LIGHT

Types of Mirrors

 

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Spherical mirrors

            Spherical mirrors are one form of curved mirrors. If the curved mirror is a part of a sphere, then it is called a ‘spherical mirror’. It resembles the shape of a piece cut out from a spherical surface. One side of this mirror is silvered and the reflection of light occurs at the other side.

 

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Concave mirrors

            A spherical mirror, in which the reflection of light occurs at its concave surface, is called a concave mirror. These mirrors magnify the object placed close to them. The most common example of a concave mirror is the make-up mirror.

 

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Convex mirror

      A spherical mirror, in which the reflection of light occurs at its convex surface, is called a convex mirror. The image formed by these mirrors is smaller than the object. Most common convex mirrors are rear viewing mirrors used in vehicles.

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Convex mirrors used in vehicles as rear-view mirrors are labeled with the safety warning: ‘Objects in the mirror are closer than they appear’ to warn the drivers. This is because inside the mirrors, vehicles will appear to be coming at a long distance.

 

Parabolic mirrors

          A parabolic mirror is one type of curved mirror, which is in the shape of a parabola. It has a concave reflecting surface and this surface directs the entire incident beam of light to converge at its focal point.

            In the same way, light rays generated by the source placed at this focal point will fall on this surface and they will be diverged in a direction, which is parallel to the principal axis of the parabolic mirror. Hence, the light rays will be reflected to travel a long distance, without getting diminished.

            Parabolic mirrors, also known as parabolic reflectors, are used to collect or project energy such as light, heat, sound and radio waves. They are used in reflecting telescopes, radio telescopes and parabolic microphones. They are also used in solar cookers and solar water heaters.

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Terms related to spherical mirrors

          Center of Curvature:

                        It is the center of the sphere from which the mirror is made. It is denoted by the letter C in the ray diagrams. (A ray diagram represents the formation of an image by the spherical mirror. You will study about them in your next class).

 

            Pole:

                        It is the geometric center of the spherical mirror. It is denoted by the letter P.

 

            Radius of Curvature:

                    It is the distance between the center of the sphere and the vertex. It is shown by the letter R in ray diagrams. (The vertex is the point on the mirror’s surface where the principal axis meets the mirror. It is also called as ‘pole’.)

 

          Principal Axis:

                        The line joining the pole of the mirror and its center of curvature is called principal axis.

 

Focus:

            When a beam of light is incident on a spherical mirror, the reflected rays converge (concave mirror) at or appear to diverge from (convex mirror) a point on the principal axis. This point is called the ‘focus’ or ‘principal focus’. It is also known as the focal point. It is denoted by the letter F in ray diagrams.

 

            Focal length:

                        The distance between the pole and the principal focus is called focal length (f) of a spherical mirror.

 

            There is a relation between the focal length of a spherical mirror and its radius of curvature. The focal length is half of the radius of curvature.

            That is, focal length = Radius of curvature/2

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Images formed by spherical mirrors

          Images formed by spherical mirrors are of two types:

i)                  Real image and

ii)                Virtual image.

       

          Real images can be formed on a screen, while virtual images cannot be formed on a screen.

            Image formed by a convex mirror is always erect, virtual and diminished in size. As a result, images formed by these mirrors cannot be projected on a screen.

 

            The characteristics of an image are determined by the location of the object. As the object gets closer to a concave mirror, the image gets larger, until attaining approximately the size of the object, when it reaches the Centre of curvature of the mirror. As the object moves away, the image diminishes in size and gets gradually closer to the focus, until it is reduced to a point at the focus when the object is at an infinite distance from the mirror.

           

            The size and nature of the image formed by a convex mirror is given in Table 1.

 

Concave mirrors form a real image and it can be caught on a screen. Unlike convex mirrors, concave mirrors show different image types. Depending on the position of the object in front of the mirror, the position, size and nature of the image will vary. Table 2 provides a summary of images formed by a concave mirror.

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Applications of curved mirrors

 

Concave mirrors

1.      Concave mirrors are used while applying make-up or shaving, as they provide a magnified image.

2.    They are used in torches, search lights and head lights as they direct the light to a long distance.

3.    They can collect the light from a larger area and focus it into a small spot. Hence, they are used in solar cookers.

4.    They are used as head mirrors by doctors to examine the eye, ear and throat as they provide a shadow-free illumination of the organ.

5.     They are also used in reflecting telescopes.

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Convex mirrors

1.      Convex mirrors are used in vehicles as rear view mirrors because they give an upright image and provide a wider field of view as they are curved outwards.

2.    They are found in the hallways of various buildings including hospitals, hotels, schools and stores. They are usually mounted on a wall or ceiling where hallways make sharp turns.

3.     They are also used on roads where there are sharp curves and turns.

Laws of reflection

          Reflection involves two rays:

i)                  incident ray

ii)                Reflected ray. The incident ray is the light ray in a medium falling on the shiny surface of a reflecting body. After falling on the surface, this ray returns into the same medium. This ray is called the reflected ray. An imaginary line perpendicular to the reflecting surface, at the point of incidence of the light ray, is called the normal.

