Lenses

LENSES• A lens is an optical device, which transmits and

refracts lights, converging or diverging the beam.

HISTORY

• The word lens comes from the Latin name of the lentil, because a double-convex lens is lentil-shaped.

• The genus of the lentil plant is lens , and the most commonly eaten species is Lens culinaris.

• The oldest lens artifact is the Nimrud Lens, dating back 2700 years to ancient Assyria.

• David Brewster proposed that it might have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight.

A Nimrud Lens

• Another early reference to magnification dates back to ancient Egyptian hieroglyphs in the 8th century BC, which depict "simple glass meniscal (a lens with the convex face facing outward.) Lenses“

• The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire).

• Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia.

• These lenses were used by artisans for fine work, and for authenticating seal impressions.

Ancient lens One used by artisans

• The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald .

• Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

TYPES OF LENSES

A simple lens consists of a single optical element.

A compound lens is an array of simple lenses

(elements) with a common axis; the use of multiple elements allows

more optical aberrations to be

corrected than is possible with a single element.

Lenses are typically made of glass or transparent

plastic.

The Visby lenses or Viking lenses

WHAT ARE VIKING LENSES ???

• The Visby lenses or Viking lenses are a collection of lens-shaped manufactured objects made of rock crystal (quartz) found in several Viking graves on the island of Gotland, Sweden, and dating from the 11th or 12th century.

• Some were in silver mounts with filigree.• It has been suggested that the lenses themselves are much

older than their mounts.• The symmetry of the lenses as well as their biconvex elliptical

form and fine polish, which resulted in a very good imaging, made a sensation when discovered by modern scientists.

• The Viking lenses were capable of concentrating enough sunlight to ignite fires.

Uses Of Viking Lenses…

• Used in jewellery• used by craftsmen for magnification in fine

work• used as reading stones• also to start fires.• Olaf Schmidt has speculated that they may

have been used as part of a telescope

A Viking lens

WHAT ARE SIMPLE LENSES???

• A simple lens or singlet lens is a lens consisting of a single simple element.

Types of Singlet Lenses

Classification of LENSES…

• Lenses are classified by the curvature of the two optical surfaces.

• A lens is biconvex (or double convex, or just convex) if both surfaces are convex.

• If both surfaces have the same radius of curvature, the lens is equiconvex.

• A lens with two concave surfaces is biconcave (or just concave).

• If one of the surfaces is flat, the lens is Plano-convex or Plano-concave depending on the curvature of the other surface.

• A lens with one convex and one concave side is convex-concave or meniscus.

Some basic instructions…• The distance from the lens to the spot is the focal length of the

lens, which is commonly abbreviated f in diagrams and equations.• The point of incidence is that point on the surface of the mirror

where the incident ray falls. A line perpendicular to the surface, at the point of incidence , is known as Normal.

• The ray of light coming towards the mirror is called the Incident Ray.

• The ray of light which turns back after ‘reflection’ is called the Reflected Ray.

• The angle between the incident ray and the normal at the point of incidence is known as the Angle of Incidence.

• The angle between the normal and the reflected ray at the point of incidence is called the Angle of Reflection.

Application of these lenses…

• If the lens is biconvex or Plano-convex, a collimated beam of light passing through the lens will be converged (or focused) to a spot behind the lens. In this case, the lens is called a positive or converging lens.

• If the lens is biconcave or Plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens.

• Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. A negative meniscus lens has a steeper concave surface and will be thinner at the centre than at the periphery.

• A positive meniscus lens has a steeper convex surface and will be thicker at the centre than at the periphery.

• An ideal thin lens with two surfaces of equal curvature would have zero optical power, meaning that it would neither converge nor diverge light.

• All real lenses have nonzero thickness, which causes a real lens with identical curved surfaces to be slightly positive.

• To obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens' thickness.

Lensmaker’s Equation

• The focal length of a lens in air can be calculated from the lensmaker's equation:

Where, is the power of the lens, is the focal length of the lens, is the refractive index of the lens material, is the radius of curvature of the lens surface closest to the light source, is the radius of curvature of the lens surface farthest from the light source, and is the thickness of the lens (the distance along the lens axis between the two surface vertices).

Thin Lens Equation

• If d is small compared to R1 and R2, then the thin lens approximation can be made. For a lens in air, f is then given by