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| Formation of Image by a Plane Mirror
- Ray Diagrams |
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| The following rays are usually considered
while constructing ray diagrams. |
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| A Ray Diagram Showing the formation of an 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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| A ray of light falling on a plane mirror at any angle gets reflected from the mirror such that the angle of incidence is equal to the angle of reflection. |
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It has been found that if the mirrors are inclined at an angle q then the number of images is given by the
relation
 If
 is not a whole
number, then the number of images will be rounded off to the nearest integer. This can be verified by actual drawing. |
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If the mirrors are inclined at 120o the number of images formed by
the mirrors is given by the relation  |
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| Let MM and MM' be two plane mirrors inclined at an angle 120o and O be the object placed in between these mirrors. In this case there will be only two images viz., O1 and O2. |
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Now let us consider the mirrors MM and MM' to be mutually perpendicular. In this case the number of images formed by the mirror
is  |
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| The Angle of Inclination Between the Mirrors is 900.
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 Place the mirrors MM' perpendicular to MM. |
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An object O is kept in between these mirrors. |
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OA and OB are the two rays, which are incident on the mirror MM. |
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OA being normal to the surface retraces its path. |
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OB makes an angle i with the normal N and gets reflected along BC according to the laws of reflection |
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Extend the rays OA and BC backwards. |
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They meet at O1, which
is the virtual image of O. |
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OD and OE represent the rays which are incident on the mirror MM'. |
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OD is perpendicular to the mirror MM' and hence gets reflected along the same path. |
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OE is the incidentray and N2 is the normal at the point of incidence and OE gets reflected along the path EF. |
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Extend OD and EF backwards. They meet at O2, which is the virtual image of O. |
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The reflected ray BC gets internally reflected by the mirror MM' along CG. |
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The ray DG appears to comes from O3, which is the image of O1, |
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Similarly EF the reflected ray gets internally reflected by the mirror MM along FH. |
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The ray FH appears to come from O4, which is the image of O2. |
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The position of O1 and O2 coincide. |
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Thus when the angle of inclination between the mirrors is 900 we get three images. |
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| Let us now calculate the number of images formed if the two mirrors are placed parallel to each other i.e., the angle of inclination between them is 00. |
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Place the mirrors MM and MM' parallel to each other. |
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An object O is kept between these mirrors. |
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OA and OO' represent the rays which are incident on the mirror MM. |
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OO' being normal retraces its path. |
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OA makes an angle i with the normal N1 and gets reflected along AB according to the laws of reflection. |
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Extend the rays AB and OM backwards. |
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They meet at I1, which is the virtual image of the object O. |
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The reflected ray AB gets reflected by the mirror MM' and forms an image I2. |
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Similarly I3, I4 etc. are formed. |
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The light from I1, I2, I3, I4 etc. gets reflected and forms their images. |
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| In this manner, many images are formed but the intensity of the remote images goes on decreasing due to absorption of light energy at every successive reflection and thus we see only finite number of images even though infinite images will be formed. |
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| A plane mirror is used: |
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as a looking glass to view ourselves |
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by interior designers to create an illusion of depth |
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to fold light as in a periscope and other optical instruments |
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to make kaleidoscope, an interesting toy |
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