It covers the topics--Spherical lenses and their types ,principal focus and focal length of convex and concave lens.

1. LEARNING OBJECTIVES --- VIDEO 6
•SPHERICAL LENSES
•TYPES OF SPHERICAL LENSES
•TERMS ASSOCIATED WITH LENSES
•PRINCIPAL FOCUS AND FOCAL LENGTH OF /CONVEX LENS
•PRINCIPAL FOCUS AND FOCAL LENGTH OF CONCAVE LENS
•RULES FOR FORMATION OF IMAGES BY CONVEX LENS

2. SPHERICAL LENSES
• A spherical lens is a piece of a transparent refracting material, which is bound
by two surfaces. Often ,both the surfaces of a lens are spherical.
• On passing through a lens, light is refracted twice, at the two surfaces of the
lens.
• Spherical lenses are of two types:
1.CONVEX LENS or Converging lens
2.CONCAVE LENS or Diverging lens

3. TYPES OF LENSES
1.Convex lens or Converging lens: Such a lens is thick at the centre and thin at
the edges. It is called converging lens, because it converges the rays of light
falling on it.
2.Concave lens or Diverging lens: Such a lens is thick at the edges and thin at the
centre. It is called diverging lens, because it diverges the rays of light falling on
it.

4. SOME TERMS ASSOCIATED WITH SPHERICAL LENSES
1.Aperture: It is the diameter of the circular edge of the lens.
2.Centre of Curvature: The centres of the two spheres of which the lens is a
part are called centres of curvatures.
3.Principal Axis: An imaginary straight line passing through centres of
curvature of the two surfaces of the lens is called principal axis of the lens.
4.Optical centre: The optical centre of a lens is a point on the principal axis
of the lens ,such that a ray of light passing through it goes undeviated.

5. PRINCIPAL FOCUS AND FOCAL LENGTH OF A CONVEX LENS
• FIRST PRINCIPAL FOCUS: of a convex lens is the position of a point object
on the principal axis of the lens, for which the image formed by the lens is at
infinity. Represented by F1.
• The distance of first principal focus of the lens from optical centre C of the lens
is called first focal length of convex lens. It is represented by f1.
• SECOND PRINCIPAL FOCUS: of a convex lens is the position of an image
point on the principal axis of the lens, when the point object is situated at
infinity. Represented by F2. It is the real point.
• The distance of second principal focus of the lens from the optical centre C of
the lens is called second focal length of convex lens. It is represented by f2.

6. PRINCIPAL FOCUS AND FOCAL LENGTH OF A CONCAVE LENS
• FIRST PRINCIPAL FOCUS: of a concave lens is the virtual position of a point
object on the principal axis of the lens, for which the image formed by the concave
lens is at infinity. Represented by the point F1.
• The distance of first principal focus of the lens from optical centre C of the lens is
called first principal focal length of concave lens. It is represented by f1.
• SECOND PRINCIPAL FOCUS: of a concave lens is the position of image point
on the principal axis of the lens ,when the point object is situated at infinity.
Represented by F2. It is a virtual point.
• The distance of second principal focus of the lens from the optical centre of the
lens is called second principal focal length of concave lens. It is represented by f2.

7. RULES FOR FORMATION OF IMAGES BY A CONVEX LENS
RULE 1: light ray incident on the lens in a direction parallel to the principal
axis of convex lens on refraction ,passes through second principal focus of
the lens located on the other side of the lens.

8. RULES FOR FORMATION OF IMAGES BY A CONVEX LENS
RULE 2: Ray passing through optical centre C of convex lens passes straight
after refraction through the lens.

9. RULES FOR FORMATION OF IMAGES BY A CONVEX LENS
RULE 3: Ray passing through first principal focus F1, of convex lens becomes
parallel to the principal axis of the lens ,after refraction through the lens.
The image is formed at a point where any two of the refracted rays actually
meet or appear to meet.

10. SUMMARY
• SPHERICAL LENSES
• TYPES OF SPHERICAL LENSES
• TERMS ASSOCIATED WITH LENSES
• PRINCIPAL FOCUS AND FOCAL LENGTH OF CONVEX LENS
• PRINCIPAL FOCUS AND FOCAL LENGTH OF CONCAVE LENS
• RULES FOR FORMATION OF IMAGES BY CONVEX LENS