Light

Table of Contents (tap to open/close)

  1. What Makes Things Visible?
    1. Introduction to Seeing
    2. What Makes Things Visible?
  2. Reflection of Light
    1. Laws of Reflection
    2. Activity 13.1: Observing Reflection
    3. More to Know
    4. Activity 13.2
    5. Features of Images by a Plane Mirror
    6. Activity 13.3
      1. Additional Information
  3. Regular and Diffused Reflection
    1. Activity 13.4: Investigating Reflection
      1. Regular vs. Diffused Reflection
      2. More to Know: Reflected Light
      3. Think it Over!
  4. Reflected Light Can be Reflected Again
    1. Key Points
  5. Multiple Images
    1. Using Two Mirrors
    2. Activity 13.5:
      1. Kaleidoscope
    3. Activity 13.6:
      1. Dispersion of Light
    4. Activity 13.7:
  6. Sunlight — White or Colored
    1. Activity 13.7:
    2. Key Points
  7. What is Inside Our Eyes?
    1. Structure of the Eye
    2. Activity 13.8
      1. Lens and Retina
    3. Activity 13.9
      1. Persistence of Vision
    4. Activity 13.10
      1. Movies
      2. Eye Protection and Function
      3. Vision Defects and Corrections
      4. Summary
  8. Care of the Eyes
    1. Tips for Eye Care
    2. Diet and Eye Health
    3. Interesting Facts About Animal Eyes
  9. Visually Impaired Persons Can Read and Write
    1. Non-Optical and Optical Aids
    2. What is the Braille System?
      1. Overview of Braille
      2. Braille Characters
      3. Achievements by Visually Impaired Individuals
  10. Chapter Summary:
  11. KEYWORDS

What Makes Things Visible?

Introduction to Seeing

  • Our sense of sight helps us see various objects like mountains, rivers, trees, and even words on a page.
  • Seeing is possible because of light.

What Makes Things Visible?

  • Objects are seen only when light from them enters our eyes.
  • Light can be emitted by the object or reflected from it.
  • In darkness, we can’t see anything because there is no light.

Reflection of Light

Laws of Reflection

Activity 13.1: Observing Reflection

  1. Setup:
    • Fix a white sheet of paper on a table.
    • Use a comb with only one opening in the middle (cover others with black paper).
    • Hold the comb perpendicular to the paper.
    • Shine a torch through the opening.
  2. Observation:
    • You will see a ray of light on the paper.
    • Place a mirror in the path of the light ray.
  3. Results:
    • The light ray changes direction after hitting the mirror.
    • The incoming ray is called the incident ray.
    • The ray that bounces back is the reflected ray.
  4. Drawing the Experiment:
    • Draw the position of the mirror, incident ray, and reflected ray on the paper.
    • Draw a line at 90º to the mirror where the incident ray strikes (this is the normal).
  5. Angles:
    • The angle between the incident ray and the normal is the angle of incidence (∠i).
    • The angle between the reflected ray and the normal is the angle of reflection (∠r).
    • Measure these angles and record them.
  6. Conclusion:
    • The angle of incidence is always equal to the angle of reflection.
    • This is the law of reflection.

By doing these activities, we learn how light behaves and how we are able to see objects through reflection.

More to Know

  • Ray of Light:
    • A ray of light is an ideal concept.
    • In reality, a narrow beam of light has several rays.
    • We use “ray” for simplicity.

Activity 13.2

  • Objective: Understand the reflection of light.
  • Materials: Stiff paper or chart paper, table, light source.
  • Steps:
    1. Perform Activity 13.1 with the new materials.
    2. Let the paper project beyond the table edge.
    3. Cut the middle of the projecting part.
    4. Observe the reflected ray on the projected portion.
    5. Bend the projected portion and check visibility of the reflected ray.
    6. Bring the paper back and check again.
  • Observation:
    • When the paper is bent, the reflected ray is not visible.
    • When flat, the reflected ray is visible.
    • Inference: Incident ray, normal, and reflected ray lie in the same plane.
    • This is a law of reflection.

