What Makes Things Visible?
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We see objects when light from them enters our eyes.
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Two ways light reaches our eyes:
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Luminous objects → Emit their own light (Sun, candle, lamp).
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Illuminated objects → Reflect light from another source (Moon, book, table).
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In darkness → no light, so objects are not visible.
Reflection of Light
Definition: The bouncing back of light when it strikes a smooth surface (like a mirror).
Important Terms
| Term | Meaning |
|---|---|
| Incident Ray | The incoming ray of light striking the surface |
| Reflected Ray | The ray bouncing back from the surface |
| Normal | A line drawn at 90° to the surface at the point of incidence |
| Angle of Incidence (∠i) | Angle between incident ray and normal |
| Angle of Reflection (∠r) | Angle between reflected ray and normal |
Laws of Reflection
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∠i = ∠r (Angle of incidence = Angle of reflection).
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Incident ray, reflected ray, and normal lie in the same plane.
Types of Reflection
| Type | Explanation | Example |
|---|---|---|
| Regular Reflection | Light reflected in one direction from a smooth surface → clear image forms | Plane mirror, calm water |
| Diffused Reflection | Light reflected in different directions from a rough surface → no clear image forms | Wall, cardboard |
🔑 Note: In both cases, laws of reflection are obeyed.
Image in Plane Mirror
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Image is erect (upright).
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Image is of same size as object.
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Image appears at same distance behind the mirror as object is in front.
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Laterally inverted (left and right interchanged).
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Image is virtual (cannot be obtained on a screen).
Multiple Images
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Using two plane mirrors at different angles → multiple images are formed.
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Formula: Number of images = (360° ÷ θ) – 1 (where θ = angle between mirrors).
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Special cases:
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At 90° → 3 images.
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At 60° → 5 images.
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At 0° (parallel mirrors) → infinite images.
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📍 Applications:
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Periscope – uses 2 mirrors (for submarines, bunkers).
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Kaleidoscope – uses 3 mirrors at angles → beautiful patterns (used by designers, artists).
Sunlight – White or Coloured?
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Sunlight appears white but is made of 7 colours (VIBGYOR).
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Splitting of light into 7 colours = Dispersion.
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Rainbow is a natural example of dispersion.
Human Eye
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Eye = spherical organ, helps us see.
Structure & Functions
| Part | Function |
|---|---|
| Cornea | Transparent front part, bends light into the eye |
| Iris | Coloured part of eye, controls pupil size |
| Pupil | Opening that allows light into eye |
| Lens | Convex lens, focuses light on retina |
| Retina | Screen of the eye; contains rods (dim light) & cones (bright light, colour) |
| Optic Nerve | Carries signals from retina to brain |
| Blind Spot | No sensory cells → no vision here |
Defects of Vision
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Myopia (near-sightedness) – cannot see distant objects; corrected by concave lens.
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Hypermetropia (far-sightedness) – cannot see nearby objects; corrected by convex lens.
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Cataract – lens becomes cloudy, treated by surgery (artificial lens).
Care of Eyes
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Regular check-up; use spectacles if advised.
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Avoid reading in too dim/too bright light.
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Do not rub eyes; wash with clean water if dust enters.
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Maintain normal reading distance (~25 cm).
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Vitamin A is essential → deficiency causes night blindness.
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Sources: Carrot, spinach, papaya, mango, milk, butter, cod liver oil.
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Visually Impaired & Braille System
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Some people are partially or completely blind. They develop touch & hearing senses more.
Braille System
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Invented by Louis Braille (1821).
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Uses 63 dot patterns (arranged in 2 columns × 3 rows).
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Each pattern = letter, word, or sign.
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Dots are raised → can be felt by touch.
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Used for books, notes, and learning in many languages including Indian languages.
Aids for Visually Impaired
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Non-optical aids: Braille slate, stylus, talking books, audio devices.
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Optical aids: Magnifiers, spectacles, telescopic lenses.
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Electronic aids: Talking calculators, screen readers, computers.
📑 Key Points to Remember
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Laws of reflection are always valid.
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Difference between regular & diffused reflection.
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Properties of image in plane mirror.
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Formula for number of images with mirrors.
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Sunlight → dispersion into VIBGYOR.
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Human eye parts + functions + defects.
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Vitamin A importance & deficiency disease.
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Braille system for visually impaired.
