Converging (convex) lens

Definition of the convex lens

One type of lens used in everyday life is the convex lens. The convex lens is a lens with a thicker middle while the edges are thinner. Convex lenses are usually circular and made of glass so that the lens has the refractive index higher than the refractive index of the air.

Types of the convex lenses

In general, there are three types of the convex lenses, in which the shape of the convex lens is as shown below (side view).

Converging lens 1

Use of the convex lenses

If a person’s eyes can not see close objects clearly or farsightedness, then he can use the convex lens or the converging lens or the positive lens to help his eyesight. The convex lenses are used on eyeglasses or contact lenses to help people with nearsightedness. The convex lens is also used as a magnifying glass, optical instruments used in cameras, binoculars and telescope optics and optical instruments microscope.

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The focal point of the convex lens (F)

Converging lens 2Observe the figure on the side. The light beam that comes from objects that are very far away like the sun is parallel to the principal axis of the lens. In the figure, the principal axis of the lens is a blue line.

A beam of light comes to the surface of the lens which is convex, and the lens refracts the beam of light. Refraction light by the convex lens obeys the law of refraction of light. All incoming light beams refracted by the convex lens towards the focal point of F2 and the light beam intersect at the focal point.


Converging lens 3The light beam refracted to the focal point. Therefore, the convex lens is called a convergent lens. Because all light intersects at the focal point of F2, it can be said that the focal point of F2 is the location of the image of a very distant object. If the beam of light refracted by the convex lens comes from the sun, the sun’s image will appear at the focal point of F2.

Because the beam of light passes through the focal point, the focal point of the convex lens is real, and the image located at the focal point is real.

Image of the convex lens

Unlike concave lenses that can only form virtual images, the convex lenses can form real images and virtual images. The virtual image does not exist but as if it exists because the human eye sees the beam of light moving straight so that the human brain concludes that the image exists. If a screen is placed at a point where there is a virtual image, there is no image on the screen. On the contrary, the real image exists. If a screen is placed at a point where there is a real image, the image appears on the screen. The formation of the image by the convex lenses has been explained in detail in the topic of the image formation by the convex lens.


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