Features of Images by a Plane Mirror

  • Questions:
    1. Is the image erect or upside down?
    2. Is it the same size as the object?
    3. Does it appear at the same distance behind the mirror?
    4. Can it be obtained on a screen?

Activity 13.3

  • Objective: Learn about image formation by a plane mirror.
  • Materials: Plane mirror, light source, paper.
  • Steps:
    1. Place a light source (O) in front of the mirror (PQ).
    2. Draw incident rays (OA and OC).
    3. Draw normals at points A and C.
    4. Draw reflected rays (AB and CD).
    5. Extend reflected rays and check if they meet.
    6. Mark the meeting point as I.
    7. For an eye at E, the reflected rays appear to come from point I.
  • Conclusion:
    • Reflected rays do not actually meet at I but appear to.
    • This forms a virtual image at I.
    • Virtual images cannot be captured on a screen.

Additional Information

  • Ray Streak Apparatus: Available in the NCERT kit for performing these activities.
  • Lateral Inversion:
    • The left of the object appears on the right in the mirror image.
    • The right appears on the left.

Regular and Diffused Reflection

Activity 13.4: Investigating Reflection

  • Objective: Understand regular and diffused reflection.
  • Steps:
    1. Imagine parallel rays hitting an irregular surface.
    2. Use the laws of reflection to draw reflected rays at various points.
    3. Observe that reflected rays are not parallel (diffused reflection).
  • Observation:
    • Reflected rays from a rough surface scatter in different directions.
    • This is called diffused or irregular reflection.
    • Laws of reflection still apply; irregularities in the surface cause diffused reflection.
    • Reflection from a smooth surface (like a mirror) is called regular reflection.
    • Images are formed by regular reflection.

Regular vs. Diffused Reflection

  • Regular Reflection:
    • Happens on smooth surfaces like mirrors.
    • Forms clear images.
  • Diffused Reflection:
    • Occurs on rough or irregular surfaces.
    • Doesn’t form clear images.

More to Know: Reflected Light

  • Reflected Light:
    • Most objects are seen due to reflected light (e.g., the Moon reflects sunlight).
    • Illuminated Objects: Objects that reflect light from other sources.
    • Luminous Objects: Objects that emit their own light (e.g., Sun, candles, lamps).

Think it Over!

  • Question: Can reflected rays be reflected again if they hit another mirror?
  • Example: At a hairdresser, you can see the back of your head using two mirrors.

Reflected Light Can be Reflected Again

  • Example:
    • Hairdresser’s Mirror: You see the back of your head using a mirror behind you.
    • Periscope: Uses two plane mirrors to see objects not directly visible.
    • Applications: Used in submarines, tanks, and bunkers.

Key Points

  • Regular Reflection: Occurs on smooth surfaces; forms clear images.
  • Diffused Reflection: Occurs on rough surfaces; scatters light in different directions.
  • Illuminated Objects: Visible due to reflected light.
  • Luminous Objects: Emit their own light.
  • Multiple Reflections: Light can reflect multiple times using mirrors.

Multiple Images

Using Two Mirrors

Activity 13.5:

  • Activity 13.5:
    • Materials: Two plane mirrors.
    • Steps:
      1. Set mirrors at right angles with edges touching.
      2. Place a coin between them and observe the images.
      3. Change angles to 45°, 60°, 120°, 180°, and observe images of an object like a candle.
      4. Set mirrors parallel and observe the number of images.
  • Observation:
    • Multiple images are formed when mirrors are placed at angles.
    • This principle helps you see the back of your head at a hairdresser’s shop.

