Notes On Images Formed By Lens - CBSE Class 7 Science
 Concave Lens A concave lens is thinner at its centre than at its edges, and is used to correct short sightedness. When light rays are incident on a concave lens, they bend outwards or diverge.  Thus, a concave lens is also called a diverging lens. The image formed by a concave lens is upright, virtual and smaller than the object. For example, the images seen through a peep hole are different from those seen through normal holes, because these peep holes contain concave lenses. Convex Lens A convex lens makes the object magnified, when viewed through it. A convex lens is thick in the middle and thin at its edge. When light rays pass through a convex lens, they bend inwards and converge at a common point to form an image of the source of light. Rays from the sun converge to form its image as a bright spot. A convex lens converges light rays. Therefore, it is also called a converging lens. The image formed when the object is placed close to a convex lens is virtual, erect and magnified. Virtual images cannot be caught on a screen. Images that are caught on a screen are called real images. When the object is placed at a distance from a convex lens, the image formed is real, inverted and diminished. Applications of Lenses Lenses are used in magnifying glasses, peep holes, cameras, bioscopes, binoculars, telescopes, microscopes and projectors. A refracting telescope uses a concave mirror and a convex lens.

#### Summary

 Concave Lens A concave lens is thinner at its centre than at its edges, and is used to correct short sightedness. When light rays are incident on a concave lens, they bend outwards or diverge.  Thus, a concave lens is also called a diverging lens. The image formed by a concave lens is upright, virtual and smaller than the object. For example, the images seen through a peep hole are different from those seen through normal holes, because these peep holes contain concave lenses. Convex Lens A convex lens makes the object magnified, when viewed through it. A convex lens is thick in the middle and thin at its edge. When light rays pass through a convex lens, they bend inwards and converge at a common point to form an image of the source of light. Rays from the sun converge to form its image as a bright spot. A convex lens converges light rays. Therefore, it is also called a converging lens. The image formed when the object is placed close to a convex lens is virtual, erect and magnified. Virtual images cannot be caught on a screen. Images that are caught on a screen are called real images. When the object is placed at a distance from a convex lens, the image formed is real, inverted and diminished. Applications of Lenses Lenses are used in magnifying glasses, peep holes, cameras, bioscopes, binoculars, telescopes, microscopes and projectors. A refracting telescope uses a concave mirror and a convex lens.

#### Activities

 Activity 1 phet.colorado.edu has developed an interactive simulation to describe the formation of image by a convex lens for various positions of the object. This simulation allows to change the radius of curvatute, refractive index of the material of the lens and the diameter (aperture) of the lens. Go to Activity Activity 2 icdadvantage.com has developed a simulation to investigate the image formation by a convex lens. It first explains the general terms associated with a convex lens and then presents the image formation. A virtual lab and a virtual test are given at the end. Go to Activity

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