Image produced by the convex lensCopy

Learning Objectives

• Describe the action of a thin converging lens on a beam of light
• Use the terms principal focus and focal length
• Draw ray diagrams for the formation of a real image by a single lens
• Describe the nature of an image using the terms enlarged/same size/diminished and upright/ inverted
• Draw and use ray diagrams for the formation of a virtual image by a single lens
• Use and describe the use of a single lens as a magnifying glass
• Show understanding of the terms real image and virtual image

Contents to learn

Convex lens

A Convex lens is used in many optical instruments like microscope, telescope, camera, etc. A convex lens is the thickest in the centre and is also called a converging lens because it bends the light inward.

Principal axis: A spherical surface has a centre of curvature, which is the centre of the sphere from which it is drawn. A convex lens has two centres of curvature. A line passing through the centers of curvature and the optical centre is called the principal axis.

Optical centre: There is a paricular point ‘C’ (at the centre of the lens) in a lens called its optical centre such that any ray passing through this point does not suffer any change in its direction. Opticle centre ‘C’ of a lens always lies on the principal axis.

Principal focus: The point where all the rays parallel to the principal axis meet after passing through a convex lens is called the principal focus ‘F’.

Focal length: It is the distance ‘f’ between the optical centre ‘C’ and the principal focus ‘F’.

Behavior of standard rays after passing through convex lens

Information about the images formed by a lens can be obtained by drawing any of two of the following standard rays from the top of the object.

Standard rays:

If a ray of light is coming parallel to the principal axis, after refracting through the convex lens it passes through the focus point or principal focus.
If ray of light is passing through the optical centre, after refracting through the convex lens, it goes straight without bending or deviating.
If a ray of light is coming from the focus point or principal focus, after refracting through convex lens, it becomes parallel to the principal axis.

Q: Why ray of light passing through the opticle centre of the convex lens does not bend?

The below diagram explains the fact.

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The ray of light, which is approaching to the lens at opticle centre has angle of incidence 26o and angle of refraction 18o. When this ray of light is emerging out from the lens (glass) in the air, It is almost taking the same path which it had before entring the lens. you can see that at both boundries, refraction ophenomenon occcurs but ultimately th eray of light remains on the same path.

Note: “The ray of light does not bend, when it passe through the opticle centre of the lens” is strictly true for thin lenses, while in thick lense you may have small deviation of the path of the incident ray and the emrging ray.

Image formed by convex lens

In each ray diagram, in the figure below two rays are drawn from the top of the object

(a) When an object is placed in front of a convex lens at distance greater than 2f, its image will be formed between F and 2F. The image will be diminished real and inverted.

(b) When an object is placed at 2F in front of a convex lens.The image will be formed at 2F as shown in he figure below. The image will be of the same size, real but inverted.

(c) When object is placed in front of a convex lens between between F and 2F. The image will be formed at a distance beyond 2F from the lens . The image will be magnified, real and inverted.

(d) When object is at F in front of the lens, its image will be at infinity, much enlarged, real and inverted.

(e) When the object is placed between the lens and its principal focus. Then the image will be formed on the same side behind the object. The image will be magnified, errect and virtual.

Nature and position of image produced by convex lens

Note: F’ is the principal focus on the object side and F is the principal focus on the other sde of the lens opposite to the object.