    Kaleidoscope

    Activity 13.6:

    • Activity 13.6:
      • Materials: Three rectangular mirror strips, circular cardboard tube, cardboard disc with a hole, transparent plastic sheet, circular plane glass plate, ground glass plate, colored glass pieces.
      • Steps:
        1. Join mirrors to form a prism and fix them in the tube.
        2. Close one end with the cardboard disc and plastic sheet.
        3. Fix the glass plate with colored glass pieces at the other end.
        4. Close with ground glass plate.
    • Observation:
      • Peeping through the hole shows beautiful, ever-changing patterns.
      • Designers use kaleidoscopes for pattern ideas.

      Dispersion of Light

      Activity 13.7:

      • Activity 13.7:
        • Materials: Plane mirror, bowl (katori), water, white paper (if needed).
        • Steps:
          1. Place mirror in a bowl filled with water.
          2. Position near a window so sunlight falls on the mirror.
          3. Adjust to reflect light on a wall or white paper.
      • Observation:
        • Reflected light shows many colors due to dispersion.
        • Mirror and water act as a prism splitting light into its colors.
        • Rainbow is an example of natural dispersion.

      Sunlight — White or Colored

      Activity 13.7:

      Activity 13.7:

      • Sunlight is white light.
      • White light consists of seven colors.
      • Dispersion shows that sunlight has multiple colors.

      Key Points

      • Multiple Images: Formed by placing mirrors at angles.
      • Kaleidoscope: Uses mirrors to create colorful patterns.
      • Dispersion of Light: Splitting of white light into its colors using a prism-like setup.

      What is Inside Our Eyes?

      Structure of the Eye

      • Shape: Roughly spherical.
      • Outer Coat: White and tough, protects the eye.
      • Cornea: Transparent front part.
      • Iris: Dark muscular structure behind the cornea; gives eye its color and controls light entering through the pupil.
      • Pupil: Small opening in the iris; size controlled by the iris.

      Activity 13.8

      Observation:

      • Pupil size changes with light intensity.
      • Larger pupil in dim light, smaller in bright light.

      Lens and Retina

      • Lens: Thicker in the center, focuses light on the retina.
      • Retina: Layer at the back of the eye, contains nerve cells (cones and rods).
        • Cones: Sensitive to bright light and color.
        • Rods: Sensitive to dim light.
      • Optic Nerve: Transmits sensations from retina to brain.
      • Blind Spot: No vision at the junction of the optic nerve and retina due to lack of sensory cells.

      Activity 13.9

      Demonstration of Blind Spot:

      • Using a marked sheet of paper, observe disappearance of marks to identify the blind spot.

      Persistence of Vision

      • Image Persistence: Lasts about 1/16th of a second on the retina.
      • Perception of Motion: If images flash faster than 16 per second, they appear as moving.

      Activity 13.10

      Bird in Cage:

      • Using a twirling cardboard with a bird and cage, observe the illusion of the bird in the cage.

      Movies

      • Explanation: Movies are sequences of pictures shown at 24 frames per second, creating the illusion of motion.

      Eye Protection and Function

      • Eyelids: Prevent objects from entering the eye, shut out light when not needed.
      • Normal Vision: Can see both distant and nearby objects clearly.
      • Comfortable Reading Distance: About 25 cm.

      Vision Defects and Corrections

      • Near and Far Vision:
        • Some people see close objects clearly but not distant ones.
        • Others see distant objects clearly but not close ones.
        • Corrective lenses can help.
      • Cataract:
        • Cloudy lens causing foggy vision, common in old age.
        • Treated by removing the opaque lens and inserting an artificial one.

      Summary

      • Key Points:
        • Eye structure includes cornea, iris, pupil, lens, and retina.
        • Pupil size adjusts to light intensity.
        • Retina has cones and rods for light sensitivity.
        • Blind spot demonstrates lack of vision at optic nerve junction.
        • Persistence of vision allows us to perceive motion.
        • Eyelids protect the eye, and vision defects can be corrected with lenses or surgery.

