Basic Physics

# Diverging (concave) lens

Definition of the concave lenses

One type of lens used in everyday life is the concave lens. A concave lens is a lens with a thinner center while thicker edges. The concave lenses are usually circular, although there are also lenses that are not circular. The concave lenses, like convex lenses, are made of glass so that the lens has a refractive index greater than the refractive index of the air.

Types of the concave lenses

In general, there are three types of the concave lenses, where the shape of the lens looks like in the figure below (side view). The use of the concave lenses

If a person’s eyes cannot see distant objects clearly or the person is nearsighted, then he utilizes a concave lens or divergent lens or a negative lens to help his vision. The concave lenses are used on eyeglasses or contact lenses to help a person see objects. In addition to being applied to optical glasses and contact lenses, concave lenses are also used in the optical telescope.

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The focal point (F) of the concave lens Observe the figure. The beam of light 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 on the lens surface which is concave, and the lens refracts the beam of light. Refraction of light by the concave lens obeys the law of refraction of light. All the rays of light are refracted as if coming from the focal point of F2 and the refracted beam of light spreads in various directions. The refracted beam of light propagates in multiple directions so that the concave lens is called the divergent lens. The focal point of F2 is the location of the image from a very distant object. If the beam of light refracted by a concave lens comes from the sun, the sun’s image will appear at the focal point of F2. The human eye considers the beam of light is moving straight. Therefore the beam of light is refracted as if coming from the F2 focal point, even though the refracted ray of light does not pass through the focal point. Because the beam of light does not pass through the focal point, the focal point of the concave lens is virtual, and the image of the object that looks as if it is at the focal point is also virtual.

The image of the concave lens

The concave lenses can only form 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. The image formation by the concave lenses explained in detail in the topic of image formation by the concave lens.