This PPT contains : lenses, Types of lenses- Spherical and cylindrical lenses, Concave and Convex lenses and their Identification and uses, Refraction through Concave and Convex lenses, Sturm's Conoid and Vergence

1. Optics- Lenses
Ms. Khushi Kansal
Assistant Professor
Subharti Medical College (Department of Paramedical Sciences)
Swami Vivekanand Subharti University

2. Lens
 A Lens is a transparent refracting medium, bounded by two surfaces that form a part of a sphere or
a cylinder.
 Classifications:
 According to the nature of its surface- plane, spherical, cylindrical and aspheric surfaces.
 According to its effect on light rays- convergent & divergent lens.
 According to its thickness- thin & thick

3. Cardinal data of a lens
1.Centre of curvature (C) is the centre of the sphere of which the refracting lens surface is a part.
2. Radius of curvature (r) is the radius of the sphere of which the refracting surface is a part.
3. Principal axis (AB) is the line joining the centres of curvature of its surfaces.
4. Optical centre (N) is a point on the principal axis of the lens, the rays passing from where do not
undergo deviation.
5. Principal focus (F) is that point on the principal axis where parallel rays of light, after passing through
the lens, converge or appear to diverge.

4. 6. Focal length (f) is the distance between the optical centre and the principal focus.
7. Power of the lens is defined as the ability of the lens to converge a beam of light falling on the lens.
 The unit of power is Dioptre (D).
 It is measured as reciprocal of the second focal length in metres, i.e. P = 1/F2

5. Refraction through spherical lenses
 Spherical lenses are bounded by two spherical
surfaces and are mainly of two types: convex and
concave.

6. Convex lens
 Convex lens or plus lens is a converging lens.
 It is of biconvex, plano-convex or concavo-convex (meniscus) type.
 Identification:
1. thick in the centre and thin at the periphery
2. An object held close to the lens appears magnified.
3. When a convex lens is moved, the object seen through it moves in the opposite direction to the lens.

7. Convex lens
 Uses:
1. correction of hypermetropia, aphakia and presbyopia
2. Indirect ophthalmoscopy, as a magnifying lens
 Image formation:
Position of object Position of image Nature and size of the image
At infinity At focus F2 Real, very small and inverted
Beyond 2F2 Between F2 and 2F2 Real, diminished and inverted
At 2F1 At 2F2 Real, same size and inverted
Between F1 and F2 Beyond 2F2 Real, enlarged and inverted
At focus F1 At infinity Real, very large and inverted
Between F1 and the optical
centre
On the same side of the lens Virtual, enlarged and erect

9. Concave lens
 Concave lens or minus lens is a diverging lens.
 It is of three types: biconcave, plano-concave and convexo-concave (meniscus).
 Identification:
1. Thin in the centre and thick at the periphery.
2. An object seen through it appears minified.
3. When the lens is moved, the object seen through it moves in the same direction as the lens.
 Uses:
1. Correction of myopia
2. Hruby lens foe fundus examination with the slit lamp

10. Image formation: a concave lens always
produces a virtual, erect and diminished image
of an object.

11. Refraction through the cylindrical lens
 A cylindrical lens/Toric surface acts only in one axis.
 The power is incorporated in one axis, the other axis having zero power.
 It may be convex or concave.
 The cylindrical lens has a power only in the direction at right angle to the axis. Therefore, the
parallel rays of light after passing through the cylindrical lens do not come to a point focus but
form a focal line.

12. Cylindrical lens
 Identification:
1. When the cylindrical lens is rotated, the object seen through it becomes distorted.
2. When it moves up and down or sideways, the object will move with (in concave cylinder) or opposite (in
convex cylinder) only in one direction.
 Uses:
1. Prescribed to correct astigmatism
2. Jackson cross cylinder
3. Maddox rod (consists of a series of powerful convex cylindrical lenses mounted together in a trial lens.

13. Cylindrical lens
 Types:
1. Simple: curved in one meridian only, either convex or concave.
2. Compound: curved unequally in both the meridian, either convex or concave and also known as
spherocylinder.
3. Mixed: one meridian is convex and other is concave.

14. Sturm’s conoid
 In toric surface, one principal meridian is more curved than the second principle meridian,
 The configuration of rays refracted through a toric surface is called the sturm’s conoid.

16. Sturm’s conoid
 At point A, the vertical rays are converging more the the horizontal rays, so the section here is
horizontal oval or an oblate ellipse.
 At point B, the vertical rays have come to focus while the horizontal rays are still converging and so
they form a horizontal line.
 At point C, the vertical rays are diverging and their divergence is less than the convergence of the
horizontal rays, so a horizontal oval is formed here.
 At the D, the divergence of vertical rays is exactly equal to the convergence of the horizontal rays.
So the section is a circle, which is called the circle of least confusion.

17. Sturm’s conoid
 At point E, the divergence of vertical rays is more than the convergence of horizontal rays. The
section is called vertical oval.
 At point F, the horizontal rays have come to focus while the vertical rays are divergent and a vertical
line is formed.
 Beyond F, both horizontal and vertical rays are diverging. So the section is vertical oval or prolate
ellipse.
 The distance between the two foci called the focal interval of sturm.

18. Vergence of light
 The term vergence describes what light rays are doing in relation to
each other.
 With respect to a given point, light rays can:
 Spread out (diverge)
 Come together (converge)
 Run parallel (vergence =0)
 Vergence is measured in diopters (D).
 Vergence power of the lens is positive for converging lens and negative
for diverging lens.
 Dioptric power is defined as the reciprocal of the distance (In meters) to
the point where light rays would intersect.
 D = 1/f2