      Care of the Eyes

      Tips for Eye Care

      • Regular Checkups: Visit an eye specialist regularly.
      • Use Spectacles: If advised, wear suitable spectacles.
      • Proper Lighting: Avoid too little or too much light; both can harm your eyes.
      • Avoid Direct Light: Do not look directly at the Sun or powerful lights.
      • No Rubbing: If dust gets in your eyes, wash them with clean water instead of rubbing.
      • Reading Distance: Keep a normal distance when reading; don’t hold books too close or too far.

      Diet and Eye Health

      • Balanced Diet: Essential for eye health.
      • Vitamin A: Important for preventing eye problems like night blindness.
        • Rich Sources: Raw carrots, broccoli, green vegetables (spinach), cod liver oil, eggs, milk, curd, cheese, butter, papaya, and mango.

      Interesting Facts About Animal Eyes

      • Crab: Small eyes, can see all around.
      • Butterflies: Large eyes(compound eyes) with thousands of tiny eyes, see in all directions.
      • Night Birds (Owls): Can see well at night due to large cornea and many rods.
      • Day Birds (Kites, Eagles): See well during the day, have more cones.

      Visually Impaired Persons Can Read and Write

      Non-Optical and Optical Aids

      • Non-Optical Aids:
        • Visual Aids: Magnify words, adjust light intensity.
        • Tactual Aids: Use touch (Braille writer, slate, and stylus).
        • Auditory Aids: Use hearing (cassettes, tape recorders, talking books).
        • Electronic Aids: Talking calculators, computers, CCTV, audio CDs, and voice boxes.
      • Optical Aids:
        • Lenses: Bifocal, contact, tinted lenses.
        • Magnifiers and Telescopic Aids: For better viewing.

      What is the Braille System?

      Overview of Braille

      • Braille System: Developed by Louis Braille in 1821.
      • Adoption: Current system adopted in 1932.
      • Languages and Notation: Braille codes for languages, mathematics, and scientific notation.
      • Learning Braille: Starts with letters, then special characters and combinations; relies on touch.

      Braille Characters

      • Dot Patterns: 63 characters using dots in cells of two vertical rows of three dots each.
      • Embossed Dots: Raised slightly for easy recognition by touch.

      Achievements by Visually Impaired Individuals

      • Diwakar: Child prodigy singer.
      • Ravindra Jain: Lyricist, singer, and composer.
      • Lal Advani: Established an association for the rehabilitation of the disabled.
      • Helen A. Keller: American author and lecturer, known for her inspiring life story.

      Chapter Summary:

      • Light is reflected from all surfaces.
      • Regular reflection happens on smooth, polished, and regular surfaces.
      • Diffused or irregular reflection occurs on rough surfaces.
      • Two laws of reflection are:
        1. The angle of incidence is equal to the angle of reflection.
        2. Incident ray, reflected ray, and the normal at the point of incidence lie in the same plane.
      • Images in a plane mirror undergo lateral inversion.
      • Two mirrors inclined to each other create multiple images.
      • Beautiful patterns in a kaleidoscope are due to multiple reflections.
      • Sunlight, called white light, consists of seven colors.
      • Splitting of light into its constituent colors is known as dispersion.
      • Parts of the eye are the cornea, iris, pupil, lens, retina, and optic nerve.
      • A normal eye can see nearby and distant objects clearly.
      • Visually impaired persons can read and write using the Braille system.
      • Visually impaired persons develop their other senses more sharply to interact with their environment.

      KEYWORDS

      1. ANGLE OF INCIDENCE
      2. ANGLE OF REFLECTION
      3. BLIND SPOT
      4. BRAILLE
      5. CONES
      6. CORNEA
      7. DIFFUSED OR IRREGULAR REFLECTION
      8. INCIDENT RAYS
      9. IRIS
      10. KALEIDOSCOPE
      11. LATERAL INVERSION
      12. LAWS OF REFLECTION
      13. PUPIL
      14. REFLECTED RAYS
      15. REFLECTION
      16. REGULAR REFLECTION
      17. RETINA
      18. RODS
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