Lenses

A lens is a piece of transparent material that is generally round in form and has two polished surfaces, one or both of which is curved and may be convex (bulging) or concave (not bulging) (depressed). Curves are usually always spherical, which means that the radius of curvature is constant. The ability of a lens to produce pictures of things in front of it is an important quality. Eyeglasses, contact lenses, pocket magnifiers, projection condensers, signal lights, viewfinders, and rudimentary box cameras all employ single lenses. To correct aberrations, a number of lenses made of various materials are frequently joined as a compound lens in a tube. Compound lenses are employed in a variety of equipment, including cameras, microscopes, and telescopes.

What is Lens?

The lens is a piece of transparent material that is formed to bend light rays in a given way as they pass through it, whether that means causing the rays to converge to or diverge from a specific location. The lens might be made of glass or plastic, and the form of the lens controls whether light rays converge or diverge. The term “lens” is derived from the Latin word for “lentil,” owing to the form similarities between a converging lens and a legume.

Because the lens material has a different index of refraction than the surrounding air, the real bending of light rays generated by a lens happens. Snell’s law of refraction describes this behaviour by relating the difference in angle between the incident and refracted light rays to the indices of refraction for the two materials.

In summary, the law states that if you move from a lower refractive index substance to a higher one (e.g., from air to glass), the light ray is deflected towards the “normal” to the surface (i.e., the direction perpendicular to the surface at that point), and the opposite is true for light rays moving from a higher refractive index material to a lower.

Types of Lenses

There are two types of lenses: convex (or converging) and concave (or diverging).

Convex

A convex lens (also known as a positive lens) has glass (or plastic) surfaces that bulge outwards in the centre, creating the iconic lentil-like form. A convex lens is also known as a converging lens because it causes parallel light rays travelling through it to bend inward and meet (converge) at the focal point, which is located immediately beyond the lens.

Convex lenses are used in telescopes and binoculars to focus distant light rays in your eyes.

Concave

“It is another form of lens that allows parallel light beams to diverge from a point.” This is referred to as a concave or diverging lens. This lens is narrow in the middle and thick at the borders.”

Important Terms

Focal Point : The focal point is the point at which two parallel rays converge after passing through a lens.

Focal Length : The focal length of a lens is the distance between its centre and its focal point, and it effectively defines a lens’s “bending power.”

Optical Axis : The optical axis is the lens’s line of symmetry.

A light ray is an approximation of the path of light in which straight lines represent the velocity of light waves (or photons). Every point on an object emits light rays in every possible direction, but the position of the final picture is generally determined by a few distinct rays.

Optical Center : The optical centre is the point on the primary axis at the centre of the lens.

After refraction, light rays travelling parallel to the primary axis of a convex lens meet at a point on the principal axis known as the principal focus of focal point F. As a result, convex lenses are also known as converging lenses. Principal focus F is the name given to a concave lens. As a result, a concave lens is also known as a diverging lens.

Pole : The pole is the centre of the lens’s spherical refracting surface. The point at which the principal axis intersects the lens’s surface.

Center of Curvature : A lens has two spherical surfaces, which together form a sphere. The centre of curvature is the point at where these spheres intersect.

Determination of Focal length of lenses

The approximation technique 

Using the convex lens, focus a crisp, inverted picture of a very distant object (e.g., a tree more than 6 metres away) on a white screen. Using a metre rule, measure the distance between the screen and the lens. This will give you a rough estimate of the focal length (f) of the lens.

No parallax method

Place a plane mirror perpendicular to the primary axis of a lens. Place an inverted pin on a cork with its tip parallel to the primary axis. Move the pin in front of the lens back and forth until its point (tip) aligns with the point of its inverted image.

When there is no parallax between the pin and its image, the coincidence is guaranteed. The focal length (f) of a convex lens is given by the distance between the lens and the pin.

Conclusion 

Because of the shape similarities between a converging lens and a legume, the name “lens” is derived from the Latin word for “lentil.” The true bending of light rays caused by a lens occurs because the lens material has a different index of refraction than the surrounding air. A convex lens is also known as a converging lens because it allows parallel light rays to bend inward and meet at the focal point, which is positioned just beyond the lens. Light rays travelling parallel to the primary axis of a convex lens meet at a point on the principal axis known as the principal focus or focal point F after refraction.