Class 8 Science Notes Chapter 16 (Light) – Science Book

Dear students, let's make notes for your quick revision of Chapter 16, 'Light'. This is a crucial topic, not just for your class but also forms the basis for many concepts tested in government exams. Revise it multiple times.

Chapter 16: Light - Detailed Notes for Exam Preparation

1. What Makes Things Visible?

• We see objects because light from an object enters our eyes.
• This light may be emitted by the object itself (luminous objects like the Sun, a bulb, fire) or may have been reflected by the object (non-luminous objects like the moon, chairs, books).
• Key Concept: Vision requires light traveling from the object to the eye.

2. Laws of Reflection

• Reflection: The bouncing back of light after striking a surface.

• Incident Ray: The ray of light that strikes the reflecting surface.

• Reflected Ray: The ray of light that bounces back from the surface after reflection.

• Normal: An imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point where the incident ray strikes (point of incidence).

• Angle of Incidence (∠i): The angle between the incident ray and the normal.

• Angle of Reflection (∠r): The angle between the reflected ray and the normal.

• Laws of Reflection:

  • First Law: The angle of incidence is always equal to the angle of reflection (∠i = ∠r).
  • Second Law: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
  • Important: These laws apply to all types of reflecting surfaces, whether smooth or irregular.

3. Regular vs. Diffused Reflection

• Regular Reflection: Occurs when parallel incident rays strike a smooth, polished surface (like a plane mirror, still water). The reflected rays are also parallel to each other. This type of reflection forms clear images.
• Diffused (or Irregular) Reflection: Occurs when parallel incident rays strike a rough or uneven surface (like cardboard, wall, wood). The reflected rays scatter in different directions because the normal at each point on the rough surface points in a different direction. While the laws of reflection are still obeyed at each point, the overall reflected rays are not parallel. This allows us to see non-luminous objects from various angles, but it doesn't form a clear image.
  • Exam Relevance: Most objects around us are visible due to diffused reflection.

4. Image Formation by a Plane Mirror

• Characteristics of the image formed by a plane mirror:
  • Virtual (cannot be obtained on a screen)
  • Erect (upright)
  • Same size as the object
  • Located as far behind the mirror as the object is in front of it.
  • Laterally Inverted: The right side of the object appears as the left side of the image, and vice versa. (This is why 'AMBULANCE' is written in reverse on vehicles).

5. Multiple Images

• When two plane mirrors are placed at an angle to each other, multiple images of an object placed between them are formed due to repeated reflections.
• The number of images depends on the angle (θ) between the mirrors: Number of images = (360°/θ) - 1 (This formula is useful but might be slightly advanced for strict Class 8; the concept is important).
• Kaleidoscope: Works on the principle of multiple reflections by mirrors inclined to each other, creating beautiful patterns.

6. Sunlight: White or Coloured?

• Sunlight, which appears white, is actually composed of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (Remembered by the acronym VIBGYOR).
• Dispersion: The phenomenon of splitting white light into its constituent colours upon passing through a transparent medium like a prism.
• A prism is typically a triangular glass block that causes dispersion.
• Rainbow: A natural phenomenon showing dispersion of sunlight by tiny water droplets suspended in the atmosphere after rain. The droplets act like small prisms.

7. Structure and Function of the Human Eye

• The eye is our primary sense organ for vision.
• Cornea: Transparent outer layer at the front of the eye. Light enters the eye through the cornea. It refracts (bends) light.
• Iris: A coloured, muscular diaphragm behind the cornea. It controls the amount of light entering the eye by adjusting the size of the pupil. (Gives the eye its colour).
• Pupil: The small opening in the centre of the iris. It appears black because light entering it is absorbed inside. Its size changes based on light intensity (smaller in bright light, larger in dim light).
• Lens: A convex lens located behind the pupil. It focuses light onto the retina. It can change its shape (become thicker or thinner) to focus on objects at different distances (accommodation). Made of living tissue.
• Retina: The light-sensitive screen at the back of the eye. It contains specialized nerve cells:
  • Rods: Sensitive to dim light.
  • Cones: Sensitive to bright light and colours.
• Optic Nerve: Transmits visual information (electrical signals generated by rods and cones) from the retina to the brain.
• Blind Spot: The junction of the optic nerve and the retina. There are no sensory cells (rods or cones) here, so no vision is possible at this spot.
• Persistence of Vision: The impression of an image stays on the retina for about 1/16th of a second even after the object is removed. This phenomenon is used in motion pictures (movies), where still images are projected at a rate faster than 16 frames per second, creating the illusion of movement.

8. Care of the Eyes

• Read in proper light (not too dim, not too bright).
• Avoid reading while lying down or in a moving vehicle.
• Maintain a proper distance (at least 25 cm) while reading.
• Wash eyes frequently with clean water.
• Do not look directly at the sun or other powerful light sources.
• Include Vitamin A-rich foods (carrots, green leafy vegetables, eggs, milk) in your diet. Deficiency causes night blindness.
• Get regular eye check-ups.

9. Visually Impaired Persons and Braille System

• Some people have limited vision or are completely unable to see due to illness, injury, or congenital conditions.
• Visually impaired individuals often develop their other senses (touch, hearing) more sharply.
• Braille System: A tactile writing system used by people who are visually impaired.
  • Developed by Louis Braille (who was himself visually impaired).
  • Uses patterns of raised dots arranged in cells of six positions (2 columns of 3 dots).
  • Each pattern represents a letter, a combination of letters, a punctuation mark, or a word.
  • Can be produced by hand using a stylus and slate or by Braille typewriters/printers.

Multiple Choice Questions (MCQs)

1. According to the laws of reflection, which statement is always true?
   (a) Angle of incidence > Angle of reflection
   (b) Angle of incidence < Angle of reflection
   (c) Angle of incidence = Angle of reflection
   (d) Angle of incidence can be unrelated to the angle of reflection

2. Image formed by a plane mirror is:
   (a) Real and Erect
   (b) Virtual and Inverted
   (c) Real and Laterally Inverted
   (d) Virtual and Laterally Inverted

3. Which part of the human eye controls the size of the pupil?
   (a) Cornea
   (b) Retina
   (c) Iris
   (d) Lens

4. The phenomenon responsible for the working of a kaleidoscope is:
   (a) Dispersion
   (b) Refraction
   (c) Multiple Reflections
   (d) Diffused Reflection

5. Sunlight is composed of:
   (a) One colour (white)
   (b) Three primary colours
   (c) Seven colours
   (d) Five colours

6. The splitting of white light into its constituent colours is called:
   (a) Reflection
   (b) Refraction
   (c) Dispersion
   (d) Scattering

7. The part of the eye where the image is formed is the:
   (a) Pupil
   (b) Lens
   (c) Cornea
   (d) Retina

8. The Braille system, used by visually impaired persons, was developed by:
   (a) Isaac Newton
   (b) Louis Braille
   (c) Helen Keller
   (d) Thomas Edison

9. When parallel rays of light strike a rough surface, the reflection is:
   (a) Regular reflection
   (b) Diffused reflection
   (c) No reflection
   (d) Multiple reflection

10. The sensation of an image persists on the retina for about:
    (a) 1/10th of a second
    (b) 1/16th of a second
    (c) 1 second
    (d) 1/25th of a second

Answer Key for MCQs:

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

Remember to revise these notes thoroughly. Understanding the basic principles of light is essential. Good luck with your preparation!