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Introduction |
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It is a matter of common experience that the objects inside a dark room, which are invisible, become visible when the room is illuminated by a source of light. Thus light can be defined as the external cause responsible for the sensation of vision. |
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Nature of Light |
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Light is a form of energy and can be transformed into other forms of energy. You would have observed the path of 'a beam of light' inside a room. This beam is nothing but the scattered light produced by the dust particles and this beam of light becomes invisible if the room is dust free. Thus light makes things visible even though light by itself is invisible. |
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Nature of Light - Theories |
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Light is a form of energy. Energy can be transferred from one point to another point either by particle motion or by wave motion. Accordingly, different theories on the nature of light have been proposed. |
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Characteristics of Light |
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Light is a form of energy produced by luminous objects. Light can travel through vacuum. |
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Rectilinear Propagation of Light |
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In a homogenous transparent medium light travels in a straight line and this is known as rectilinear propagation of light. |
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Reflection of Light |
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When a ray of light falls on any surface, a part of the light is sent back to the same medium. This phenomenon where the incident light falling on a surface is sent back to the same medium is known as reflection. |
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Reflection of Light by a Plane Surface |
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The figure shows how a ray of light is reflected by a plane surface. Let MM' represent a reflecting surface. When a ray of light is incident on MM' in the direction IO it gets reflected along the direction OR. IO is the incident ray; O is the point of incidence and OR is the reflected ray. |
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Verification of the Laws of Reflection |
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Fix a sheet of white paper on a drawing board. Draw a line MM' on it and mark a point O at the center of the line and a normal ON on MM'. |
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Nature of the Image Formed by a Plane Reflecting Surface |
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An image can be real or virtual. A real image is formed when the rays of light actually intersect after reflection. A virtual image is formed when the light rays after reflection do not actually intersect but appear to diverge from it (these rays of light intersect when produced backwards). |
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Formation of Image by a Plane Mirror |
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A ray of light incident on a plane mirror at 90o gets reflected from the mirror along the same path. |
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Application of Plane Mirrors - Periscope |
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It is an instrument in which plane mirrors are used to fold light so that the image of an object can be brought down to a lower level. It is used for observing enemy movements from trenches without any danger of being seen. Sailors on submarines use periscopes to see things above the water level. |
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Spherical Mirrors |
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A mirror whose polished, reflecting surface is a part of a hollow sphere of glass or plastic is called a spherical mirror. |
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Relation Between f and R |
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To show that f = R/2 where f is the focal length of a mirror and R its radius of curvature. |
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Sign Convention for Spherical Mirrors |
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All distances are measured from the pole of the mirror. Distances measured in the direction of the incident ray are positive and the distances measured in the direction opposite to that of the incident rays are negative. |
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Formation of Images by Spherical Mirrors |
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When an object is placed in front of a concave mirror, light rays from the object fall on the mirror and get reflected. The reflected rays produce an image at a point where they intersect or appear to intersect. Formation of an image by mirrors is usually shown by constructing ray diagrams. To construct a ray diagram, we need at least two rays whose paths after reflection from the mirror are known. These rays must be chosen according to our convenience. Any two of the rays can be considered to obtain the image. |
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Formation of Images by a Concave Mirror |
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When an object is placed at infinity, the rays coming from it are parallel to each other. Let us consider two rays, one striking the mirror at its pole and the other passing through the center of curvature. |
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Uses of Concave Mirror |
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The use of a concave mirror depends upon the distance of the object from the mirror. |
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Formation of Image in a Convex Mirror |
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Formed between the pole and the focus. |
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Mirror Formula (Concave Mirror) |
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Mirror formula is the relationship between object distance (u), image distance (v) and focal length. |
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Mirror Formula (Convex Mirror) |
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Let AB be an object placed on the principal axis of a convex mirror of focal length f. u is the distance between the object and the mirror and v is the distance between the image and the mirror. |
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Magnification |
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The ratio of the height of the image to the height of the object is called the linear magnification. It is denoted by the letter m. While deriving the mirror formula it has been proved that D ACB and D A1CB1 are similar and so also D FB1A1 and D FED are similar. |
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Summary |
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Light is a form of energy which can be reflected either by a plane or spherical mirror. Depending upon the type of mirror used for reflection of light the use of reflecting surface also varies. In the next chapter let us see what happens when light moves from one medium to another. |
