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📘 Chapter 2: Light, Shadows and Reflections


 

🔹 1. How Do We See Things?

  • We can see objects only when light reflects from them and enters our eyes.

  • Luminous objects: Emit their own light (e.g. Sun, bulb, fire).

  • Non-luminous objects: Reflect light from other sources (e.g. chair, moon).

🔹 2. Types of Objects Based on Transparency

Type Description Examples
Opaque Light cannot pass Wood, stone
Transparent Light passes completely Glass, clean water
Translucent Partial light passes Oiled paper, frosted glass

🔹 3. Shadows

A shadow is formed when an opaque object blocks light.

✅ Conditions to form a shadow:

  • A light source

  • An opaque object

  • A screen (wall, ground)

Characteristics:

  • Always dark

  • Always on the opposite side of the light

  • Size and shape depend on distance and angle

🔹 4. Pinhole Camera

A simple camera using a box with a small hole (aperture).

  • It forms a real, inverted image on a screen.

  • Works on rectilinear propagation of light (light travels in straight lines).

Real-life example:

  • Sunlight filtering through leaves creates natural pinhole images.

🔹 5. Reflection and Mirrors

Reflection: Light bounces off a surface (e.g., mirror).

  • Plane Mirror: Forms a virtual, erect, same-size image.

  • Used in: Periscopes, solar cookers, vanity mirrors, etc.

Key Properties of Mirror Image:

  • Laterally inverted (left ↔ right)

  • Same size and distance as object

  • Erect and virtual

🔹 Summary Table

Concept Explanation
Light Needed to see; reflects into eyes
Shadow Formed by opaque objects blocking light
Pinhole Camera Shows light travels in straight line
Reflection Bouncing of light from surface
Transparent Full light passes
Opaque No light passes

🧠 UPSC-Style Quick Questions

  1. Can shadows form with translucent objects? → ❌ No

  2. What is needed to form a shadow? → Light source + opaque object + screen

  3. Why are images in pinhole camera inverted? → Because of straight-line path of light

  4. Does a mirror form a real image? → ❌ No, it's virtual

✅ MCQs to Practice

  1. Which object is not luminous?
    (a) Sun (b) Torch (c) Star (d) Moon

  2. What allows only some light to pass through?
    (a) Transparent (b) Opaque (c) Translucent (d) None

  3. What is required for forming a shadow?
    (a) Light (b) Opaque object (c) Screen (d) All of these

  4. The image formed in a plane mirror is:
    (a) Real (b) Inverted (c) Enlarged (d) Virtual

  5. The property of light travelling in a straight line is called:
    (a) Reflection (b) Rectilinear propagation (c) Refraction (d) Dispersion