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Plane mirror is a mirror having a flat reflective surface with no inward or outward curves that reflects light and forms a virtual image. The angle of the reflection of light rays striking the plane mirror is equal to its angle of incidence. The image formed by a plane mirror is always upright, and the size and shape of the image is exactly the same as that of the object.
Mirrors, in general, are polished surfaces coated with mercury. These polished surfaces reflects most of the light falling on them leading to the formation of an image. Depending upon the type of reflecting surface they have, mirrors can be classified into concave, convex and plane mirrors.
Read Also: Reflection on a Plane Mirror
What is a Plane Mirror?
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Plane mirrors are the type of mirrors whose reflecting surface is flat not curved. These mirrors produces an exact virtual image of an object. The maximum light falling on a plane mirror gets reflected back due to its polished surface.
- The surface of the plane mirror is polished with silver and aluminium in a process called silvering.
- After the process of silvering, a thin layer of red lead oxide is applied at the back of the mirror.
- The ability of the mirror to reflect most of the light falling on it is maintained as long as their surface is not contaminated by tarnishing or oxidation.
- To prevent tarnishing, the plane mirrors are now equipped with a thin piece of plate glass that protects the surface of the mirror.
Image Formation by a Plane Mirror
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When a ray of light falls on a surface, it undergoes one of the following three phenomena:
- Reflection
- Refraction
- Absorption
Most of the light falling on a normal surface is absorbed. Reflection of light enables us to see an object. Therefore, to form an image, we need at least two rays from the object which meet or appears to meet at a point.
In a plane mirror, one side of the mirror is polished due to which the incident rays falling on the surface of the mirror gets reflected back. If the reflected rays are extended behind the screen they meet at a certain point leading to the formation of the image. Since, these images are not formed by actual meeting of the reflected rays, they are called virtual.
Image Formation by a Plane Mirror
Real and Virtual ImagesThere are two types of images formed by the mirrors:
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Properties of Image Formed by Plane Mirror
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The characteristics of the image formed by a plane mirror are as follows:
- The image formed by a plane mirror is of the same magnification as the object.
- These images are always virtual in nature.
- The size of the image formed by a plane mirror is the same as that of the object.
- One distinct feature of a plane mirror is that the image formed by a plane mirror is laterally inverted, that is, if you raise your left hand then the image of the plane mirror will show the right hand going up.
- These images are always straight or erect.
- The distance between the object and the plane mirror is the same as the distance between the image and the mirror.
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Types of Reflection in Plane Mirror
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In a plane mirror, the light gets reflected off, it does not curve inwards or outwards. Thus, images from a plane mirror lack the interference seen from concave and convex mirrors. There are two types of reflection in the plane mirror:
Regular/Specular Reflection
Regular/specular reflection, provides the correct image of the object without any distortion. When light strikes smooth surfaces the ray of light gets reflected along the reflected ray whose angle is equal to the angle of the incident ray. For example, the image of the mountains displayed in the lake is same as the lake is a smooth surface.
Irregular/Diffused Reflection
Irregular or diffused reflection is the reflection in which light hits a rough surface and gets reflected in all directions, this includes any light that we can see with our eyes. In this type of reflection, the reflected ray does not have the same angle as the angle of the incident ray. For example, when light strikes a bird which is a rough surface, the reflected light gets scattered in all directions, when it reaches our eyes and hits the retina, it gets processed in the brain by an electrical signal to form the image of a bird.
Laws of ReflectionThe two laws of reflection that are also followed by a plane mirror are as follows:
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Uses of Plane Mirror
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Every person uses mirrors in their daily life for various purposes. The most commonly used mirror is the plane mirror. A plane mirror can be used for the following purposes:
- In Periscopes and Kaleidoscopes: Plane mirrors are widely used while making kaleidoscopes and periscopes (used in submarines). The plane mirrors and colored glass used in kaleidoscopes represent many colored patterns and the plane mirror used in periscopes in submarines reflects images of all ships on the surface of the water.
- In Torch Lights: Plane mirrors are very commonly used in flashlights and the torchlight in order to reflect beams of light. They are even used for the same purpose in overhead projectors.
- Used by Dentist: Dentists widely use mirrors to view and examine the image of the teeth. Plane mirrors are also used especially in microscopes to reflect the image of the object being monitored.
- Used in Solar Cookers: Plane mirrors are used in solar cookers because they are capable of reflecting most of the sunlight falling on their surface. Solar cookers are the most effective and efficient way of using renewable sources of energy for cooking-related purposes.
- Used for Safety and Security: Plane mirrors are widely used to locate explosives under vehicles. They are also used in stores and shops to keep an eye on customers to prevent incidents of robbery. The mirror is also used to see the vehicles coming from the opposite directions on the blind turns of busy roads.
Check Also: NCERT Solutions for Class 10 Science Chapter 10 – Light: Reflection and Refraction
Things to Remember
- The mirrors that have a flat reflecting surface are called plane mirrors.
- Plane mirrors follows the laws of reflection.
- The image obtained by the plane mirror is virtual, erect and equal in size.
