A convex lens is thicker at the center and thinner at the top and bottom edges. When light rays parallel to its principal axis fall on the surface of a convex lens, they converge at a point called the principal focus or focal point because of its shape and the difference in refractive index between the air and the lens. The distance between the optical center and the main focal point is called the focal length. Because of their converging properties, they are also called converging lenses.
Convex lenses are broadly classified into the following categories.
It consists of a spherical surface and a flat surface, i.e. it flares outward from one side and has a flat surface on the other side.
They are also known as biconvex lenses. These lenses flare outward from both sides. Their focal length is shorter than that of a plano-convex lens of equal radius and diameter.
Concave Convex Lenses
They are also called bent-moon lenses because they are concave-convex lenses formed by combining a concave lens side and a convex lens side. It has one surface curved inward and the other surface curved outward. It has two radii, one radius is the convex lens surface and the other radius is the concave lens surface.
Depending on the position of the object, a convex lens can form a real and an imaginary image. When the image is formed on the same side as the object, it is called an imaginary image. When the image is formed on the opposite side of the object, it is called a real image. Let us discuss the image formation by convex lens.
Objects at infinite distances
When an object is placed at an infinite distance, a point-sized real and inverted image is formed at the focal point of a convex lens.
Objects beyond the center of curvature
When an object is positioned behind the center of curvature, a true reduced image is formed between the center of curvature (2F) and the focal point (F) on the opposite side of the convex lens. The center of curvature is also called 2F, because it is twice the focal length.
Object at the center of curvature
When an object is placed at the center of curvature in front of a convex lens, a real and inverted image of equal size is formed at the center of curvature on the other side of the convex lens.
Objects between the center of curvature and the focal point
When an object is placed between the center of curvature and the focal point of a convex lens, a real and inverted image of larger size than the object is formed behind the center of curvature on the other side of the convex lens.
Objects at the focal point
When an object is placed in front of the focal point of a convex lens, a real image much larger than the object is formed at infinity on the other side of the convex lens.
Objects at a distance smaller than the focal length
When an object is placed at a distance smaller than the focal length of a convex lens, a virtual and positive image much larger than the object is formed on the same side of the object.
1. Human eye
The most typical example of a convex lens in daily life is that it helps us see the world with our eyes. When an object emits light at a visible wavelength (380-700 nm), it passes through a lens into our eyes and falls on a light-sensitive membrane called the retina. A real and inverted image is formed on the retina; nerve impulses attached to the retina send this data to the brain in the form of signals that are eventually interpreted by the brain and we can see the object clearly. It is very important that the image formed on the retina should be of high intensity, because only then we can see a clear image of the object. This task is performed by the convex lens in our eyes. The convex lens converges the light at a point on the retina, so it provides a clear and sharp image of the object.
2. Magnifying Lenses
The most common use of a convex lens is for a magnifying lens. Magnifiers trick our eyes by creating the illusion of a larger image behind the lens. This illusion is actually the virtual image formed by the convex lens. The magnifying lens converges the light at one point. When the distance between the object and the convex lens is less than the focal length, a magnified image of the object can be observed.
3. Spectacles
Convex lenses are used in eyeglasses to correct farsightedness or hyperopia. People with farsightedness can see distant objects clearly, but they have difficulty seeing nearby objects. In this case, the ciliary muscle is unable to adjust the focal length of the lens, so the image formed is far beyond the retina. The image thus obtained is not focused on the retina and the person sees a blurred image. This can occur due to a weakened ciliary muscle for a number of reasons. To correct this, a convex lens is used in the glasses to converge and focus the light on the retinal surface, resulting in a bright and clear image.
4. Cameras
The type of lens used in a camera may vary depending on the type of photography desired. The lenses used in cameras are usually convex. The convex lens in the camera controls the intensity of the light and the magnification of the object. Light is reflected from the object and enters the camera. The convex lens controls the intensity of the light and focuses it on the photographic film behind the lens; the higher the intensity of the light, the sharper the image. The image can be magnified by adjusting the focal length of the convex lens. When the position of the object is between the center of curvature and the focal point of the convex lens, we get a magnified image of the object.
5. Telescope
A telescope is used to view a clear image of a distant object. Usually, this is done by placing two convex lenses parallel to each other. The first convex lens serves to collect the maximum amount of light from the light source, which is why it is larger than the second convex lens, which ensures a brighter image. The function of the second lens is to magnify the image. Light from distant objects converges on a point between the focal point of the second lens and the center of curvature of the first lens, where a reduced image is formed. When viewed through the second lens, the image appears to be magnified. This magnified image is real and inverted, but a virtual and orthogonal image can also be obtained with the help of another convex lens placed parallel to the first two lenses.
6. Microscope
Microscopes enable us to see magnified images of very small objects that cannot be seen with the naked eye. Microscopes are made by combining two or more convex lenses, which is the reason they are also called compound microscopes. The greater the number of convex lenses used in a microscope, the greater the magnification of the microscope. The simplest compound microscope consists of two lenses. The first lens, which is closer to the object, is called the objective lens, and the second lens is called the eyepiece or ocular lens. The distance between the objective lens and the eyepiece is shorter than the focal length of the eyepiece. The magnified image formed by the objective lens acts as an eyepiece for the object and further magnifies that image, thus providing a double magnified image. By using a compound microscope, the
7. Projector
A convex lens is also used to make a projector. A projector is an optical device that projects an image or video onto a screen. The primary function of a projector is to provide a magnified image of an object, which can be obtained by placing the object between the center of curvature and the focal point of a convex lens. Film or data projectors form an image that is inverted, which is why film is always loaded into the projector upside down so that it can be seen on the projection screen in the correct manner.
8. Multi-junction solar cells
Today, convex lenses are used in concentrating lens systems for multi-junction solar cells. A 3-day study showed that using a convex lens in a concentrating lens system produced a 1.94% increase in light focus compared to when it was not used. The addition of a convex lens above the Fresnel lens increased the output power and reduced the need to use a solar tracker. The convex lens focuses more radiation on the solar cell, which increases power production.
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