CBSE – 7th Standard Science

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Chapter 15 – Light

CHAPTER 15

LIGHT

  • White light is composed of seven colours, light travels along straight lines, to change the direction of light is to let it fall on a shiny surface.
  • Change of direction by a mirror is called reflection of light, it is reflected from a plane mirror.

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  • Object which appears behind the mirror, is the image of the object.
  • An image formed by a plane mirror is erect and of the same size as the object.

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  • In the mirror the right appears left and the left appears right, only sides are interchanged; the image does not appear upside down.
  • The curved mirror is a spherical mirror. If the reflecting surface of a spherical mirror is concave, it is called a concave mirror. If the reflecting surface is convex, then it is a convex mirror

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  • An image formed on a screen is called a real image. the image formed by a plane mirror could not be obtained on a screen. Such image is called a virtual image.
  • The image formed by a concave mirror can be smaller or larger in size than the object. The image may also be real or virtual.
  • Concave mirrors are used for many purposes. Dentist and doctors use concave mirrors for examining eyes, ears, nose and throat.  The reflectors of torches, headlights of cars and scooters are concave in shape

 

  • Convex mirrors can form images of objects spread over a large area. these help the drivers to see the traffic behind them

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  • The magnifying glass is actually a type of a lens the lenses are transparent and light can pass through them .
  • Lenses are widely used in spectacles, telescopes and microscopes.
  • lenses which feel thicker in the middle than at the edges are convex lenses
  • Lenses which feel thinner in the middle than at the edges are concave lenses
  • A convex lens converges (bends inward) the light generally falling on it so it is called converging lens.
  •  A concave lens diverges (bends outward) the light and is called a diverging lens

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  • That the image formed by a concave lens is always virtual, erect and smaller in size than the object.
  • Light travels along straight lines.
  • Any polished or a shining surface acts as a mirror.
  • An image which can be obtained on a screen is called a real image.

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  • An image which cannot be obtained on a screen is called a virtual image.
  • The image formed by a plane mirror is erect. It is virtual and is of the same size as the object.
  •  The image is at the same distance behind the mirror as the object is in front of it.
  • In an image formed by a mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image.
  • A concave mirror can form a real and inverted image.
  • When the object is placed very close to the mirror, the image formed is virtual, erect and magnified.
  • Image formed by a convex mirror is erect, virtual and smaller in size than the object.
  • A convex lens can form real and inverted image.
  • When the object is placed very close to the lens, the image formed is virtual, erect and magnified.
  • When used to see objects magnified, the convex lens is called a magnifying glass.
  • A concave lens always forms erect, virtual and smaller image than the object.

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