Formation of Image by Spherical Lenses – Explanation

Image Formation by Spherical Lenses

Introduction

By using the concept of predictable refracted rays, it becomes convenient to locate images formed by the spherical lenses at various positions of an object. Both the laws of refraction are obeyed in all the ray diagrams. The image of an object placed at a particular position in front of a spherical lens can be located by using the incident rays with predictable refracted rays. 

Explanation

Image formation by a convex lens: 

Object at infinity: 

The light rays approaching the lens from an object located at an infinitely large distance from it appear to be parallel to each other. These parallel light rays, after refraction from the convex lens, pass through the focus F₂. 

The image is, 

1. Formed at F₂ 

2. Point sized 

3. Real and inverted 

Object located behind 2F₁: 

Two rays are chosen to arise from the head of the extended object. 

1. The first one is parallel to the principal axis, which passes through F₂ after refraction. 

2. The second is through the optic center, which goes along the same path after refraction. 

The image is, 

1. Formed between the 2F₂ and the focus (F₂), 

2. Diminished  

3. Real and inverted 

Object at 2F₁: 

Two rays are chosen to arise from the head of the extended object. 

1. The first one is parallel to the principal axis, which passes through F₂ after refraction. 

2. The second is through the optic center, which goes undeviated after refraction. 

The image is, 

1. Formed at 2F₂ 

2. Same size as the object  

3. Real and inverted 

Object located between 2F₁ and F₁: 

Two rays are chosen to arise from the head of the extended object. 

1. The first one is parallel to the principal axis, which passes through F₂ after refraction. 

2. The second is through the optic center, which goes undeviated after refraction. 

The image is, 

1. Formed behind 2F₂ 

2. Magnified 

3. Real and inverted 

Object located at F₁: 

Two rays are chosen to arise from the head of the extended object. 

1. The first one is parallel to the principal axis, which passes through F₂ after refraction. 

2. The second is through the optic center, which goes undeviated after refraction. 

The two refracted rays turn out to be parallel to each other, which are said to meet at infinity. 

The image is, 

1. Formed at infinity (far away) 

2. Highly magnified 

3. Real and inverted 

Object located between F₁ and the optic center: 

Two rays are chosen to arise from the head of the extended object. 

1. The first one is parallel to the principal axis, which passes through F₂ after refraction. 

2. The second is through the optic center, which goes undeviated after refraction. 

The two refracted rays appear to diverge from a point at the same side of the lens as the object, where the image of the object is formed. 

The image is, 

1. Formed at the same side as the object 

2. Magnified 

3. Virtual and upright 

Image formation by a concave lens: 

Object at infinity: 

The light rays approaching the lens from an object located at an infinitely large distance from it appear to be parallel to each other. 

These parallel light rays after refraction from the concave lens appear to diverge from the focus F₁. 

The image is, 

1. Formed at F₁ 

2. Point sized 

3. Virtual and upright 

Object located at a finite distance from the concave lens: 

Two rays are chosen to arise from the head of the extended object. 

1. The first one incidents parallel to the principal axis and appears to diverge from the focus F₁ after reflection. 

2. The second one is through the optic center, which goes undeviated after refraction. 

The refracted rays seem to diverge from a point at the same side of the lens as the object, where the image of the object is formed.  

The image is, 

1. Formed at the same side of the lens as the object 

2. Diminished 

3. Virtual and upright