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Lenses are transparent pieces of glass or plastic that refract light. It is based on the principle that as light moves from air into glass or plastic, its direction changes due to changes in density. Lenses are used in cameras, microscopes, telescopes, binoculars, and corrective glasses. A lens can either be convex or concave, depending on the shape. To put it simply, Lenses are specialised pieces of glass used in an array of equipment. After cutting the lens, when two lenses are in combination, the first one forms an image that is used as an object for the second one. This is known as a combination of lenses.
Cutting of lens
Through the optical axis
Symmetric convex lenses are cut into two equal parts along the optical axis. Since the intensity of the image formed by each part is the same as the intensity of the entire lens, the focal length for each part is double the original. Convex lenses have two circular arcs, and when the two arcs of the lens have an equal radius, it is known as a symmetrical convex lens.
Through the principle axis
Symmetric lenses are cut along the principal axis into two equal parts. The intensity of the image formed by each part will be less when compared to that of a complete lens (the aperture of each part is 1/√2 times that of the complete lens). Thereby, the focal length remains the same for each part because the focus and radius of the curvature will remain the same.
Combination of lenses
To calculate the net power, net focal length, and magnification for a system of lenses, the following formula is used:
p=p1+p2+p3………
m=m1+m2+m3…….
1/F=1/f1+1/f2+1/f3……
When two thin lenses are in contact, the combination of lenses will be a lens that has either more power or less focal length.
1/F=1/f1+1/f2+1/f3 = F= f1f2/f1+f2
When two lenses of similar focal length but reverse nature are in contact, then the combination will be a plane glass plate, and thereby;
F(combination) = ∞
In the case of two lenses co-axially placed at a distance ‘d’ from each other, then the equivalent focal length (F) will be:
1/F=1/f1+1/f2-d/f1f2 and p= p1+p2-dp1p2
When one surface of the lens is silvered, the lens forms a mirror. Thereby, the focal length of the formed silver lens is given by:
1/F= 2/f1+1/fm
where f1= focal length of lens where refraction is produced and fm= focal length of the glass where reflection is produced.
How does a lens work?
A lens produces a focusing effect because light travels more slowly in the lens than it does in the surrounding air. As a result, refraction, which is an abrupt bending of a light beam, occurs on both sides where the beam enters the lens and where it emerges from the lens to the air. A single lens has two regular opposite surfaces; either both surfaces are curved, or one is curved and the other is plane. Because of the curvature of the lens surfaces through different angles, different rays of an incident light beam are refracted to make an entire beam of parallel rays converge to or seem to diverge from a single point. This point is known as the focal point. Refraction of the rays of light emitted by or reflected from an object causes the rays to form a visual image of the object.
Conclusion
Lenses are significantly found in a huge array of optical instruments, ranging from simple magnifying glasses to a camera lens to the lens of the human eye. The word lens is derived from the Latin word for ‘lentil bean’—the shape of a bean is similar to that of a convex lens that is converging in nature, whereas a concave lens is diverging in nature. Simply put, a lens, in optics, is referred to as a piece of glass or other transparent substance that is used to form an image of an object generated by focusing rays of light from the object.
