Chapter 11:
Light

Class 7 Science | CBSE Curriculum

Learn about reflection, plane mirrors, spherical mirrors (concave and convex), lenses (convex and concave), dispersion of light, rainbow, Newton's disc explained with more than 100 practice questions.

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Time:
05:00

Quick Revision: Light

  • Light travels in straight lines. This property is demonstrated by the bent pipe experiment.
  • Reflection: Bouncing back of light from a shiny surface. Angle of incidence = Angle of reflection.
  • Plane Mirror: Image is virtual, erect, same size, laterally inverted, and at same distance behind mirror as object is in front.
  • Spherical Mirrors: Concave (curved inward) and Convex (curved outward).
  • Concave Mirror: Can form real/virtual images. Used in torches, headlights, dentists' mirrors, shaving mirrors.
  • Convex Mirror: Always forms virtual, erect, diminished image. Provides wider field of view. Used as side mirrors in vehicles.
  • Lenses: Convex (converging, thicker in middle) and Concave (diverging, thinner in middle).
  • Convex Lens: Used as magnifying glass, in cameras, microscopes, telescopes, spectacles for hypermetropia.
  • Concave Lens: Always forms virtual, erect, diminished image. Used in spectacles for myopia.
  • Real Image: Can be obtained on a screen (e.g., image from concave mirror/convex lens).
  • Virtual Image: Cannot be obtained on a screen (e.g., plane mirror, convex mirror, concave lens).
  • Dispersion: Splitting of white light into seven colours (VIBGYOR) using a prism.
  • Rainbow: Natural dispersion of sunlight by water droplets in the atmosphere.
  • Newton's Disc: Rotating disc with seven colours appears white due to persistence of vision.
  • Lateral Inversion: Left appears right and right appears left in a plane mirror. Used in AMBULANCE writing.

Chapter Summary:

Light is a form of energy that travels in straight lines. The chapter begins by demonstrating rectilinear propagation through the bent pipe experiment. Reflection is the bouncing of light from shiny surfaces. A plane mirror forms a virtual, erect, same-sized image that is laterally inverted and appears at the same distance behind the mirror. Lateral inversion explains why 'AMBULANCE' is written in reverse on emergency vehicles.

Spherical mirrors include concave (curved inward) and convex (curved outward) mirrors. Concave mirrors can form both real and virtual images depending on object position; they are used in torches, headlights, and by dentists. Convex mirrors always form virtual, erect, diminished images and provide a wider field of view, making them ideal for vehicle side mirrors.

Lenses are transparent optical devices. Convex (converging) lenses are thicker in the middle and can form real or virtual images; they are used as magnifying glasses. Concave (diverging) lenses are thinner in the middle and always form virtual, erect, diminished images. White light (sunlight) is composed of seven colours (VIBGYOR), demonstrated by dispersion through a prism or naturally in a rainbow. Newton's disc shows that these colours combine to appear white when rotated rapidly. Understanding these optical phenomena helps us appreciate everything from simple mirrors to complex scientific instruments.