Diverging (concave) lens

Article about 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).

Diverging lens 1

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.

The focal point (F) of the concave lens

Diverging lens 2Observe 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.

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Diverging lens 3The 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.

10 conceptual questions and answers about diverging (concave) lenses.

1. What is a diverging lens and what are its key characteristics?

A diverging lens, also known as a concave lens, is a lens that spreads out light rays that have been refracted. Key characteristics include thinner center than edges, light rays diverge after refraction, and it always forms a virtual, upright, and reduced image.

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2. How does a diverging lens affect parallel light rays?

A diverging lens will cause parallel light rays to spread apart (diverge) and seem to come from a single point called the focal point. The focal point is on the same side of the lens as the incoming light.

3. What is the formula for lens power and how is it applied to a diverging lens?

The formula for lens power is P = 1/f, where P is the power of the lens and f is the focal length. For a diverging lens, the focal length is considered negative, hence the power of a diverging lens is also negative.

4. Can a diverging lens produce a real image? Explain.

No, a diverging lens cannot produce a real image. This is because the light rays diverge after passing through the lens and they appear to come from a point on the same side of the lens as the light source, hence creating a virtual image.

5. How does the image formed by a diverging lens differ from that formed by a converging lens?

A diverging lens forms a virtual, upright, and reduced image while a converging lens can form real, inverted images (if the object is beyond the focal point) or virtual, upright images (if the object is at a position closer than the focal point).

6. How does the lensmaker’s formula differentiate between a converging and diverging lens?

The lensmaker’s formula is 1/f = (n2/n1 – 1) x (1/R1 – 1/R2), where f is the focal length, n2 and n1 are the refractive indices, and R1 and R2 are the radii of curvature. For a diverging lens, R1 is negative and R2 is positive (considering the convention that the radius is positive if the center of curvature is on the opposite side of the light), while for a converging lens, both are positive.

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7. How does a diverging lens affect the path of a light ray passing through its optical center?

A light ray that passes through the optical center of a diverging lens continues in a straight line, unaffected by the lens. This is because the light’s incidence angle is 0° at the optical center.

8. What is the focal length of a diverging lens and how is it conventionally represented?

The focal length of a diverging lens is the distance between the lens and the point from which diverging light rays appear to originate. Conventionally, it’s represented as a negative value, indicating that the focus is on the same side of the lens as the incoming light.

9. What happens to the image size and position when an object moves further away from a diverging lens?

As an object moves further away from a diverging lens, the image size decreases and the image position moves closer to the lens. However, the image remains upright and virtual.

10. Can diverging lenses be used for correction of certain eye defects? If so, which ones?

Yes, diverging lenses can be used for the correction of myopia or nearsightedness. This condition is characterized by the eye’s inability to focus distant objects, causing them to appear blurred. The diverging lens helps by spreading out the light before it enters the eye, effectively moving the image back to the retina.

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