LIGHT , COLOUR AND SIGHT

7.1 FORMATION OF IMAGES BY PLANE MIRROR AND LENSES

A. Formation of Images by Plane Mirrors Image formed by a plane mirror are caused by

reflection of light from the mirror surface. The characteristics of an image formed by a plane

mirror are:

1. Virtual : Images that cannot be displayed on a screen.

2. Laterally inverted : The right side and the left side interchanged.

3. Upright4. Same size as the object5. Image distance and object

distance from the plane mirror are the same

LATERALLY INVERTED

B. IMAGES FORMED BY CONVEX LENSES

The characteristics of images formed by a convex lens depend on the distance between the lens and the object :

(a) For a distant object, the image formed is :(i) real(ii) inverted (iii) diminished

CONVEX LENS

(b) For a near object, the image formed is :

(i) virtual(ii) erect(iii) magnified(iv) on the same side of the object

C. THE IMAGE FORMED BY CONCAVE LENS

The image formed by concave lens is always :

(i) Virtual(ii) erect/ Upright(iii) diminished(iv) on the same side as the object

PARALLEL RAYS THROUGH CONVEX LENSES When a parallel rays pass through a convex lens, they

converge and meet at a point called THE FOCAL POINT (F)

The centre of the lens is called the OPTICAL CENTRE (O).

The distance between the focal point and the optical centre is called the FOCAL LENGTH (f)

A convex lens has two focal points, one on each side of the lens

The line passing through the 2 focal points and the optical centre is called the PRINCIPLE AXIS

Principal axis

Optical Centre (O)Optical Centre (O)

PARALLEL RAYS THROUGH THE CONCAVE LENS

When parallel rays pass through the concave lens , they appear to DIVERGE from a point on the PRINCIPLE AXIS.

This is the FOCAL POINT (F) of the concave lens.

CONCAVE LENS

RAY DIAGRAM

Rules for convex lens

1) A ray parallel to the principle axis is converged to focal point, F after passing through the lens.

2) A ray passing through the optical centre

3) A ray passing through the Focal Point, F becomes parallel after passing through the lens

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Rules for concave lens

1) A ray parallel to the principle axis is DIVERGED in such a way that it appears to come from focal point, F

2) A ray directed towards the optical centre,O travels straight on.

3) A ray directed towards the Focal Point, F becomes parallel after passing through the lens

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