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Concave Lens and Application of Lenses- Physics Guide for Class 8

Concave Lens and Application of Lenses Class 8 Science Guide

Information about Concave Lens

Title

Concave Lens and Application of Lenses

Class

Class 8

Subject

Class 8 Physics

Topics Covered

  • Convex Lens
  • Image formation by Concave Lens
  • Application of Lens

Concave Lens

Unlike a convex lens, the nature and (general) position of the image, formed by a concave lens, does not depend upon the distance of the object from the lens.
  • A concave lens always forms a virtual, erect and diminished image.
  • Also, the image always appears to be located between the optical centre and the 'F' point of the lens, on the same side as the object is.
We illustrate these features of the concave lens by drawing ray diagrams for three different distances of the object from the lens.
In all cases, the image formed is a virtual, erect and diminished one. Also, it always appears to be formed between the optical centre and the 'F' point of the lens.

Activity 9 
Recall the activity, performed in Class-VII, to study the details of image formation by a concave mirror.
  •  A lighted candle and a screen was used for that activity.
  • We can now perform the same activity using a convex lens. Take a convex lens, a small candle and a thick sheet of paper.
  • Use moulding clay/stand to hold the lens as well as the candle in vertical positions.
  • Keep the lighted candle at some distance away from the lens.
  • Put the thick sheet of paper behind the lens, on the side opposite to the side on which the candle has been put.
  • Adjust the position of the paper screen till you get a sharp image of the candle flame on the sheet.
  • Now, move the candle, first gradually towards the lens and then gradually away from the lens.
When a light ray propogates, from a medium of high refractive index to a medium of lower refractive index, it bends away from normal.
At a certain (critical) angle, the refracted ray bends away so much as to just graze the boundary between the two media.
  • For an angle greater than the critical angle, the ray is (totally) reflected back into the medium of higher refractive index. This phenomenon is known as the phenomenon of total internal reflection.
Optical fibres, used for transmitting and receiving (electric) signals, make use of this phenomenon.
The phenomenon of Mirage, observed in deserts, is also due to total internal reflection.

Applications of Lenses

Lenses find a number of uses and applications in our day to day life. These include the magnifying glass, the microscope, the telescope and the photographic camera, among others. The human eye also has a 'built in' lens that plays a very important role in its functioning. 

Some Points to remember

Lenses have been put to use in designing 'Great Refractors'. These are large telescopes that use specially designed lenses. Some of the great refractors, built over years, have been the following: (i) A 61 cm aperture refracting telescope in Flagstaff, Arizona. (ii) A 69 cm aperture 'great refractor' at Vienna observatory. (iii) A 100 cm aperture refracting telescope, built in 1895, at the Yerks observatory. (iv) A 125 cm aperture demostration telescope, known as the 'Great Paris Exhibition Telescope' of 1900. However, because of very many practical difficulties, astronomers now no longer prefer to use large telescopes made from lenses. Now-a-days, it is the reflecting telescopes—made from mirrors—that are used by astronomers for exploring the far depths of space. These are very much larger in size than the great refractors. 
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