Spherical Mirrors: Structure, Types & Terminologies

Spherical Mirrors are those mirrors that have the shape of a piece cut out of a spherical surface. Spherical mirrors are generally constructed from glass. The curved surface of the glass has a silver coating on one side and a polished surface on the other side for reflection. There are two main types of spherical mirrors namely Convex Mirrors and Concave Mirrors.

• Concave Mirror: When the reflective surface is on the side curved inwards, it is a concave mirror.
• Convex Mirror: When the reflective surface is curved outwards, it is a convex mirror.

Both concave and convex mirrors have a wide variety of applications in our daily lives. Convex mirrors are used in the rear-view mirror to provide a larger view of the road while Concave mirrors are used in telescopes, satellite dishes, and by dentists to create images of body parts that are larger than the actual.

Read More: NCERT Solutions For Class 12 Physics Ray Optics and Optical Instruments

Key Terms: Spherical Mirrors, Concave Mirror, Convex Mirror, Reflection, Sphere, Image, Principal Focus, Rearview Mirror, Center of Curvature

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What are Spherical Mirrors?

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Spherical Mirrors are those mirrors whose reflecting surface is part of a hollow sphere of glass. 

• Spherical mirrors are the most common curved reflecting surfaces.
• They are made by cutting out a piece from a hollow spherical glass structure. 
• One of the surfaces of the cut-out piece is then silvered. 
• The other side is polished which is then used for the reflection of light.
• There are two types of spherical mirrors namely Concave Mirrors and Convex Mirrors. 

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History of Spherical Mirrors

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A natural mirror used by humans was a pool of water. 

• The very first mirror observed was made from polished volcanic glass, such as Obsidian (Black colored rock).
• Around the years 3000 BC and 4000 BC, respectively copper mirrors polished were created in Ancient Egypt and Mesopotamia.
• Spherical Mirrors have been of great importance since the Ancient ages till this day. 
• Polished stone mirrors from the year 2000 BC, have been making bronze mirrors in China. 
• In 500 AD Chinese people made mirrors with silver mercury amalgams. 
• Claudius Ptolemy has conducted various experiments with these curved polished iron mirrors. 

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Structure of Spherical Mirrors

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A spherical mirror is a part of a bigger virtual sphere. 

• A line called the Principal Axis passes from the center of the sphere that touches the reflection surface at the center. 
• The point touched at the center of the surface by the principal axis is known as the vertex.
• A spherical mirror can either be a convex mirror or a concave mirror depending on the surface of the reflection. 
• If the mirror is bulged out, then it means it is a convex mirror whereas if the mirror is bent inwards then it is termed as a concave mirror. 

Structure of Spherical Mirrors

Read More: Ray Optics and Optical Instruments

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Types of Spherical Mirrors

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A spherical mirror is defined as a portion of the hollow sphere whose one side endorses a reflecting surface whereas the other side has been silvered. 

Spherical Mirrors are of two types, and they can be classified as follows:

• Concave Mirror 
• Convex Mirror

If the cut-out part of the sphere is silvered on the outer surface and the inner part is used for reflection then it is called a concave mirror whereas if the part of the sphere is silvered on the inner surface and the outer surface is used for reflection, then it is known as a convex mirror.

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Concave Mirror

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A concave mirror has a reflecting curved surface bent inwards which means it faces the center of the sphere. 

• The back of the mirror is shaded so that reflection could take place from the inward bulged surface.
• These mirrors are commonly used as shaving mirrors or in telescopes or by dentists. 
• The reflected image seems magnified but the view of the field is limited. 
• Therefore, it is known as the converging mirror as the ray of light after bouncing back from it appears at the points where one can obtain a real, diminished, inverted, or diminished image depending on the location of the object.

Uses of Concave Mirrors

The important uses of concave mirrors are

• Converging mirrors are most widely used in shaving because they have reflective and curved surfaces.
• A concave mirror is used in the ophthalmoscope.
• These mirrors are also widely used in making astronomical telescopes. A converging mirror of a diameter that is about 5 meters or more is used as an objective.
• Converging mirrors are widely used in headlights of automobiles and in motor vehicles, torchlights, railway engines, etc. as reflectors.
• Large converging mirrors are used to focus the sunlight to produce heat in the solar furnace.

