Convex Lens

A convex lens is a type of lens that converges light rays, bringing them to a single point. It is made by joining two outwardly curved spherical surfaces, where the lens is thicker at the centre and thinner at the edges. Due to its converging property, it is also called a converging lens.

When parallel rays of light pass through a convex lens, they meet at a focal point on the other side. The distance from the centre of the lens to this focal point is called the focal length. If one surface of the lens is flat and the other is convex, it is called a plano-convex lens.

A convex lens may be of the following three kinds : 

1. Bi-convex or double-convex or equi-convex,
2. Piano-convex, and
3. Concavo-convex.

1. Vision and Magnification

• Human Eye: The natural lens in our eyes is convex and helps us see clearly.
• Magnifying Glass: A convex lens magnifies objects when they are placed closer than its focal length.
• Correcting Vision: It is used in spectacles to correct farsightedness (hypermetropia).

2. Optical Devices

• Cameras: Convex lenses focus light to form clear images.
• Projectors and Spectrographs: Used to focus and manipulate light.

3. Microscopes

A. Simple Microscope: It uses a single convex lens with a short focal length to magnify objects.

• Object within focal length → The image is virtual, erect, and magnified.
• Object at the focal point → The image is at infinity (parallel rays).
• Magnifies up to 20 times.
• It is used in watchmaking, jewellery inspection, and gem testing.

Simple microscope

B. Compound Microscope: It uses two convex lenses for higher magnification, making it suitable for observing  cells, bacteria, and microorganisms.

1. Objective Lens: It has a small focal length and a small cross-section. Forms the first magnified image of the object.
2. Eyepiece Lens:  It has a larger focal length and a bigger cross-section. Further magnifies the image formed by the objective lens.
3. Magnification Process: Magnification occurs in two stages.

• The image from the objective lens acts as the object for the eyepiece lens.
• Both lenses are aligned along the same optical axis and fitted inside a metallic tube, allowing the distance between them to be adjusted for focus.

A compound microscope

4. Telescopes

It is used for observing stars and planets by magnifying distant objects.

1. Objective Lens: It has a large diameter and long focal length to collect maximum light.
2. Eyepiece Lens: Smaller in size with a shorter focal length for enhanced magnification.
3. Image Formation:

• The objective lens forms an image, which acts as the object for the eyepiece.
• The eyepiece further magnifies the image, producing the final view.
• Both lenses are aligned along the same optical axis in a metallic tube with an adjustable distance for focus.

Refracting telescope

Types of Telescopes:

• Refracting Telescope: Uses only lenses for magnification.
• Reflecting Telescope: Uses mirrors along with lenses for image formation.

5. Spectacles

Convex lenses are used in glasses to correct farsightedness, allowing clear vision for nearby objects.

Persistence of Vision Activity:

1. Incense Stick Rotation

• When a burning incense stick is rotated quickly in a circle, it appears as a continuous ring of fire instead of a single moving point.
• This happens due to persistence of vision, where the human eye retains an image for about 1/16th of a second after it disappears.

2. Cardboard with Cage and Bird

• A cage is drawn on one side of a cardboard and a bird is on the other side.
• When the thread is twisted and released, the cardboard spins rapidly, making it appear as if the bird is inside the cage.
• This is again due to persistence of vision, where the eye merges both images due to rapid motion.