Spherical Surfaces

We come across plenty of spherical surfaces unknowingly in our daily lives, but we hardly realise how they work. For example, the spectacles you are wearing and the vehicle mirror you have come with a spherical surface.

A change in the light direction or just a bending in the light wave moving from transparent mediums is caused due to wave speed. This phenomenon is known as refraction. You will come across this phenomenon in nature. Lenses tend to converge or diverge the light rays passing through it only due to refraction. 

Refraction of light is a surface phenomenon in which the light transfers from one medium to another and changes its direction. In such a phenomenon, when the beam of light changes its medium, the speed of the light also changes. The refraction occurs when the light rays bend at the interface of two different media when the light enters the second medium with different optical densities.

Now let’s delve into the spherical surfaces.

Reflection of Light from Spherical Surfaces 

A mirror or surface that resembles the shape of a sphere is termed a spherical mirror or surface. In other words, a spherical mirror looks like a part that has been cut from a sphere. Spherical mirrors are of two types: concave mirrors and convex mirrors. 

Concave mirror – Also referred to as converging mirror, this type of mirror is known to converge the rays that fall on it. When a light beam strikes at the concave mirror, the light rays converge at a single point. And this type of mirror curves inward. Such mirrors are used as vehicle mirrors as well as in street lights. 

Convex mirror – Unlike a concave mirror, this type of mirror has an outward curve. The images formed by the convex mirror are erect, virtual, and diminished. When the beam of light strikes the surface, the lines get reflected by either diverging or spreading out. For this reason, these mirrors are known as diverging mirrors. 

Refraction of Light at Spherical Surfaces 

When two spherical lenses come in contact with each other (face to face), it forms a spherical lens shape. Spherical lenses are of two types: concave lenses and convex lenses. When a ray of light passes through the concave lens, it forms a virtual, erect, and diminished image. 

Lens

There are two main types of lens: convex lens and concave lens. We will learn about them in detail. Glasses or plastics make lenses, and they are polished or moulded in the required shape. A lens can focus light to form an image, but a prism cannot. 

Characteristics of Lens 

• It is a transparent medium bounded by two surfaces, and one of them must be curved. The lens is thin if the gap between two surfaces is small, and a lens will be converging with the positive focal length and diverging when the focal length is negative.

• The assumptions made of the lens are that the lens is thin. Therefore, the lens has small apertures, the object lies close to the principal axis, and the incident ray makes a small angle with the surface of the lens or the principal axis.

• Devices that focus or disperse waves and radiation other than the visible light are also called lenses, such as microwave, electron, acoustic, or explosive. 

Principal Axis and Optical Centre of a Lens

The central point of the lens is called the lens’s optical centre. The line that passes by the optical centre of the lens is commonly known as the principal axis. The principal axis is perpendicular to the convex and concave surface of the lens.

Convex lens: Principal focus and focal length – A parallel beam of light that travels by the principal axis and passes by the convex surface converges to a point on the principal axis. This point of refraction is known as the principal focus. As the lens has two spherical surfaces, the lens has two foci. 

Concave lens: Principal focus and focal length – Principal focus is the point on the principal axis from where the light travels parallel to the principal axis, appearing to diverge after it passes through the concave lens. Focal length is the distance between the optical centre and principal focus. 

Image Formation by a Lens

To determine the position of an image from the spherical lenses; it is important to understand image formation through lenses.

When any observer views the picture of an item via the lens, images are produced at the spots where they are seen. As a result, if the passage of numerous light beams via a lens is tracked, the light rays intersect at a point where the image is formed.

Lens Formula: 

The formula for spherical lenses is as follows:

1/v – 1/u = 1/f

This formula describes the relationship between object distance (u), image distance (v), and the focal length (f).

Conclusion

Both convex and concave mirrors are used in daily lives, and both of them act differently. The size of the image formed by the mirrors depends on the distance between the mirror and the object. We have understood some of the facts and figures related to the reflection and refraction of light at spherical surfaces.