CHAPTER - 15 LIGHT

1
CHAPTER - 15LIGHT
2
1) Light travels along a straight line -

• Activity -
• Light a candle and fix it on a table. Take a
  piece of straight pipe or a rubber tube. Look at
  the candle through the pipe. You can see the
  candle flame. If the pipe is bent, you cannot see
  the candle flame. This shows that light travels
  along a straight line.

3
2) Reflection of light -

• When light falls on a mirror, the direction of
  light changes. This change in the direction of
  light by a mirror is called reflection of light.
• Activity - Take a torch and cover its glass with
  a chart paper. Having a small hole. Spread a
  chart paper on a wooden board. Keep a plane
  mirror vertically on it. Direct a beam of light
  at an angle to the mirror. The direction of light
  changes. If the torch is moved slightly to either
  side, the direction of light also changes.

4
3) Image formed by a plane mirror -

• i) The image is erect.
• ii) The image is same size as the object.
• iii) The image is at the same distance from the
  mirror as the object is
• in front of it.
• iv) The image is virtual (cannot be obtained on a
  screen).
• v) In the image the right side appears left and
  the left side appears
• right.

5
4a) Spherical mirrors -

• Spherical mirrors are curved mirrors.
• If the reflecting is inside, the spherical
  mirror is a concave mirror.
• If the reflecting surface is outside, the
  spherical mirror is a convex mirror.
• Eg - A stainless steel spoon also act like a
  mirror. The inner side acts like a concave mirror
  and the outer side acts like a convex mirror

Convex
Concave
6
b) Image formed by Concave mirror -

• i) The image formed by a concave mirror may be
  smaller or larger than
• the object.
• ii) The image may also be real (can be obtained
  on a screen) or virtual
• (cannot be obtained on a screen).
• iii) The image may inverted or erect.
• Activity - Fix a concave mirror on a screen.
  Light a candle and keep it at a distance of 50 cm
  from the mirror. Try to obtain the image of the
  candle flame on a screen by moving the screen.
  Observe the image. Then move the candle closer to
  the mirror at different distances and observe the
  images.

Distance of object from the mirror Smaller / Larger than the object Inverted / Erect Real / Virtual
50 cm
40 cm
30 cm
20 cm
10 cm
5 cm
7
c) Uses of Concave mirrors -

• Concave mirrors are used as reflectors in
  torches, headlights of cars, scooters etc.
• Concave mirrors are used by dentists to see
  enlarged images of teeth.
• Concave mirrors are used by doctors for
  examining eyes, ears, nose and throat.

8
d) Image formed by Convex mirror -

• i) The image formed by a convex mirror is
  virtual (cannot
• be obtained on a screen).
• ii) The image is smaller in size than the object.
• e) Uses of Convex mirror -
• Convex mirrors are used as side mirrors in
  cars, scooters etc.
• Convex mirror has a wider view to help
  drivers to see the traffic behind them.

9
5a) Lenses -

• Lenses are of two main types. They are
  Convex lenses and concave lenses.
• i) Convex lens is thick in the middle and thin
  at the
• edges. A convex lens bends light inwards. So
  it is called
• converging lens.
• ii) Concave lens is thin in the middle and thick
  at the edges.
• A concave lens bends light outwards. So it
  is called
• diverging lens.

10
b) Image formed by Convex lens -

• i) The image formed by a convex may be smaller
  or larger
• than the object.
• ii) The image may also be real (can be obtained
  on a screen)
• or virtual (cannot be obtained on a screen).
• iii) The image may inverted or erect.
• c) Image formed by Concave lens -
• i) The image formed by a convex mirror is
  virtual (cannot
• be obtained on a screen).
• ii) The image is smaller in size than the object.
• d) Uses of lenses -
• Lenses are used in spectacles, microscopes,
  telescopes,
• cameras etc.

11
6a) Sunlight White or coloured ?

• A rainbow is usually seen in the sky after a
  rain when the sun is low in the sky. A rainbow
  has seven colours. They are red, orange,
  yellow, green, blue, indigo and violet.
• Activity -
• Take a glass prism and allow a narrow beam of
  sunlight to fall on one side of the prism. Keep a
  screen on the other side of the prism. We can see
  that seven rainbow colours on the screen. This
  shows that sunlight or white light is a mixture
  of seven colours.

12
b) Mixing the rainbow colours produces white
light -

• If the seven rainbow colours are mixed
  together, it produces white light.
• Activity - Take a circular cardboard and divide
  it into seven segments. Paint the seven rainbow
  colours on it. Make a small hole in the centre of
  the disc. Fix the disc on the tip of a refill of
  a ball pen. When the disc is rotated fast, the
  colours get mixed and appears white in colour.
  This disc is known as Newtons colour disc.