Light and Vision Notes

Seeing Objects and Light

  • We see objects through reflected light entering our eyes.
  • Rods and cones in the retina convert light into electrical impulses.
  • The brain interprets these signals as objects.
  • Light consists of energy waves emitted by vibrating electrons, similar to sound waves.
  • Sound waves are mechanical waves.
  • Light is an electromagnetic wave and does not require a medium to travel, it is a bundle of energy.

Reflection of Light

  • Objects are visible because they absorb and re-emit light.
  • Shiny surfaces (e.g., metals) re-emit almost all light, allowing reflections.
  • Mirrors re-emit maximum light, providing clear reflections.
  • Opaque surfaces absorb light and partially re-emit it, resulting in dull surfaces with no clear reflection.
  • Reflection is the bouncing back of light from opaque objects.

Laws of Reflection

  • Incident Ray: Light ray striking a mirror.
  • Reflected Ray: Light ray bouncing off the mirror.
  • Normal: Line perpendicular to the mirror surface at the point of incidence.
  • Angle of Incidence: Angle between the incident ray and the normal.
  • Angle of Reflection: Angle between the reflected ray and the normal.
First Law of Reflection
  • The incident ray, reflected ray, and normal all lie in the same plane.
Second Law of Reflection
  • The angle of incidence equals the angle of reflection.

Experiment to Verify Laws of Reflection

  1. Place white paper on a wooden board and secure with pins.
  2. Draw a line MM' and the normal (90°) to MM'.
  3. Measure 50° from the normal and draw the incident ray.
  4. Place a plane mirror upright along line MM'.
  5. Place two pins (P₁ and P₂) upright along the incident ray, at least 5 cm apart.
  6. Looking into the mirror, place two pins (P₃ and P₄) on the other side of the normal, aligned with the reflections of P₁ and P₂.
  7. Remove the mirror and pins; join the pin marks to the normal to draw the reflected ray.
  8. Measure the angle the reflected ray makes with the normal.
  • Observation: The angle of reflection equals the angle of incidence (e.g., 50°).
  • Conclusion: Verifies both laws of reflection.
  • ∠i=∠r\angle i = \angle r

Regular and Diffused Reflections

Regular Reflection

  • Occurs on smooth, shiny surfaces.
  • Parallel incident rays result in parallel reflected rays.
  • Also known as clear reflection.

Diffused Reflection

  • Occurs on rough surfaces.
  • Parallel incident rays scatter in different directions after reflection.

Note: The laws of reflection hold true for both regular and diffused reflections.

Reflection on a Plane Mirror

  • Rays of light from an object are reflected in different directions.
  • Reflected rays appear to diverge from a point behind the mirror, forming a virtual image.
  • Image size equals object size.
  • Object distance from the mirror equals image distance behind the mirror.

Lateral Inversion

  • Left-right inversion of an object in a plane mirror.
  • Example: Shaking your right hand appears as shaking the left hand in the mirror.
  • Application: The word 'AMBULANCE' is written laterally inverted so it can be read correctly in a vehicle's rear-view mirror.

Multiple Reflection

  • Mirrors placed parallel to each other create infinite multiple images.

Experiment: Number of Images with Two Mirrors

  1. Draw two lines forming an angle (θ) on paper.
  2. Place mirrors upright on the lines.
  3. Place an object between the mirrors.
  4. Count the number of images (N) formed.
  5. Vary the angle (θ) and record the number of images.
  • Conclusion: As the angle θ between the mirrors changes, the number of images N changes.
  • The general equation for the number of images formed by two mirrors is given by:
  • N=360θ−1N = \frac{360}{\theta} - 1

Kaleidoscope

  • Uses multiple reflection to create colorful patterns.
  • Construction: Three mirrors form a triangular prism, with colored beads at one end.
  • Multiple reflections create symmetrical patterns.

Periscope

  • Used to see objects at a higher level.
  • Construction: Two mirrors placed parallel to each other at 45° angles.
  • Applications: Submarines and military spying.

Refraction of Light

  • Refraction is the phenomenon where light deviates from its path when passing from one optical medium to another.

  • Light bends towards the normal when passing from air to water.

  • Light bends away from the normal when passing from glass to air.

  • Light slows down when entering an optically denser medium.

  • Definition: Refraction occurs when a light ray passes obliquely from one optical medium to another, causing it to deviate from its path.

Dispersion of Light

  • Dispersion is the splitting of white light into its constituent colors.
  • Rainbows are formed due to the dispersion of sunlight.
  • Dispersion can be demonstrated using a prism.
  • Sir Isaac Newton demonstrated dispersion in his book Opticks in 1704.

Experiment: Dispersion through a Prism

  • Direct sunlight through a small hole into a dark room.

  • Pass the narrow beam of light through a prism.

  • Observation: White light splits into seven colors.

  • Conclusion: White light can be dispersed into seven colors through a prism.

The Human Eye

  • The human eye is a remarkable optical instrument.
  • It converts light energy into electrical impulses for the brain to interpret.

Parts of the Eye

  • Cornea: Transparent outer cover; protects the eye and focuses light (about 70%).
    • Shaped by the aqueous humour.
  • Pupil: Opening through which light enters; size controlled by the iris.
  • Iris: Controls the size of the pupil and gives the eye its color.
  • Lens: Focuses light onto the retina; reshaped by eye muscles for focusing.
  • Retina: Screen at the back of the eye where the image is formed.
    • Contains rods (detect intensity) and cones (detect color).
    • Some animals only have rods and see in black and white.
  • Optic Nerve: Transmits signals from the retina to the brain.
  • Blind Spot: Point where the optic nerve connects to the retina (no rods or cones).
  • Vitreous Humour: Jelly-like fluid that fills and shapes the eye.

Defects of the Eye

  • Common defects include myopia (short-sightedness) and hypermetropia (long-sightedness).
  • Myopia:
    • Can see nearby objects clearly, but not distant objects.
    • Corrected with concave lenses.
  • Hypermetropia:
    • Can see faraway objects clearly, but not nearby objects.
    • Corrected with convex lenses.
    • Can be due to proper reading habits, watching television up-close or reading books by keeping it close to our eyes.

Eye Care

  • Take proper diet rich in vitamins, especially vitamin A.
  • Avoid reading books or watching television in low light.
  • Expose the eye to intense light source such as lasers, bright halogen lights or direct sunlight.
  • Visit the doctor in case of eye injury or when foreign particles enter the eye.

Corrective Measures

  • Wear spectacles in case of a defect in vision
  • Eating vitamin-rich, nutritious diet.
  • Visiting a doctor or ophthalmologist immediately in case of any eye problem.

Visually Challenged People

  • Visually challenged people have limited or no vision due to various reasons.

  • Eye donation involves transplanting a cornea to restore vision.

Braille System
  • A method for visually challenged people to read and write.
  • Invented by Louie Braille, who was visually challenged.
  • Consists of raised dots representing alphabets and numerals.