Class 6 Science Notes Chapter 11 (Chapter 11) – Examplar Problem Book

Detailed Notes with MCQs of Chapter 11: Light, Shadows and Reflections from your Science Exemplar book. This is an important chapter, not just for your class exams, but concepts from here often form the basis for questions in various government exams. Pay close attention!

Chapter 11: Light, Shadows and Reflections - Detailed Notes

1. What is Light?

• Light is a form of energy that enables us to see objects.
• Objects are visible either because they emit their own light or because they reflect light falling on them from another source.

2. Sources of Light:

• Luminous Objects: Objects that produce or emit their own light.
  • Examples: Sun, stars, fire, candle flame, electric bulb, torch.
• Non-Luminous Objects: Objects that do not produce their own light but become visible when light from a luminous source falls on them and gets reflected towards our eyes.
  • Examples: Moon (reflects sunlight), planets, chairs, tables, books, trees, humans.

3. Classification of Objects based on Interaction with Light:

• Transparent Objects: Allow light to pass through them completely. We can see clearly through them.
  • Examples: Clear glass, clean water, air, cellophane paper (clear).
• Translucent Objects: Allow light to pass through them partially. We cannot see clearly through them; objects on the other side appear blurry.
  • Examples: Butter paper, frosted glass, ground glass, thin plastic sheet, tissue paper.
• Opaque Objects: Do not allow any light to pass through them. We cannot see through them at all.
  • Examples: Wood, metal sheet, brick wall, cardboard, thick book, stones.

4. Rectilinear Propagation of Light:

• This is a fundamental property of light. It means that light travels in a straight line.
• Evidence:
  • We cannot see around corners.
  • The formation of sharp shadows.
  • Light beams from a projector or torch appear straight.
  • Activity: Looking at a candle flame through a straight pipe versus a bent pipe. You can only see the flame through the straight pipe.
• This principle is the basis for the pinhole camera and shadow formation.

5. Shadows:

• Formation: A shadow is formed when an opaque object is placed in the path of light. The object blocks the light, creating a dark region behind it on a screen or surface.
• Requirements for Shadow Formation:
  1. A source of light.
  2. An opaque object.
  3. A screen or surface behind the object where the shadow can be formed. (The ground, a wall, or a sheet of paper can act as a screen).
• Characteristics of Shadows:
  • Colour: A shadow is always dark (black or grey), regardless of the colour of the opaque object or the light source.
  • Shape: The shadow gives some information about the shape (outline) of the object.
  • Size: The size of the shadow depends on:
    • The distance between the light source and the object. (Closer the source, larger the shadow).
    • The distance between the object and the screen. (Closer the screen, smaller the shadow).
  • Shadows are formed on the side opposite to the source of light.
  • Transparent objects do not form distinct shadows; translucent objects form faint shadows.

6. The Pinhole Camera:

• Principle: Works on the principle of rectilinear propagation of light.
• Construction: A simple device, usually a box with a tiny hole (pinhole) on one side and a translucent screen (like tracing paper or ground glass) on the opposite side.
• Image Formation: Light rays from different points on an object travel in straight lines through the pinhole and form an image on the screen.
• Characteristics of the Image formed by a Pinhole Camera:
  • Real: The image is formed on a screen.
  • Inverted: The image is upside down and sideways reversed compared to the object. (Rays from the top of the object pass through the pinhole and reach the bottom of the screen, and vice-versa).
  • Size: The size of the image depends on the distance of the object from the pinhole and the distance of the screen from the pinhole. It is often smaller than the object but can be larger if the object is very close to the pinhole.
  • Coloured: The image formed is coloured, same as the object's colour.

7. Reflection of Light:

• Definition: The bouncing back of light rays after striking a surface is called reflection.
• Smooth vs. Rough Surfaces:
  • Smooth, shiny surfaces like mirrors reflect light uniformly in a particular direction (specular reflection), forming clear images.
  • Rough surfaces reflect light irregularly in different directions (diffused reflection). Most objects around us are visible due to diffused reflection.

8. Mirrors and Images:

• Mirror: A highly polished surface (usually glass with a silvered backing) that reflects almost all the light falling on it.
• Plane Mirror: A mirror with a flat reflecting surface.
• Image: An appearance or likeness of an object formed due to reflection (or refraction) of light.
• Characteristics of the Image formed by a Plane Mirror:
  1. Virtual: The image cannot be obtained on a screen. It appears to be formed behind the mirror.
  2. Erect: The image is upright, the same way up as the object.
  3. Same Size: The image is exactly the same size as the object.
  4. Laterally Inverted: The image is sideways reversed. The left side of the object appears as the right side of the image, and vice-versa. (This is why the word 'AMBULANCE' is written in reverse on the front of ambulances).
  5. Same Distance: The image is formed as far behind the mirror as the object is in front of it.

Key Takeaways for Exams:

• Know the definitions: Luminous/Non-luminous, Transparent/Translucent/Opaque.
• Understand the principle: Rectilinear Propagation of Light.
• Remember the requirements and characteristics of shadows.
• Understand the working principle and image characteristics of a Pinhole Camera (especially 'inverted').
• Memorize the characteristics of the image formed by a Plane Mirror (especially 'virtual', 'erect', 'same size', 'laterally inverted', 'same distance').
• Be able to differentiate between shadows and images. (Shadows are dark areas where light is blocked; images are formed by reflection/refraction and can show colour and detail).

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the chapter for your practice:

1. Which of the following is a natural source of light?
   a) Torch
   b) Sun
   c) Candle
   d) Electric Lamp

2. An object that allows light to pass through it completely is called:
   a) Opaque
   b) Translucent
   c) Transparent
   d) Luminous

3. Shadows are formed because:
   a) Light travels in curved lines.
   b) Opaque objects reflect light.
   c) Light travels in straight lines and opaque objects block it.
   d) Transparent objects absorb light.

4. Which statement is TRUE about the image formed by a pinhole camera?
   a) It is erect and virtual.
   b) It is inverted and real.
   c) It is erect and real.
   d) It is inverted and virtual.

5. The phenomenon where the left side of an object appears as the right side in its image formed by a plane mirror is called:
   a) Reflection
   b) Refraction
   c) Rectilinear propagation
   d) Lateral inversion

6. Which of the following is required to form a shadow?
   a) A source of light, a transparent object, a screen
   b) A source of light, an opaque object, a screen
   c) A luminous object, a translucent object, a mirror
   d) A non-luminous object, an opaque object, a lens

7. If you stand 2 meters in front of a plane mirror, how far behind the mirror does your image appear to be?
   a) 1 meter
   b) 2 meters
   c) 4 meters
   d) 0.5 meters

8. Which of these objects is translucent?
   a) A wooden door
   b) Clear glass
   c) Butter paper
   d) A steel plate

9. The colour of a shadow formed by a red opaque object in white light is:
   a) Red
   b) White
   c) Black/Dark
   d) Depends on the screen colour

10. Which property of light is demonstrated by the working of a pinhole camera?
    a) Light is a form of energy.
    b) Light travels very fast.
    c) Light travels in straight lines.
    d) Light is composed of seven colours.

Answers to MCQs:

1. b) Sun
2. c) Transparent
3. c) Light travels in straight lines and opaque objects block it.
4. b) It is inverted and real.
5. d) Lateral inversion
6. b) A source of light, an opaque object, a screen
7. b) 2 meters
8. c) Butter paper
9. c) Black/Dark
10. c) Light travels in straight lines.

Study these notes thoroughly. Understanding these fundamental concepts of light is crucial. Let me know if any part needs further clarification. Good luck with your preparation!