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# Light

## 07 Formation of Image By Different Lenses

#### 7.1 FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONVEX LENS

Case1. Object placed between optical centre and focus (object between C and F’)

The image formed is:

(i) behind the object (on the left side of lens),
(ii) virtual and erect, and
(iii) larger than the object (enlarged or magnified).

Case2. Object placed at the focus of a convex lens (Object at F’)

The image formed is:

(i) at infinity
(ii) real and inverted, and
(iii) highly enlarged.

Case3. Object placedbetween F’ and 2F’ (Object between f and 2f)

The image formed is:

(i) beyond 2F,
(ii) real and inverted, and
(iii) larger than the object (or magnified).

Case4. Object placed at 2F’ (Object at 2f)

The image formed is:

(i) at a distance 2f on the other side of the lens,
(ii) real and inverted, and
(iii) of the same size as the object.

Case5. The object placedbeyond 2F’ (object beyond 2f)

The image formed is:

(i) between f and 2f on the other side of the lens,
(ii) real and inverted

Case6. The object placed at infinity

The image formed is:

(i) at the focus,
(ii) real and inverted, and
(iii) much smaller than the object (or highly diminished).

Summary of the Image Formed by a Convex Lens

Position of object Position of image Size of image Nature of image
1. Between f and lens On the same side Enlarged Virtual and erect
2. At focus (F) At infinity Highly enlarged Real and inverted
3. Between F and 2F Beyond 2F Enlarged Real and inverted
4. At 2F At 2F Same size as object Real and inverted
5. Beyond 2F Between F and 2F Diminished Real and inverted
6. At infinity At focus (F) Highly diminished Real and inverted

#### 7.2 USES OF CONVEX LENSES

(i) Used in spectacles to correct the defect of vision called hypermetropic (or long-sightedness).
(ii) Used for making a simple camera.
(iii) Used as a magnifying glass (or magnifying lens) (by palmists, watchmakers, etc.)
(iv)Used in making microscopes, telescopes and slide projectors (or film projectors).

#### 7.3 FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONCAVE LENS

Position of object Position of image Size of image Nature of image
1. Anywhere between optical centre (C) and infinity. Between optical centre (C) and focus (F) Diminished Virtual and erect
1. At infinity At focus (F) Highly diminished Virtual and erect

#### 7.4 USES OF CONCAVE LENSES

(i) They are used in spectacles to correct the defect of vision called myopia (or short-sightedness).
(ii) They are used as eye-lens in Galilean telescope.
(iii) Concave lenses are used in combination with convex lenses to make high quality lens systems for optical instruments.
(iv) They are used in wide-angle spyhole in doors.