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📖 Science

Std 7
Curriculum Book Guide
1 2 3 4 5 6 7 8 9 10 11 12
Chapters
1 The Ever-Evolving World of Science 2 Exploring Substances: Acidic, Basic, and Neutral 3 Electricity: Circuits and their Components 4 The World of Metals and Non-metals 5 Changes Around Us: Physical and Chemical 6 Adolescence: A Stage of Growth and Change 7 Heat Transfer in Nature 8 Measurement of Time and Motion 9 Life Processes in Animals 10 Life Processes in Plants 11 Light: Shadows and Reflections 12 Earth, Moon, and the Sun
11
Chapter 11
Skill: 50%

Light: Shadows and Reflections

Light: Shadows and Reflections

Chapter at a Glance

This chapter explores the properties of light, including its sources, rectilinear (straight-line) propagation, interaction with different materials, shadow formation, and reflection in mirrors. Students learn the distinction between transparent, translucent, and opaque objects. The Z-shaped periscope, multiple-reflection kaleidoscope, and pinhole camera are introduced as practical applications. The chapter also details plane mirror image properties like lateral inversion, features of shadows, and cultural practices like traditional Indian shadow puppetry.

Key Definitions & Terminology

  • Luminous Object: An object that emits its own light (e.g., the Sun, stars, fireflies, lit candles, light bulbs).
  • Non-luminous Object: An object that does not emit its own light but becomes visible by reflecting light from luminous sources (e.g., the Moon, mirrors, trees, tables).
  • Rectilinear Propagation: The physical property of light showing that it travels along straight-line paths.
  • Transparent Material: A material that allows light to pass through it almost completely, making objects behind it clearly visible (e.g., clear glass, pure water).
  • Translucent Material: A material that allows light to pass through it only partially, making objects behind it appear blurry or indistinct (e.g., tracing paper, frosted glass).
  • Opaque Material: A material that does not allow light to pass through it at all, blocking the light completely (e.g., cardboard, wood, metals).
  • Shadow: A dark region formed on a screen behind an opaque or translucent object when it blocks the path of light.
  • Reflection: The process by which a surface (especially a shiny flat surface like a mirror) changes the direction of light rays that fall on it.
  • Image: A visual likeness of an object formed by light rays reflecting from a mirror or passing through a lens.
  • Erect Image: An upright image where the top of the object appears at the top of the image.
  • Lateral Inversion: The left-right reversal where the left side of an object appears as the right side of its mirror image, and vice versa.
  • Pinhole Camera: A simple, lensless camera consisting of a box with a tiny hole on one side, which projects an inverted, colored image of an object onto a screen.

Formulas, Rules & Properties

  • Requirements for Shadow Formation:
  • A source of light, an opaque/translucent object, and a screen (like a wall, floor, or ground).
  • Shadow Size Rules:
  • Keeping the light source and screen fixed:
    • Moving the object closer to the light source $\rightarrow$ shadow becomes larger.
    • Moving the object closer to the screen $\rightarrow$ shadow becomes smaller.
  • Changing the color of the opaque object does not change the color of the shadow; shadows are always dark/black.
  • Plane Mirror Image Properties:
  • Image is erect (upright).
  • Image is of the same size as the object.
  • Image distance behind the mirror is equal to the object distance in front of the mirror.
  • Image is laterally inverted.
  • Image is virtual (cannot be obtained on a screen placed in front of or behind the mirror).
  • Travel Time of Sunlight:
  • Light from the Sun takes approximately 8 minutes and 20 seconds to reach the Earth.

