- Note: - In order to achieve a large magnification of a small object (hence the name microscope), the objective and eyepiece should have small focal lengths. In practice, it is difficult to make the focal length much smaller than 1 cm.
Class 12 Physics Ray Optics and Optical Instruments NCERT Chapter 9 Free Notes for Best Revision
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