Difference Between Converging Lenses And Diverging Lenses
Converging Lenses
Imagine some parallel rays headed directly toward a converging lense. After passing through the lense, all except the middle one change direction. The ray that passes directly through the middle keeps going straight ahead. All the others bend toward the middle. They refract (bend aside) after passing through one surface of the lense, and refract a little more after passing through the second surface, that is, when exiting the lense.
A converging lens is very useful because it can send the light rays toward a single point, called the principal focus. The length of the focal distance depends on the material that the lens is made of and the way that the surfaces are curved. The focal distance tells a great deal about the way the lens will bend light rays.
Rays coming from a very distant source are nearly parallel. A converging lens can bring them together at the principal focus to form a bright point of light. You can catch this spot of light on a card held at the principal focus. If the card is not there, the rays simply go on and fan out again after crossing at the principal focus.
Axis is the centre of the lens. The rays described earlier are parallel to each other and to the axis. They pass through the lens and converge at the principal focus. What happens to parallel rays going in some other direction? They converge at a different place, a little off to one side.
Any object, such as a tree or a lamp, is really a collection of many points. Each of these points reflects light or sends out light of its own. A set of light rays coming from any one of the points can be gathered together again by the lens and sent to a certain place on a card held at the proper distance from the lens. The whole set of these bright points forms a “light-picture” of the object from which the rays came.
Diverging Lenses
If you try to do the lamp-lens-card experiment with a diverging lens, you find that you cannot get a real image anywhere on the card. This is because this lens spreads the rays apart.
Imagine Parallel rays moving toward a diverging lens parallel to its axis. After passing through the lens, the rays fan out in a way that makes all of them seem to be coming from a single point in front of the lens. This point is the principal focus of the diverging lens. Its distance from the lens is the focal distance.
Other bundles of parallel rays come in at a slant with the axis. They fan out in the same way after going through the lens. Each bundle seems to come from a different point off to one side or the other of the principal focus. A whole collection of such bundles, coming from various points of an object, forms a complete image. But this image is different in an important way from the real image formed by a converging lens—it cannot be caught on a card. This kind of image is only a collection of points from which the rays seem to come.
A virtual image formed by a diverging lens does not show up on a card. But it can be seen directly through the lens. That is because the eye furnishes an additional set of lenses to bring the rays together.
Look at any object through a diverging lens, such as the lens in a nearsighted person’s eyeglasses. You will see a small, upright image. It seems to be located nearer to the lens than to the object itself. Diverging lenses always form virtual images that are right side up and smaller than the object. Diverging lenses are almost always used in combination with other lenses.