Read More: Ray Optics and Optical Instruments Important Questions

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Convex Mirror

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If the reflective surface is curved outwardly it is the Convex Mirror. 

• According to the focal length, the reflected image is reduced but the view of the field is more. 
• This is very useful for drivers as blind spots are reduced drastically. 
• The back of the mirror is shaded so that the reflection takes place from the outward part that is bulged. 
• The surface of the spoon is a convex mirror. 
• It is also known as the diverging mirror as the light reflecting through the surface diverges in any direction. 
• Though it meets at some points where the virtual image that is diminished is formed.

Uses of Convex Mirrors

The uses of convex mirrors are

• Convex mirrors can be used inside the buildings so that people can see all around the building at once.
• Convex mirrors are used as rear-view mirrors in automobiles and vehicles because they can diverge light beams and make virtual images.
• These mirrors are usually used for constructing magnifying glasses. To construct a magnifying glass in Industries, two convex mirrors are placed back to back.
• Diverging mirrors are also used for security purposes in many places. They are placed near the ATMs for the bank customers to check whether someone is behind them or not.
• Convex mirrors are also used as streetlight reflectors because they can spread light over bigger areas.

Rear-View Mirrors in Cars (Convex Mirrors)

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Basic Terminologies for Spherical Mirrors

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Here are a few terms that are frequently used in the concept of Spherical Mirrors: 

• Center of Curvature: The point in the center of the mirror passes through a curve of the mirror and has the same tangent and curvature at a point.
• Radius of Curvature: It is the linear distance between the pole and the center of curvature.
• Principal Axis: The imaginary line passes through the optical center & the center of curvature of any lens or a spherical mirror.
• Pole: It is the midpoint of the spherical mirror.
• Aperture: An aperture of a mirror or the lens is a point from which the reflection of light takes place. 
• Principal Focus: It is also called Focal Point. It’s on the axis of a mirror or lens where the rays of light parallel to the axis converge or appear to converge after reflection or refraction.
• Focus: It’s an assigned point, where light rays that are parallel to the principal axis, will converge after getting reflected from the mirror.

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Uses of Spherical Mirrors

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The uses of Spherical Mirrors are as follows:

• Concave mirrors are used torch headlights to disperse light over a larger surface, hence enhancing the field of vision.
• Convex mirrors are used in the car’s rearview mirror, as it gives a wider field of view, which helps the driver to see most of the traffic behind him.
• Concave mirrors are used in solar cookers as reflectors to focus sunlight on a specific area and raise the box’s temperature.
• Concave mirrors are used in telescopes, satellite dishes, and by dentists to create images of the ears, skin, teeth, ears, skin, and other body parts that are larger than the actual.

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Things to Remember

• Spherical Mirrors are mirrors that have the shape of a piece cut out of a spherical surface.
• Convex Mirrors and Concave Mirrors are the two types of Spherical Mirrors.
• A spherical mirror whose reflecting surface is curved inwards is a concave mirror.
• A spherical mirror whose reflecting surface is curved outwards is a convex mirror.
• Center of curvature, Radius of curvature, Pole, Principal axis, Focus, and Focal length are a few terms that are crucial to understand the functioning of spherical mirrors.

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Previous Years’ Questions

• The focal length of a spherical mirror made of steel is… (AP EAPCET)
• A concave mirror of focal length 15 cm forms an image… (Haryana PMT 2004)
• A student performed the experiment of determination of focal length… (KEAM 2009)
• The image formed by a concave mirror… (UPSEE 2008)
• When a convex mirror of focal length f produces an image…
• To find the focal length of a convex mirror, a student records… (JEE Main 2016)
• An object is placed at the focus of a convex mirror. If its focal… (BCECE 2009)
• A clock hung on a wall has marks instead of numerals on its dial… (BHU UET)
• A car is fitted with a convex side-view mirror of focal length… (AIEEE 2011)