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Class 10, Science, Chapter-9, Lecture-5, Lens (Notes)

RULES FOR IMAGE FORMATION BY SPHERICAL LENS:

  1. An incident ray parallel to the principal axis either pass through the focus of a convex lens or appear to diverge from the focus of a concave lens.
  2. An incident ray passing through focus of a convex lens or approaching the focus of a concave lens becomes parallel to the principal axis after refraction.
  3. An incident ray passing through the optical centre goes without any deviation after refraction.

CHARACTERISTICS OF AN IMAGE FORMED BY A CONVEX LENS:

POSITION OF OBJECT

CHARACTERISTICS OF IMAGE

RAY DIAGRAM

At infinity

At focus F2

Real and Inverted

Highly diminished

FIG. 10.16 (a) OF NCERT TEXT BOOK
Beyond C1

Between F2 and C2

Real and Inverted

Diminished

FIG. 10.16 (b) OF NCERT TEXT BOOK
At C1

At C2

Real and Inverted

Same size

FIG. 10.16 (c) OF NCERT TEXT BOOK
Between C1 and F1

Beyond C2

Real and Inverted

Magnified

FIG. 10.16 (d) OF NCERT TEXT BOOK
At F1

At infinity

Real and Inverted

Highly magnified

FIG. 10.16 (e) OF NCERT TEXT BOOK
Between F1 and O

On the same side of lens

Virtual and Erect

Magnified

FIG. 10.16 ( f ) OF NCERT TEXT BOOK

CHARACTERISTICS OF AN IMAGE FORMED BY A CONCAVE LENS:

 
POSITION OF OBJECTCHARACTERISTICS OF IMAGERAY DIAGRAM
At infinity

At focus F1

Virtual and erect

Highly diminished

FIG. 10.17 (a) OF NCERT TEXTBOOK
Between infinity and the optical centre O of the lens

Between F1 and O

Virtual and erect

Diminished

FIG. 10.17 (b) OF NCERT TEXTBOOK

SIGN CONVENTION FOR LENS:

  1. Light is incident on the lens from the left-hand side of the lens.
  2. All distances are measured from the optical centre (O).
  3. Distances in the direction of incident ray are taken as positive.
  4. Distances against the direction of incident ray are taken as negative.
  5. Distances measured upward and perpendicular to the principal axis are taken as positive.
  6. Distances measured downward and perpendicular to the principal axis are taken as negative.
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