Class 7 Notes

Class 7 Science Notes Chapter 15 (Chapter 15) – Examplar Problems Book

Philoid

Philoid

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Examplar Problems
Alright class, let's delve into Chapter 15, 'Light', from your Science Exemplar book. This is a crucial chapter, and understanding these concepts well will be very helpful for various competitive examinations. Pay close attention to the properties and behaviours of light, mirrors, and lenses.

Chapter 15: Light - Detailed Notes for Exam Preparation

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

  • Concept: Light travels in straight lines. This property is called rectilinear propagation.
  • Evidence:
    • We cannot see around corners.
    • Formation of sharp shadows.
    • Working principle of a Pinhole Camera.
  • Pinhole Camera:
    • A simple device demonstrating rectilinear propagation and image formation.
    • Image formed is inverted (upside down) and usually smaller than the object.
    • The image is real as it can be formed on a screen (the tracing paper/ground glass screen of the camera).

2. Reflection of Light

  • Concept: The bouncing back of light after striking a surface is called reflection.
  • Polished Surfaces: Smooth and shiny surfaces (like mirrors) reflect light well (regular reflection). Rough surfaces reflect light diffusely (irregular reflection).
  • Plane Mirror:
    • A flat, smooth reflecting surface.
    • Image Characteristics:
      • Virtual: Cannot be obtained on a screen; appears to be behind the mirror.
      • Erect: Upright, same orientation as the object.
      • Same Size: Image size is equal to the object size.
      • Laterally Inverted: Left appears right, and right appears left (e.g., the word AMBULANCE written in reverse on vehicles).
      • Same Distance: The image is formed as far behind the mirror as the object is in front of it.

3. Spherical Mirrors

  • Mirrors whose reflecting surfaces are curved (part of a sphere).
  • Types:
    • Concave Mirror: Reflecting surface curves inwards (like the inside of a spoon). It is a converging mirror.
    • Convex Mirror: Reflecting surface curves outwards (like the back of a spoon). It is a diverging mirror.
  • Key Terms (Conceptual Understanding):
    • Pole (P): Centre of the reflecting surface.
    • Centre of Curvature (C): Centre of the sphere of which the mirror is a part.
    • Principal Axis: Straight line passing through the Pole and Centre of Curvature.
    • Focus (F): Point on the principal axis where parallel rays converge (concave) or appear to diverge from (convex) after reflection.
  • Image Formation & Uses:
    • Concave Mirror:
      • Can form real (can be obtained on screen) and inverted images.
      • Can form virtual (cannot be obtained on screen) and erect images (when the object is very close to the mirror).
      • Image can be magnified, diminished, or the same size depending on the object's position.
      • Uses: Shaving mirrors, dentists' mirrors (magnified virtual image), reflectors in torches/headlights (to produce a parallel beam), solar furnaces (to concentrate sunlight).
    • Convex Mirror:
      • Always forms virtual, erect, and diminished (smaller than the object) images, irrespective of the object's position.
      • Provides a wider field of view.
      • Uses: Rear-view mirrors/Side-view mirrors in vehicles, security mirrors in shops.
  • Real vs. Virtual Image:
    • Real Image: Formed by the actual intersection of reflected/refracted rays. Can be projected onto a screen. Usually inverted (with respect to the object).
    • Virtual Image: Formed when reflected/refracted rays appear to meet but do not actually intersect. Cannot be projected onto a screen. Usually erect.

4. Lenses

  • A transparent medium bounded by one or two spherical surfaces. Light passes through lenses (refraction).
  • Types:
    • Convex Lens: Thicker at the centre, thinner at the edges. It is a converging lens.
    • Concave Lens: Thinner at the centre, thicker at the edges. It is a diverging lens.
  • Image Formation & Uses:
    • Convex Lens:
      • Can form real and inverted images (when the object is placed beyond the focus).
      • Can form a virtual, erect, and magnified image (when the object is placed very close, between the lens and its focus - this is how it works as a magnifying glass).
      • Image can be magnified, diminished, or the same size.
      • Uses: Magnifying glass, cameras, telescopes, microscopes, spectacles (to correct hypermetropia/far-sightedness).
    • Concave Lens:
      • Always forms virtual, erect, and diminished images.
      • Uses: Spectacles (to correct myopia/near-sightedness), some telescopes, peepholes in doors.

5. Sunlight: White or Coloured? (Dispersion of Light)

  • Concept: Sunlight, which appears white, is actually composed of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (acronym: VIBGYOR).
  • Dispersion: The splitting of white light into its constituent colours when it passes through a transparent medium like a prism.
  • Rainbow: A natural phenomenon showing dispersion. It is formed when sunlight passes through raindrops, which act like tiny prisms. A rainbow is always formed in the direction opposite to the Sun.
  • Newton's Disc:
    • A cardboard disc painted with the seven colours of VIBGYOR in segments.
    • When rotated rapidly, the disc appears whitish/grey.
    • Demonstrates that the seven colours can recombine to form (nearly) white light.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 15 for your practice:

  1. Which of the following statements is true about the image formed by a plane mirror?
    (a) It is real and erect.
    (b) It is virtual and inverted.
    (c) It is virtual, erect, and laterally inverted.
    (d) It is real, inverted, and same size.

  2. A convex lens is used as:
    (a) A rear-view mirror in cars.
    (b) A magnifying glass.
    (c) A reflector in torches.
    (d) A shaving mirror.

  3. The phenomenon responsible for the formation of a rainbow is:
    (a) Reflection
    (b) Rectilinear propagation
    (c) Dispersion
    (d) Lateral inversion

  4. Which type of mirror can form a real image?
    (a) Plane mirror
    (b) Convex mirror
    (c) Concave mirror
    (d) Both convex and concave mirrors

  5. An image that cannot be obtained on a screen is called:
    (a) Real image
    (b) Virtual image
    (c) Inverted image
    (d) Diminished image

  6. A concave lens always forms an image which is:
    (a) Virtual, erect, and magnified
    (b) Real, inverted, and diminished
    (c) Virtual, erect, and diminished
    (d) Real, erect, and same size

  7. The property of light travelling in straight lines is demonstrated by:
    (a) A rainbow
    (b) A Newton's disc
    (c) A pinhole camera
    (d) A magnifying glass

  8. For which purpose are convex mirrors commonly used?
    (a) As dentists' mirrors
    (b) In solar furnaces
    (c) As side-view mirrors in vehicles
    (d) In projectors

  9. When a Newton's disc is rotated fast, the different colours:
    (a) Separate out clearly
    (b) Blend together to appear whitish
    (c) Form a rainbow pattern
    (d) Disappear completely

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

Answer Key for MCQs:

  1. (c)
  2. (b)
  3. (c)
  4. (c)
  5. (b)
  6. (c)
  7. (c)
  8. (c)
  9. (b)
  10. (b)

Revise these notes thoroughly. Focus on the characteristics of images formed by different types of mirrors and lenses, and their practical applications. Good luck with your preparation!

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