Class 7 Science Notes Chapter 15 (Light) – Science Book

Okay, let's focus on the key concepts from NCERT Class 7 Science Chapter 15, 'Light', structured for government exam preparation.

NCERT Class 7 Science: Chapter 15 - Light: Detailed Notes

1. Light Travels Along Straight Lines (Rectilinear Propagation of Light)

• Core Concept: Light travels in a straight line path. This property is called rectilinear propagation.
• Evidence:
  • We cannot see around corners.
  • Shadows are formed when an opaque object blocks the path of light.
  • Light from a torch or laser pointer appears to travel straight.
  • Pinhole camera image formation relies on this principle.
• Key Terms:
  • Ray: A very narrow path of light, represented by a straight line with an arrow indicating direction.
  • Beam: A broader collection of light rays.

2. Reflection of Light

• Definition: The bouncing back of light after striking a surface.
• Mechanism: When light falls on a surface, it changes direction and returns to the same medium.
• Types of Surfaces:
  • Smooth/Polished Surfaces (like mirrors): Cause regular reflection, where parallel incident rays remain parallel after reflection. This forms clear images.
  • Rough/Uneven Surfaces: Cause diffuse or irregular reflection, where parallel incident rays scatter in different directions after reflection. No clear image is formed.
• Laws of Reflection (Implicit in Plane Mirror Activity):
  • The angle of incidence is equal to the angle of reflection. (Angle i = Angle r)
  • The incident ray, the reflected ray, and the normal (a line perpendicular to the surface at the point of incidence) all lie in the same plane.

3. Mirrors

• Definition: A polished surface that reflects most of the light falling on it.
• Types:
  • Plane Mirrors
  • Spherical Mirrors (Concave and Convex)

A. Plane Mirrors

• Surface: Flat reflecting surface.
• Image Characteristics:
  • Virtual: The image cannot be formed on a screen. It appears behind the mirror.
  • Erect: The image is upright, the same way up as the object.
  • Same Size: The image size is equal to the object size.
  • Laterally Inverted: The left side of the object appears as the right side of the image, and vice versa. (Example: The word 'AMBULANCE' written laterally inverted on vehicles).
  • Same Distance: The image is formed as far behind the mirror as the object is in front of it.
• Uses: Looking glass, periscopes, kaleidoscopes.

B. Spherical Mirrors

• Definition: Mirrors whose reflecting surfaces are part of a sphere.

• Types:

  • Concave Mirror: Reflecting surface curves inwards (like the inside of a spoon). Also known as a converging mirror.
  • Convex Mirror: Reflecting surface curves outwards (like the back of a spoon). Also known as a diverging mirror.

• Key Terms (Simplified for Class 7):

  • Real Image: An image that can be formed on a screen. It is usually inverted.
  • Virtual Image: An image that cannot be formed on a screen. It is usually erect.

• Image Formation by Concave Mirror:

  • Can form real or virtual images depending on the object's distance.
  • Can form inverted or erect images.
  • Can form magnified (larger), diminished (smaller), or same size images.
  • Example: When the object is very close, it forms a virtual, erect, and magnified image (used as shaving/makeup mirror). When the object is far, it forms a real, inverted, and diminished image.
  • Uses: Shaving/makeup mirrors, dentists' mirrors, reflectors in torches, headlights of vehicles, solar furnaces.

• Image Formation by Convex Mirror:

  • Always forms a virtual, erect, and diminished (smaller than the object) image.
  • Provides a wider field of view compared to a plane mirror of the same size.
  • Uses: Rear-view mirrors (side mirrors) in vehicles (allows drivers to see a wider area behind), security mirrors in shops.

4. Lenses

• Definition: A piece of transparent material (like glass or plastic), usually with at least one curved surface, that refracts (bends) light passing through it.

• Types:

  • Convex Lens: Thicker in the middle and thinner at the edges. It converges parallel rays of light to a point (focus). Also known as a converging lens.
  • Concave Lens: Thinner in the middle and thicker at the edges. It diverges parallel rays of light, making them appear to come from a point (focus). Also known as a diverging lens.

• Image Formation by Convex Lens:

  • Can form real or virtual images depending on the object's distance.
  • Can form inverted or erect images.
  • Can form magnified, diminished, or same size images.
  • Example: Used as a magnifying glass (forms a virtual, erect, magnified image when the object is close). Used in cameras and projectors (forms real, inverted images).
  • Uses: Magnifying glass, spectacles (for farsightedness), cameras, microscopes, telescopes, projectors.

• Image Formation by Concave Lens:

  • Always forms a virtual, erect, and diminished image.
  • Uses: Spectacles (for nearsightedness), peepholes in doors, some telescopes.

5. Sunlight: White or Coloured?

• Composition: Sunlight, which appears white, is actually composed of seven different colours.
• Dispersion: The phenomenon of splitting white light into its constituent colours when it passes through a transparent medium like a glass prism.
• Spectrum: The band of seven colours obtained by dispersion.
• Colours (Sequence - VIBGYOR):
  • Violet
  • Indigo
  • Blue
  • Green
  • Yellow
  • Orange
  • Red
  • (Mnemonic: VIBGYOR) Red light bends the least, Violet light bends the most.
• Rainbow: A natural phenomenon showing the spectrum of sunlight. It is formed when sunlight passes through raindrops in the atmosphere, which act like tiny prisms, causing dispersion and reflection of light. A rainbow is always formed in the direction opposite to the Sun.
• Newton's Disc: A cardboard disc painted with the seven colours of the spectrum in segments. When rotated rapidly, the disc appears whitish or greyish, demonstrating that mixing the seven colours can produce white light.

Key Differences Summary:

Points to Remember for Exams:

• Definitions of key terms (Reflection, Refraction, Dispersion, Real/Virtual Image, Concave/Convex, etc.).
• Properties of images formed by plane mirrors, concave mirrors, convex mirrors, concave lenses, and convex lenses.
• Specific uses of different types of mirrors and lenses.
• The concept of rectilinear propagation.
• The composition of white light and the VIBGYOR sequence.
• How a rainbow is formed (basic principle).
• The difference between real and virtual images.
• Lateral inversion in plane mirrors.

This detailed breakdown covers the essential concepts from Chapter 15 'Light' as per the NCERT Class 7 syllabus, focusing on aspects relevant for competitive government exams. Good luck with your preparation!