A lens is defined as a portion of a refracting medium bordered by two curved surfaces which have a common axis.

When each surface forms a part of sphere, the lens is called a spherical lens.

SPHERICAL LENS

SPHERICAL LENS - CONVEX LENS - CONCAVE LENS

CYLINDRICAL LENS

CONVEX LENS

CONCAVE LENS

A convex lens causes convergence of incident light, whereas a concave lens causes divergence of incident light.

CONVEX LENS

CONCAVE LENS

Center of curvature (C) The centre of the sphere of which the refracting lens surface is a part.

Radius of curvature (r)

It is the radius of the sphere of which the refracting surface is a part.

The principal axis (AB) It is the line joining the centers of

curvature of its surfaces.

Optical centre (N) of the lens corresponds to the nodal point of a thick lens.

The principle focus (F) of a lens is that point on the principal axis where parallel rays of light, after passing through the lens ,converge (in convex lens) or appear to diverge (in concave lens)

The Focal length of a lens is the distance b/w the optical centre and the principal focus.

Power of a lens (P) The ability of the lens to

converge a beam of light falling on the lens.

Converging (convex) – positive Diverging (concave) – Negative Unit is dioptre (D)

Lenses of shorter focal length are more powerful than lenses of longer focal length.

F = 1/ f2

Where F = Vergence power of lens in dioptres.

f2 = Second focal length in metres

So, in the above vergence at the lens is:

F = 1 / 0.25 = 4.00 D.

vergence at the lens is: F = 1/ 0.10 = 10.00 D.

Vergence is the term that determines the direction and power of light transmission.

The vergence of light is defined by:

V = n/LWhere n = refractive index of the material. L = the distance in accordance

Since the distance L1 is measured from the wavefront and the light is traveling from left to right, it is a negative distance and the vergence is negative (divergent).

L2 is positive since it is directed to the right from the wavefront (convergent)

Parallel light rays are said to have zero vergence.

The unit to measure vergence is “dioptre”. The change in vergence when the light encounters a refracting surface is equal to the power of the surface.

Most of the optical lenses have curved surfaces; the surface may be described as convex, if it bulges out of the material, and concave if it is depressed into the material.

The measure of the shape of a curved surface is known as curvature.

The curvature of a spherical surface can be read with a simple instrument called “spherometer”.

The common form of spherometer normally available is the “Optician’s lens measure”.

More accurate version of this simple lens measure is used in surfacing laboratory and is known as “Sagometers”.

Trial lens hand neutralizationTrial lens hand neutralization

Lensometer / FocimeterLensometer / Focimeter

Geneva lens measureGeneva lens measure or or Lens clock Lens clock

It is possible to determine the spherical lens by studying the image formed when two lines crossed at 90º, are viewed through the lens.

Spherical lens causes no distortion of the cross. However, when the lens is moved from side to side and up and down along the axis of the cross, the cross also appears to move.

Against movement:- In the case of a convex lens, the cross appears to move in the opposite direction to the lens .

With movement :- a movement in the same direction, is observed if the lens is concave.

Rotation of a spherical lens has no effect upon the image of the crossed lines.

Against movement:-

With movement

Two lenses neutralize each other when placed Two lenses neutralize each other when placed in contact with each other so that the combined in contact with each other so that the combined power of the two lenses is equal to zero.power of the two lenses is equal to zero.

An unknown lens is neutralized by a known An unknown lens is neutralized by a known trial lens of equal power but opposite in sign.trial lens of equal power but opposite in sign.

This is performed in the absence of a lensometerThis is performed in the absence of a lensometer

It more accurately estimates low plus and minus It more accurately estimates low plus and minus power lenses than toric lenses.power lenses than toric lenses.

It is used to measure the It is used to measure the front vertex power front vertex power of of the lensthe lens

View a large distant cross target through the View a large distant cross target through the lens.lens.

Hold lens on visual axis, at arm’s distance.Hold lens on visual axis, at arm’s distance.Align lens such that the cross target is Align lens such that the cross target is

continuous.continuous.Move lens vertically, observe movement of Move lens vertically, observe movement of

horizontal line.horizontal line.Move lens horizontally, observe movement of Move lens horizontally, observe movement of

vertical line.vertical line.

For a plus or a minus lens, movement is used For a plus or a minus lens, movement is used to neutralize powerto neutralize power

If observe “If observe “against movement against movement - use - use MINUSMINUS lens lens If observe If observe “with movement “with movement -use -use PlusPlus lens lens

Place known trial lens against front surface of Place known trial lens against front surface of unknown lensunknown lens

No movement indicates No movement indicates neutrality.neutrality.The dioptric power of the unknown lens must

equal that of the trial lens of opposite sign, e.g, a + 2.00 D lens neutralizes a – 2.00 D.

To measure this accurately, the neutralizing lens must be placed in contact with the back surface of the spectacle lens.

with highly curved lenses, this is not possible and an air space intervenes. It is, therefore, better to place the neutralizing lens against the front surface of the spectacle lens.

Neutralization is, thus, somewhat inaccurate for curved lenses of more than about 2.00D

Large distant cross target Large distant cross target

Plus lensPlus lens

Minus lensMinus lens

Focimeter is an optical instrument for determining the vertex power, axis direction and optical center of an ophthalmic lens.

It is used to measure the It is used to measure the back vertex powerback vertex power or or front front vertex powervertex power of the lensof the lens

The image of the target is seen as a ring of dots when a spherical lens is tested

Target as seen in Focimeter

Lensometer

Can be used to find the surface powers of a lens by measuring the surface curvature.

The total power of a thin lens equals the sum of its surface powers.

It is called as Geneva Lens Gauge, Lens measure or Lens Clock