Class 6 Science Ch 11: Light, Shadows, Reflections

Chapter 11: Light, Shadows and Reflections

CBSE Class 6 Science Notes

Chapter Summary

• Light: It is a form of energy that enables us to see objects. We cannot see anything in the dark.
• Luminous Objects: These are objects that emit their own light.
  • Examples: Sun, stars, electric bulb, torch, firefly, candle.
• Non-Luminous Objects: These are objects that do not emit their own light. We see them because they reflect light from luminous sources.
  • Examples: Moon, chairs, tables, books, plants, humans.
• Path of Light: Light generally travels in a straight line. This is known as the rectilinear propagation of light.
• Classification of Objects Based on Light Passage:
  • Transparent Objects: Objects that allow light to pass through them completely, so we can see clearly through them.
    • Examples: Clear glass, air, pure water.
  • Translucent Objects: Objects that allow some light to pass through them partially, making objects appear blurred or hazy when viewed through them. We cannot see clearly through them.
    • Examples: Frosted glass, butter paper, tracing paper, thin plastic sheet.
  • Opaque Objects: Objects that do not allow any light to pass through them at all. We cannot see through opaque objects.
    • Examples: Wood, stone, metal, cardboard, wall.
• Shadows: A shadow is a dark area formed when an opaque object blocks the path of light.
  • Conditions for Shadow Formation: A shadow requires three things:
    1. A source of light.
    2. An opaque object to block the light.
    3. A screen or surface behind the object for the shadow to be cast upon.
  • Characteristics of Shadows:
    • Shadows are always dark, regardless of the color of the object.
    • Shadows only show the outline or shape of the object, not its details or colors.
    • The size and shape of a shadow can change depending on the position of the light source relative to the object and the screen.
  • Pinhole Camera: A simple device that demonstrates the rectilinear propagation of light.
    • It forms an inverted (upside down) and real image of an object on a screen by allowing light rays to pass through a tiny hole.
    • The image is often smaller than the object and can be seen on a screen inside the camera.
  • Reflection: It is the phenomenon of light bouncing back when it strikes a smooth, polished surface like a mirror.
    • Mirrors: Surfaces that are specifically designed to reflect light effectively.
    • Plane Mirror: A flat mirror that forms an image with the following characteristics:
      • It is erect (upright).
      • It is virtual (cannot be obtained on a screen).
      • It is of the same size as the object.
      • It is laterally inverted (the left side of the object appears as the right side in the image, and vice-versa).

Questions and Answers

1. Rearrange the boxes given below to make a sentence that helps us understand opaque objects.

R, O, D, U, C, E, P, A, Q, O, B, J, E, C, T, S, H, A, D, O, W, S, A, K.

Answer: OPAQUE OBJECTS MAKE SHADOWS.

2. Classify the objects or materials given below as opaque, transparent or translucent and luminous or non-luminous:

Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a piece of red hot iron, an umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper, the flame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a wire mesh, kerosene stove, sun, firefly, moon.

Answer:

• Opaque: A piece of rock, a sheet of aluminium, a wooden board, a CD, an umbrella, a wall, a sheet of carbon paper, a sheet of cardboard, a mirror, kerosene stove (when not lit), moon.
• Transparent: Air, water, a sheet of plane glass.
• Translucent: A sheet of polythene, smoke, fog, a sheet of cellophane, a wire mesh.

• Luminous: A piece of red hot iron, a lighted fluorescent tube, the flame of a gas burner, a lighted torch, sun, firefly.
• Non-luminous: Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, an umbrella, a wall, a sheet of carbon paper, a sheet of cardboard, a sheet of cellophane, a wire mesh, kerosene stove, moon.

3. Can you think of creating a shape that would give a circular shadow if held in one way and a triangular shadow if held in another way?

Answer: Yes, we can. A cone can produce both a circular and a triangular shadow.

• If the cone is held such that its base is facing the light source, it will cast a circular shadow.
• If the cone is held such that its pointed end is towards the light source (or if the light falls perpendicular to its side profile), it will cast a triangular shadow.

4. In a completely dark room, if you hold up a mirror in front of you, will you see your reflection in the mirror?

Answer: No, you will not see your reflection in a completely dark room. A mirror works by reflecting light. In order for you to see your reflection, there must be a source of light to illuminate you and then reflect off the mirror back into your eyes. In complete darkness, there is no light to be reflected.

5. It is known that the Moon is not a luminous object. How do we see the Moon?

Answer: The Moon is a non-luminous object, meaning it does not produce its own light. We are able to see the Moon because it reflects the light of the Sun. Sunlight falls on the surface of the Moon, and this reflected light then travels to our eyes, making the Moon visible to us from Earth.