Ray diagram - Rules for image formation by convex and concave

The image is formed at that point where at least two refracted light rays meet ( or appear to meet ) .

Rules for convex lens : 

1. Light ray incident parallel to the principal axis , meet at focus. 

Light ray incident parallel to the principal axis , meet at focus

2. Light ray incident from focus , become parallel to the principal axis after Refraction. 

Light ray incident from focus , become parallel to the principal axis after Refraction.

3. Light ray incident on the pole of the lens passes without any deviation.

Light ray incident on the pole of the lens passes without any deviation

Formation of Different Types of images by a convex Lens : 

Case 1. When the object is at Infinity from a convex lens , then the image formed is : 

When the object is at Infinity convex lens
When the object is at Infinity convex lens

(i) At the focus, 

(ii) Real and Inverted

(iii) Much smaller than the object ( or Highly Diminished).

Case 2. When the object is beyond 2f , then the image formed is :

When the object is beyond 2F - convex lens

(i) Between f and 2f on the other side of the lens.

(ii) Real and Inverted

(iii) smaller than the object ( or diminished ).

Case 3. When the object is at 2f , then the image formed is :

When the object is at 2f - convex lens

(i) At a distance 2f on the other side of lens.

(ii) Real and inverted 

(iii) of the same size as the object.

Case 4. When the object is between f and 2f , then the image formed is :

When the object is between f and 2f - convex lens

(i) Beyond 2f.

(ii) Real and Inverted 

(iii) Larger than the object ( or Magnified ) 

Case 5. When the object is placed at the focus of a convex lens :

When the object is placed at the focus of a convex lens

(i) At infinity

(ii) Real and Inverted

(iii) Highly enlarged.

Case 6. When the object between focus and  pole or optical Centre : 

When the object between focus and optical Centre

(i) Behind the object ( on the left side of lens )

(ii) Virtual and Eract.

(iii) Large than the object ( Enlarged or Magnified ).

Important point for convex lens :

1. A convex lens is also known as a converging lens because it converges , a parallel beam of light rays passing through it.

2. Higher the refractive index , shorter will be the focal length , similarly more the curvature, shorter is the focal length.

3. As the radius of curvature for a lens increases, its curvature decrease and, the focal length increase





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