Core Concepts & Topics

  • Rectilinear Propagation Experiments:
  • Matchbox holes (Activity 11.4): Light from a torch only passes through to the screen when the holes in three aligned matchbox trays are in a straight line.
  • Bent tube (Activity 11.2): A candle flame is visible through a straight hollow tube but disappears when the tube is bent.
  • Indian Shadow Puppetry:
  • A traditional storytelling and cultural art form using flat cut-out puppets. Variations include:
    • Charma Bahuli Natya: Maharashtra
    • Keelu Bomme and Tholu Bommalata: Andhra Pradesh
    • Togalu Gombeyaata: Karnataka
    • Ravana Chhaya: Odisha
    • Tholpavakoothu: Kerala
    • Bommalattam: Tamil Nadu
  • Aranmula Kannadi:
  • A unique hand-crafted metal alloy mirror made in Aranmula, Kerala, using secrets of metallurgy passed down over generations.
  • The Flying Bird Shadow Paradox:
  • A bird flying high in the sky does not cast a visible shadow on the ground. This is because the sun is a large light source, and as the bird flies higher, the dark shadow region (umbra) tapers off and disappears before reaching the ground. When the bird flies close to the ground, the shadow becomes visible.

Worked Examples

  • Identifying Luminous Objects:
  • Problem: Classify Mars, Moon, Pole Star, Sun, Venus, and Mirror.
  • Solution: Pole Star and Sun are luminous because they emit their own light. Mars, Venus, and the Moon are planets/satellites that reflect sunlight; a mirror only reflects incident light, so they are non-luminous.
  • Match the Columns:
  • Pinhole camera $\rightarrow$ Forms an inverted image.
  • Opaque object $\rightarrow$ Blocks light completely.
  • Transparent object $\rightarrow$ Light passes almost completely through it.
  • Shadow $\rightarrow$ The dark region formed behind the object.
  • Straight Pipe Observation:
  • Problem: Sahil, Rekha, Patrick, and Qasima look at a candle flame through pipes. Who can see it?
  • Solution: Only Rekha can see it because her pipe is completely straight.
  • Shadow of a Ball (Fig 11.18):
  • Problem: Choose correct shadows when: (i) ball is close to torch, (ii) ball is close to wall.
  • Solution: (i) matches (a) (shadow is larger). (ii) matches (b) (shadow is smaller).
  • Torch and Ball Positions:
  • If the torch is close to the ball $\rightarrow$ The shadow would be larger.
  • If the torch is far away $\rightarrow$ The shadow would be smaller.
  • If the ball is removed $\rightarrow$ A bright spot would appear on the screen.
  • If two torches are present on the left side $\rightarrow$ Two shadows would appear on the screen.
  • Lateral Inversion statements:
  • Statement A: Image formed by a plane mirror is laterally inverted (True).
  • Statement B: Images of letters T and O appear identical to themselves in a plane mirror (True, because T and O have lateral symmetry).
  • Correct option: (i) Both statements are true.
  • Z-Shaped Tube Periscope:
  • Question: Can a Z-shaped tube and two plane mirrors be used to make a periscope?
  • Solution: Yes. Fix the two mirrors at the two bends of the Z-tube, facing each other at an angle of $45^\circ$ to the tube walls.
  • Shortest Shadow Time:
  • Question: At what time (9 AM, 12 PM, or 4 PM) is your shadow the shortest, and why?
  • Solution: At 12 PM (noon), because the sun is directly overhead. The angle of incidence of sunlight is vertical, which minimizes the length of the blocked light region on the ground.

Practical Activities & Experiments

  • Activity 11.1 (Matchbox trays): Verifying straight-line light paths.
  • Activity 11.2 (Bent tube): Viewing candles through straight and bent tubes.
  • Activity 11.3 (Materials check): Deciding transparency/translucency/opacity of items.
  • Activity 11.4 (Shadow factors): Testing the effects of distance, tilt, and screen removal.
  • Activity 11.5 & 11.6 (Redirecting light): Using mirrors to redirect sunlight and testing reflections with comb slits.
  • Activity 11.7 & 11.8 (Plane mirror properties): Examining erectness, equal sizing, and lateral inversion.
  • Activity 11.9 & 11.10 (Pinhole camera): Projecting inverted images of a candle and tree.
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