 

          The relation between the incident ray, the reflected ray and the normal is given as the law of reflection. The laws of reflection are as follows:

1.     The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

2.     The angle of incidence and the angle of reflection are always equal.

 

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Types of reflection

Regular reflection

          When a beam of light (collection of parallel rays) falls on a smooth surface, it gets reflected. After reflection, the reflected rays will be parallel to each other. Here, the angle of incidence and the angle of reflection of each ray will be equal. Hence, the law of reflection is obeyed in this case and thus a clear image is formed. This reflection is called ‘regular reflection’ or ‘specular reflection’. Example: Reflection of light by a plane mirror and reflection of light from the surface of still water.

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Irregular reflection

In the case of a body having a rough or irregular surface, each region of the surface is inclined at different angles. When light falls on such a surface, the light rays are reflected at different angles. In this case, the angle of incidence and the angle of reflection of each ray are not equal. Hence, the law of reflection is not obeyed in this case and thus the image is not clear. Such a reflection is called ‘irregular reflection’ or ‘diff used reflection’. Example: Reflection of light from a wall.

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Multiple reflections

Kaleidoscope

          It is a device, which functions on the principle of multiple reflection of light, to produce numerous patterns of images. It has two or more mirrors inclined with each other. It can be designed from inexpensive materials and the colorful image patterns formed by this will be pleasing to you. This instrument is used as a toy for children.

Periscope

It is an instrument used for viewing bodies or ships, which are over and around another body or a submarine. It is based on the principle of the law of reflection of light. It consists of a long outer case and inside this case mirrors or prisms are kept at each end, inclined at an angle of 45°. Light coming from the distant body, falls on the mirror at the top end of the periscope and gets reflected vertically downward. This light is reflected again by the second mirror kept at the bottom, so as to travel horizontally and reach the eye of the observer. In some complex periscopes, optic fiber is used instead of mirrors for obtaining a higher resolution. The distance between the mirrors also varies depending on the purpose of using the periscope.

Uses

1.      It is used in warfare and navigation of the submarine.

2.    In military it is used for pointing and firing guns from a ‘bunker’.

3.    Photographs of important places can be taken through periscopes without trespassing restricted military regions.

4.    Fiber optic periscopes are used by doctors as endoscopes to view internal organs of the body.

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Refraction of light

          When light falls on a transparent material, it is not reflected completely, but a part of it is reflected and a part of it is absorbed and most of the light passes through it. Through air, light travels with a speed of 3 × 108 m s-1, but it cannot travel with the same speed in water or glass, because, optically denser medium such as water and glass offer some resistance to the light rays. So, light rays travelling from a rarer medium like air into a denser medium like glass or water are deviated from their straight line path. This bending of light about the normal, at the point of incidence; as it passes from one transparent medium to another is called refraction of light. When a light ray travels from the rarer medium into the denser medium, it bends towards the normal and when it travels from the denser medium into the rarer medium, it bends away from the normal.

Refractive Index

          Refraction of light in a medium depends on the speed of light in that medium. When the speed of light in a medium is more, the bending is less and when the speed of light is less, the bending is more.

            The amount of refraction of light in a medium is denoted by a term known as refractive index of the medium, which is the ratio of the speed of light in the air to the speed of light in that particular medium. It is also known as the absolute refractive index and it is denoted by the Greek letter ‘μ’ (pronounced as ‘mew’).

                                                                             μ =

 

          Refractive index is a ratio of two similar quantities (speed) and so, it has no unit. Since, the speed of light in any medium is less than its speed in air, refractive index of any transparent medium is always greater than 1.

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In general, the refractive index of one medium with respect to another medium is given by the ratio of their absolute refractive indices.

                                                                   1μ2 =

                                                                             1μ2 =        or      1μ2 =       

           

Thus, the refractive index of one medium with respect to another medium is also given by the ratio of the speed of light in first medium to its speed in the second medium.

 

Snell’s Law of Refraction

          Refraction of light rays, as they travel from one medium to another medium, obeys two laws, which are known as Snell’s laws of refraction. They are:

I) The incident ray, the refracted ray and the normal at the point of intersection, all lie in the same plane.

II) The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is equal to the refractive index of the medium, which is a constant.

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Dispersion

            Splitting of white light into its seven constituent colors (wavelength), on passing through a transparent medium is known as dispersion of light.

            Dispersion occurs because, light of different colors present in white light have different wavelength and they travel at different speeds in a medium. Refraction of a light ray in a medium depends on its speed. As each colored light has a different speed, the constituent colored lights are refracted at different extents, inside the Prism. Moreover, refraction of a light ray is inversely proportional to its wavelength. Thus, the red colored light, which has a large wavelength, is deviated less while the violet colored light, which has a short wavelength, is deviated more.