Difference Between Mirror and Lens

Last Updated : 18 Oct, 2023

Mirror and Lens are optical instruments that are used to manipulate and focus light in various ways, making them essential components in fields such as photography, astronomy, microscopy, and vision correction. Mirrors and Lenses are the two most critical optical devices that play a vital role in our lives. Whether it's the mirror we use to check our appearance regularly or the lenses in our eyeglasses, we can't go a day without; these optical tools serve distinct purposes.

In this article, we will explore what is a mirror and a lens, their types, and ultimately, the key differences between a mirror and a lens to examine their unique characteristics.

Table of Content

What is a Mirror?

A mirror is an optical device usually made of glass and a smooth reflective surface with one side polished with a thin layer of metal, that allows for the reflection of light.

Mirror works on the reflection principle i.e., when light rays fall on a mirror, they get reflected from its surface at the same angle at which they were incident. When these reflected rays fall in the eyes of the viewer, they appear to be coming from the back of the mirror, forming an image behind the mirror.

Types of Mirrors

Mirrors type are classified into the following types:

Plane Mirrors: These mirrors are flat, having a reflective coating at the back. They form virtual, upright images of the same size as the object. Plaine Mirrors are commonly used as dressing mirrors.
Concave Mirrors: These curved mirrors have an inward curve on the reflecting side. They can produce real and upright or virtual and inverted images, of different sizes, depending upon the position of the object. Concave Mirrors are used as makeup mirrors, solar power plants and telescopes.
Convex Mirrors: These curved mirrors have an outward curve on the reflecting side. They produce diminished, virtual and upright images. Convex Mirrors are frequently used as rear-view mirrors in vehicles, wide-view mirrors in stores and in optical instruments for magnification.

Types of Mirrors

What is a Lens?

A lens is a transparent optical device that has the ability to refract (bend) and focus light rays. Lenses are typically made of glass or transparent plastic and have curved surfaces.

Lens works on the principle of refraction i.e. When light rays fall on a lens, they bend due to differences in the refractive index of the atmosphere and material of the lens. Lenses come in different physical properties like size, shape, and curvatures, which significantly impact the image formed by them.

Types of Lens

There are two types of lenses i.e.,

Convex Lens: It is also called a converging lens as it converges the incoming light rays to its focal point on the other side of the lens. It is thinner at the edges and thicker at the center. It can produce real and inverted or virtual and upright images, of different sizes, depending on the position of the object. Convex Lens is used in glasses, cameras and telescopes.
Concave Lens: It is also called a diverging lens as it diverges the incoming light rays. It is thicker at the edges and thinner at the center. Concave Lens produces virtual and upright images. It is found in glasses, flashlights and telescopes.

Types of Lens

Read more about Convex and Concave Lens.

Mirror and Lens Formula

The mirror and lens formulae are equations used to relate the focal length (f), object distance (u), and image distance (v) for mirrors and lenses. Let's discuss both formulas below:

Mirrors Formula

The mirror formula establishes a relation between the object distance (u), image distance (v) and focal length of a spherical mirror. Mirror formula is given as,

\frac{1}{v}+\frac{1}{u}=\frac{1}{f}

Where,

v is the distance of image from the pole (O) of the mirror.
u is the distance of object from the pole (O) of the mirror.
f is the focal length of the spherical mirror.

Read more about Mirror Formula.

Lens Formula

The lens formula gives a relation between the object distance (u), image distance (v) and focal length of a given lens. Lens formula is given as,

\frac{1}{v}-\frac{1}{u}=\frac{1}{f}

Where,

v is the distance of image from the center (O) of the lens.
u is the distance of object from the center (O) of the lens.
f is the focal length of the lens.

Read more about Lens Formula

Sign Convention for Mirror and Lens

To use the mirror and lens formula correctly, we need to follow standard sign conventions. These are some rules which are applied while solving optical numerical:

All the distances are measured from the pole and optical center (O) of the mirror and lens respectively.
The distances measured along the direction of incident rays are positive. While the distances measured opposite to the direction of incident rays are considered negative.
The object is always placed on the left side of mirror or lens, so the value of distance of object from the optical center (u) is always negative.
If the image is formed on the right side of the optical center of mirror or lens, then the distance of the image (v) is positive. Likewise, if the image is formed on the left side of the optical center, 'v' is negative.
The focal lengths (f) of a concave mirror and convex lens are considered positive. The focal lengths of a convex mirror and concave lens are considered negative.
Heights measured above the principal axis are positive, and below the principal axis are considered negative.
The image height for a erect image is considered positive, while for a inverted image, it is negative.

Read more about Sign Convention of Spherical Mirrors