Light - Part XII

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Images by concave lenses
To find out how images are formed with the concave lenses, we have to consider certain rules as regards to rays of light coming from different directions on the surface of the lens and how they get refracted.

First rule : A ray of light parallel to the principle axis XY, on refraction diverges. The line diverging ray can be extended backwards. It can be seen that the line meets XY at the focal point F1. This is shown in the figure given below.  

Second rule :  A ray of light passing through the optical centre O of a concave lens, goes  un-deviated along the same path.  

Third rule :  The third rule is inverse of the first rule. When a ray of light passes through the focus F1 and strikes a concave lens, the refracted ray is parallel to the principal axis XY.

Unlike the different cases seen in case of convex lens, the images formed by concave lenses are always the same!

Consider the figure given  below.  

Place the candle AB anywhere in front of a concave lens. A light ray AD is parallel to the principal axis. On refraction through the concave lens it will take the path DZ (rule 1). Line DZ can be extended behind the concave lens where it appears to emerge from the focus F1. Another ray of light AO will go un-deviated (rule 2) and take the path AOW.  The rays meet behind the concave lens at A’.  Rays of light from the other end of the candle B will be refracted along BO line itself. The image of the candle AB is A’B’ as shown. The image is formed within the focal length of the lens.

Thus whatever the position of the object in front of a concave lens, the image is always :

  • Formed behind the mirror between the optical centre O and the focus F1.

  • The image is erect and virtual

  • The image is diminished and reduced in size.

The use of concave lenses is most widely seen in the use of spectacles for correcting short sightedness. This we shall see in details in later sections.

 

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