- The distance between the image obtained is equal to the distance between the object and the mirror.
- The magnification of a plane mirror is always 1.
- Plane mirrors are used for various purposes like in periscopes, torch light, solar cookers etc.
Sample Questions
Ques. What kind of image is formed by a plane mirror? (1 Mark)
Ans. The images formed by the plane mirrors are virtual and erect.
Ques. How many reflecting surfaces are there in a kaleidoscope? (1 Mark)
Ans. In a kaleidoscope, there are two or more reflecting surfaces.
Ques. What is the focal length of a plane mirror? (1 Mark)
Ans. The focal length of a plane mirror is infinite.
Ques. The magnification produced by a plane mirror is +1. What does this mean? (2 Marks) [Delhi 2013]
Ans. The magnification produced by a plane mirror is +1 which means that the size of the image formed is exactly equal to the size of the object behind the mirror.
Ques. Explain the difference between the real and virtual images of an object. (2 Marks)
Ans. Real images are formed by the actual intersection of the reflected rays at a point. Whereas, a virtual image is formed when the diverging rays reach the eye. They seem to come from a point, however, there is no real intersection of the rays.
Ques. List 4 properties of the image produced by a plane mirror? (3 Marks) [Foreign 2013, Delhi 2012, Al 2011]
Ans. Properties of the image produced by a plane mirror are as follows:
- The image is erect and virtual.
- Image is as far from the mirror as the object is in front of it.
- The image formed is laterally inverted.
- The size of the image is the same as the size of the object.
Ques. A plane mirror is approaching you at the rate of 10 cm per second. You can see your image in it. At what speed will your image come to you? (3 Marks)
Ans. The distance of the image in a plane mirror is equal to the distance of the object from the mirror. Let the distance of the object from the mirror be X cm. Hence, the distance of the image from the object will be 2X.
Mirror approaching speed = X / t = 10 cm/sec
Image approaching speed = 2X / t = 2 × 10 = 20 cm/sec.
Ques. State the laws of refraction of light. Explain the term absolute refractive index of a medium’ and write an expression to relate it with the speed of light in vacuum. (3 Marks)
Ans. Laws of refraction are:
- The incident ray, reflected ray and the normal at the point all lie on the same place.
- The ratio remains constant between the sine of angle of incidence to the sine of angle of refraction for a particular pair of medium and light wavelengths (n = sin i / sin r).
Absolute refractive index can be referred to as the ratio of the speed of light in vacuum/air to the speed of light in the medium. It is denoted by n.
n = speed of light in air (c) / speed of light in medium (v)
Ques. An object is placed at a distance of 60 cm from a concave lens of focal length 30 cm. (5 Marks)
(i) Use the lens formula to find the distance of the image from the lens.
(ii) List four characteristics of the image (nature, position, size, erect/inverted) formed by the lens in this case.
(iii) Draw a ray diagram to justify your answer of the part (ii).
Ans. (i) 1/f = 1/v - 1/u (Lens Formula)
1/f +1/u = 1/v
U = -60
F = -30 (Concave Lens)
Substituting the values in the formula we get,
1/-30 + 1/-60 = 1/v
(-2-1)/60 = 1/v
-3/60 = 1/v
1/v = -1/20
V = -20 cm
M = v/u = -20/-60 = 0.33
(ii) Four characteristics of the image formed are:
- Virtual
- Erect
- Small
- Formed between F1 and O
(iii) 
Ques. What is meant by the power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. Write the nature and power of each lens. (5 Marks)
Ans. The power of a lens refers to the measurement of the degree of convergence or divergence of the ray of light falling on it. It is also explained as the reciprocal of its focal length and expressed in meters. The SI of power is Dioptre (D).
Convex lenses have a positive focal length, hence,
Fconvex = 40 cm = (40/100) m
Power = 1/ Focal length = (1/40)/100
= 100/40 = +2.5 D
Therefore, the lens which has a focal length of 40 cm is convex.
Concave lenses have a negative focal length,
Fconcave = -20 cm = (-20/100) m
Power = 1/Focal length = (1/-20)/100
= -100/20 = -5 D
Therefore, the lens which has a focal length of -20 cm is concave.
Ques. An object is placed at a distance of 30 cm from a concave lens of focal length 15 cm. List four characteristics (nature, position, etc.) of the image formed by the lens. (5 Marks)
Ans. For concave lens, given u = -30 cm, f = -15 cm
1/v - 1/u = 1/f
1/v - 1(-30) = 1/(-15)
1/v = -1/15 - 1/30
1/v = (-2-1)/30 = -3/30 = -1/10
V = -10 cm
Four characteristics of the image formed are:
- Image formed at a 10 cm distance from the concave lens on the left side.
- Virtual image
- Erect image
- The image formed has a diminished size.
Ques. If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens is 10 D, what is its focal length in the Cartesian system ? (5 Marks)
Ans. Concave lens (diverging lens)
Power of the lens given by the relation
P = 1/f(in metre)
P = 10 D
F = 1/P = 1/10 = 0.1 m
Or -10 cm because the lens is a concave lens.
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