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Important Points of this chapter
  • Light seems to travel in straight lines.
  • Mirrors and lenses form images of objects. Images can be either real or virtual, depending on the position of the object.
  • The reflecting surfaces, of all types, obey the laws of reflection. The refracting surfaces obey the laws of refraction.
  • New Cartesian Sign Conventions are followed for spherical mirrors and lenses.
  • Mirror formula,1/v + 1/u = 1/f , gives the relationship between the object-distance (u), image-distance (v), and focal length (f) of a spherical mirror.
  • The focal length of a spherical mirror is equal to half its radius of curvature.
  • The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object.


  • A light ray travelling obliquely from a denser medium to a rarer medium bends away from the normal. A light ray bends towards the normal when it travels obliquely from a rarer to a denser medium.
  • Light travels in vacuum with an enormous speed of 3×108 m s-1. The speed of light is different in different media.
  • The refractive index of a transparent medium is the ratio of the speed of light in vacuum to that in the medium.
  • In case of a rectangular glass slab, the refraction takes place at both air-glass interface and glass-air interface. The emergent ray is parallel to the direction of incident ray.
  • Lens formula, 1/v - 1/u = 1/f , gives the relationship between the object-distance (u), image-distance (v), and the focal length (f) of a spherical lens.
  • Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is dioptre.

 REFLECTION OF LIGHT

  • The angle of incidence is equal to the angle of reflection, and The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
  • Image formed by a plane mirror is always virtual and erect. The size of the image is equal to that of the object. The image formed is as far behind the mirror as the object is in front of it.

REFRACTION
  • When a beam of light encounters another transparent medium, a part of light gets reflected back into the first medium while the rest enters the other. A ray of light represents a beam.
  • The direction of propagation of an obliquely incident ray of light that enters the other medium, changes at the interface of the two media. This phenomenon is called refraction of light.
  • The incident ray, the refracted ray and the normal to the interface at the point of incidence, all lie in the same plane.
  • The ratio of the sine of the angle of incidence to the sine of angle of refraction is constant


SPHERICAL MIRRORS
  • A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.
  • A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror
  • The centre of the reflecting surface of a spherical mirror is a point called the pole. It lies on the surface of the mirror.
Uses of concave mirrors
  • Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light.
  • They are often used as shaving mirrors to see a larger image of the face.
  • The dentists use concave mirrors to see large images of the teeth of patients. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Uses of convex mirrors
  • Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles.
  • These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving.
  • Convex mirrors are preferred because they always give an erect, though diminished, image.
  • Also, they have a wider field of view as they are curved outwards.
  •  Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.





TOTAL INTERNAL REFLECTION

 When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection.

 Total internal reflection in nature and its technological applications:-

 1)MIRAGE:
  • In hot summer days, the air near the ground becomes hotter than the air at higher levels.
  • To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object.
  • Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called mirage.
  • This type of mirage is especially common in hot deserts. Some of you might have noticed that while moving in a bus or a car during a hot summer day, a distant patch of  road, especially on a highway, appears to be wet. But, you do not find any evidence of wetness when you reach that spot. This is also due to mirage.

 2)DIAMOND:
  • Diamonds are known for their spectacular brilliance. Their brilliance is mainly due to the total internal reflection of light inside them.
  • Diamonds found in nature rarely exhibit the brilliance for which they are known.
  • It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly.
  • By cutting the diamond suitably, multiple total internal reflections can be made to occur

 3)PRISM:
  • Prisms designed to bend light by 90º or by 180º make use of total internal reflection Such a prism is also used to invert images without changing their size

 4)OPTICAL FIBRE:
  • optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection
  • Optical fibres are extensively used for transmitting and receiving electrical signals
  • optical fibres can also be used for transmission of optical signals.

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