What are Mirrors and Lenses?
A mirror is an optical device which does not allow light to pass through it and reflects it back. It is made by silvering one side of a glass. Mirrors can be plane mirror or spherical mirrors. Lens on the other hand is made up of two curved parts. It lets the light to pass through it but bends the path of light. This is called refraction. Lenses are broadly of two types convex lens and concave lens.
Difference between Mirrors and Lenses
| Mirror | Lenses |
| The most basic difference is that mirrors work on the principle of reflection | Where as lenses function based on the principle of refraction. |
| Mirrors can have either plane or curved surfaces | But in the case of lenses, at least one surface must be curved if not both |
| Plane mirrors do not have a focal point | Each type of lens has two focal points |
| They are manufactured from the raw material glass or any type of metal | And these are made from the materials glass or plastic |
Anatomy of Mirror and Lens Diagram
Principal axis: It is a horizontal straight line passing through the centre of the lens.
Optical axis or vertical axis: A vertical line passing through the centre of the curvature of a lens.
Optical centre: This is the central point of the lens. Where the principal and optical axis intersect.
Focal point (F): It is a point on the principal axis at which the parallel rays converge.
Focal length: It is the distance between the centre of the convex lens and the focal point.
When the image formed is inverted as compared to the object, the image formed is called a real image.
When the image formed is upright as compared to the object, the image formed is called a virtual image. These images cannot be captured on a screen
Mirrors
Mirrors are practically just a surface made of glass that is coated with a metal amalgam on one side due to which the light ray incident on the surface undergoes reflection and not refraction.
Similar to lenses, even mirrors can form images that have a variety of properties. For instance, makeup mirrors form an enlarged version of the object in front it. While, car side view mirrors and shop security mirrors form a miniature version of the objects in front.
It follows the law of reflection which is – the angle of incidence is equal to the angle of reflection.
The images formed by plane mirrors are always virtual images and is of the same size as the object. One major feature is that the image is laterally inverted. So, if you raise your right hand the image on the mirror will show the left hand being held upwards.
Other than general use of plane mirrors, they are also used to construct devices such as a periscope and a kaleidoscope.
There are two types of mirrors
Concave mirror
Concave mirrors are a type of spherical mirrors that have curved surfaces and are painted on the outer side. This becomes an inner surface reflecting mechanism. Light converges at a point when it first strikes the surface and then is reflected back.
In a concave mirror, if it has a long focal length (the distance of the object must be less than the focal length) the image produced is magnified
This theory is put into practical use for the making of makeup mirrors, which requires a magnified version of the incident image. On the same lines is the making of the dentist’s mirrors to magnify the image of the teeth so it is seen clearly.
Convex mirror
Convex mirrors are spherical mirrors that have curved surfaces but unlike concave, the inner side is painted and not the outside. So, this has an outer surface reflecting mechanism. This type of mirror diverges light when it strikes the reflecting surface.
Virtual, erect and diminished images are formed.
Side wing mirrors and rear-view mirrors of a car use convex mirrors. This is because the diminished image formed gives the driver a larger field of view than the normal human eye. Helps the driver have a better check on the road and traffic behind.
Parabolic mirrors
It is a concave mirror whose cross-sectional area is shaped like the tip of a parabola.
They are used to gather light in reflecting surfaces and project a beam of light rays. This is why it is used in torches and car headlamps. A small lamp is kept at the focus point of the concave mirror so that the parallel rays of light are produced.
Also parabolic antennas are used for high energy point to point communication.
Lens
Lens is just a piece of transparent material, generally glass, which has at least one curved surface. It is used for either concentrating the lay rays on one point or for dispersing the light rays.
Convex lens, also known as converging lens, is used to bend the parallel light ray incident on it so that they converge at a spot the beyond the lens. This spot is termed as the focal point. These are most commonly used in eyeglasses to correct farsightedness.
Types of convex Lenses
- Plano convex lens : This lens is curved outwards from one side and the other side is plane. The positive focal length elements of the lens have one curvature surface and one flat surface. When used it will deliver images with less spherical distortion. They are most commonly used when the object is far away from the lens. In optical physics it is termed as infinite conjugate. An example for the same is when you want to focus light form a faraway object like the star.
- Double convex lens : Double convex lens, also known as biconvex lens or just convex lens, is curved outwards at both sides of the lens. These are also used to focus light but are preferred when the object is closer to the lens. This is termed as finite conjugate in optical physics. An example for the same is microscopy.
- Concave-convex lens : Also known as the Meniscus lens, the concave-convex lens has a structure where one side is curved inwards and the other side is curved outwards. This means that the surface on one side of the lens is concave and the other side’s surface is convex. This lens is used in controlling laser beams.
Concave lens, also known as diverging lens, is used to spread out the light rays that incident on the lens and getting refracted. These have a structure where they are thinner at the centre and thicker at the edges. Some of the uses of convex lenses include the rectification of myopia, which is short sightedness and are also used in flashlights and torches.
Types of concave lenses
- Plano concave lens : This is an optical lens that has one plane, flat surface and the other surface is concave in nature. The focal length of this type of lens is always negative. It several uses that include expansion of beams and light projection.
- Double concave lens : Also known as the bi-concave lenses, they have a curvature of equal radius on both sides of the lens. These lenses are generally used to increase the focal length of existing systems.
Common Mistakes
- It is important to be extremely thorough with the difference between mirror and lens and concave and convex.
- Its respective subtypes should also be remembered with definition.
- Recognize the similarities between the concave and convex less with the prefix Plano and double respectively. This might provide you with a better understanding of the same.
Similar Topics
- Application of reflection of light
- Converging lens
- Diverging lens
- Lateral magnification
- Optical physics
Context and Applications
This topic is mainly studied in 9th, 10th, 11th and 12th standard, as well as in Bachelors of Science (Physics)
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