Content text 05 PHY ICSE X Refraction through lens.pdf
FALGUNI MISS Falguni Thakker, 03, Sapna bldg. S.V Road, Vile Parle (w), Mumbai 56. Tel 9820428493 Page 1 PHYSICS SUMMARY 05. REFRACTION THROUGH A LENS 22 – 23. (A) LENS & REFRACTION OF LIGHT THROUGH LENS. ➢ Scope: o Types of lenses (converging and diverging). o Convex and concave action of a lens as a set of prism. o Technical terms - center of curvature, radius of curvature principal axis, foci, focal plane and focal length. ➢ Lens: o Definition: A lens is a transparent refracting medium bounded by one or two spherical surfaces. o Kinds of lenses: Lenses are of two types: ▪ Converging or Convex lens ▪ Diverging or Concave lens ➢ Action of lens as a set of prism: o Refraction through a convex lens: ▪ The central part has parallel sides hence the ray passes undeviated. ▪ The prism forms a convex lens and converges the parallel light beam. o Refraction through a concave lens: ▪ The central part has parallel sides hence the ray passes undeviated. ▪ The prism forms a concave lens and diverges the parallel light beam.
FALGUNI MISS Falguni Thakker, 03, Sapna bldg. S.V Road, Vile Parle (w), Mumbai 56. Tel 9820428493 Page 2 ➢ Technical terms related to a lens: o Centre of curvature: The center of the sphere, which forms the lens surface, is called the center of curvature of that surface of the lens. o Radius of curvature: The radius of the sphere, which forms lens surface, is called the radius of curvature of that surface of the lens. o Principal axis: The line joining the centers of curvature of the two surfaces of the lens is called the principal axis. o Optical center: It is a point on the principal axis of the lens such that an incident ray of light emerges parallel to its direction of incidence after passing through this point. ▪ If the lens is thick: The light ray while passing through the optical centre is slightly displaced parallel to its original direction as follows: ▪ If the lens is thin: The light ray passing through the optical centre passes undeviated as follows: ▪ Definition of optical centre of thin lens: The optical centre of a thin lens is the point on the principal axis of a lens,such that an incident ray of light, passes undeviated through it.
FALGUNI MISS Falguni Thakker, 03, Sapna bldg. S.V Road, Vile Parle (w), Mumbai 56. Tel 9820428493 Page 3 o Principal foci: ▪ Two principal foci are situated at equal distances from the optical center one on either side of the lens. ▪ They are called first focus F1 and second focus F2. ▪ First focal point: • For a convex lens: The first focal point is a point F1 on the principal axis of the lens such that the rays of light starting from it or passing through it, after refraction through the lens, become parallel to the principal axis of the lens. • For a concave lens: The first focal point is a point F1 on the principal axis of the lens such that the incident ray of light appearsto meet at it, after refraction from the lens and becomes parallel to the principal axis of the lens. ▪ Second focal point: • For a convex lens: The second focal point is a point F2 on the principal axis of the lens such that the ray of light incident parallel to the principal axis, after refraction, passes through it. • For a concave lens: Second focal point is a point F2 on the principal axis of the lens such that the ray of light incident parallel to the principal axis, appearsto to diverge from this point, after refraction.
FALGUNI MISS Falguni Thakker, 03, Sapna bldg. S.V Road, Vile Parle (w), Mumbai 56. Tel 9820428493 Page 4 o The focal length of the lens: ▪ There are two focal lengths of a lens. • First focal length (f1): It is the distance from the optical centre of the lens to its first focal point F1. • Second focal length(f2): It is the distance from the optical centre of the lens to its second focal point F2. ▪ The factors on which the focal length of a lens depends: • The refractive index of material of lens – Directly proportional: o The refractive index is relative to the surrounding medium. o Ex: If a lens is placed in water (denser) instead of air (rarer) the focal length increases. • The radii of curvature of lens – Inversely proportional: o A thick lens has lessfocal length. o A thin lens has greater focal length for the same material. ▪ If a part of the lens is covered: • Its focal length remains unchanged and • The intensity of light entering the lens decreases due to which the intensity of the image formed by it also decreases. o The focal plane of the lens: ▪ First focal plane: A plane passing through the first focal point and is normal to the principal axis of the lens is called the first focal plane. ▪ Second focal plane: A plane passing through the second focal point and is normal to the principal axis of the lens is called the second focal plane. o
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