The Human Eye and Colourful World

The Human Eye

• Introduction to the Human Eye:
  • The eye uses light to see objects.
  • It contains a lens that helps us focus on objects.
  • Spectacles correct vision defects using lenses.
• Optical Phenomena in Nature:
  • Includes rainbow formation, splitting of white light, and the blue color of the sky.

The Human Eye

• Importance of the Human Eye:
  • A valuable and sensitive sense organ.
  • Enables us to see the world and its colors.
  • Without the eye, we cannot identify colors.
• Structure of the Human Eye:
  • Similar to a camera, with a lens system that forms an image on the retina.
  • Cornea: A thin membrane where light enters, forming a transparent bulge on the eyeball.
  • Eyeball: Spherical with a 2.3 cm diameter.
  • Refraction: Mostly occurs at the cornea’s surface.
  • Crystalline Lens: Adjusts focal length for focusing on objects at different distances.
  • Iris: A dark muscular diaphragm controlling the pupil size.
  • Pupil: Regulates the amount of light entering the eye.
• Functioning of the Eye:
  • The lens forms an inverted real image on the retina.
  • Retina: Delicate membrane with light-sensitive cells.
  • Light-sensitive cells generate electrical signals when illuminated.
  • Signals are sent to the brain via the optic nerves.
  • The brain interprets these signals so we perceive objects as they are.
• Pupil Adjustment:
  • In bright light, the iris contracts the pupil to reduce light entry.
  • In dim light, the iris expands the pupil to allow more light.

Additional Information

• Visual Impairment:
  • Damage or malfunction in any part of the visual system can lead to visual impairment.
  • Includes damage to the cornea, pupil, lens, aqueous humour, vitreous humour, retina, or optic nerve.
• Adaptation to Light Changes:
  • Moving from bright light to dim light makes it hard to see initially.
  • The pupil adjusts size to regulate light entry, allowing better vision over time in different lighting conditions.

Power of Accommodation

• Eye Lens and Curvature:
  • The eye lens is made of fibrous, jelly-like material.
  • Its curvature can change with the help of ciliary muscles.
  • Changing curvature adjusts the focal length of the lens.
• Focal Length Adjustment:
  • Distant Objects:
    • Ciliary muscles relax.
    • Lens becomes thin.
    • Focal length increases.
  • Nearby Objects:
    • Ciliary muscles contract.
    • Lens becomes thick.
    • Focal length decreases.
• Accommodation:
  • The eye lens’s ability to adjust its focal length.
  • Cannot decrease focal length below a certain limit.
• Least Distance of Distinct Vision:
  • Minimum distance for clear vision without strain.
  • About 25 cm for young adults with normal vision.
• Far Point:
  • The farthest distance where objects can be seen clearly.
  • For a normal eye, it is infinity.
  • A normal eye can see clearly between 25 cm and infinity.
• Cataract:
  • Condition where the lens becomes milky and cloudy, often in old age.
  • Causes partial or complete loss of vision.
  • Vision can be restored through cataract surgery.

Do You Know?

• Why Two Eyes?
  • Wider Field of View:
    • One eye: about 150° horizontal view.
    • Two eyes: about 180° horizontal view.
  • Enhanced Detection:
    • Two eyes can detect faint objects better than one.
  • Stereopsis (Depth Perception):
    • Two eyes provide a three-dimensional view.
    • Each eye sees a slightly different image.
    • The brain combines these images to perceive depth.
  • Comparison with Animals:
    • Prey animals have eyes on opposite sides for a wider field of view.
    • Humans have front-facing eyes for better depth perception.

Defects of Vision and Their Correction

Sometimes, the eye loses its power to focus properly, leading to blurred vision. The main defects are myopia, hypermetropia, and presbyopia. These can be corrected with suitable lenses.

(a) Myopia (Near-sightedness)

• What is it?
  • Can see nearby objects clearly.
  • Cannot see distant objects distinctly.
  • Far point is closer than infinity.
• Cause:
  • Excessive curvature of the eye lens.
  • Elongation of the eyeball.
• Correction:
  • Use of a concave lens to bring the image back on the retina.

(b) Hypermetropia (Far-sightedness)

• What is it?
  • Can see distant objects clearly.
  • Cannot see nearby objects distinctly.
  • Near point is farther than normal (beyond 25 cm).
• Cause:
  • Focal length of the eye lens is too long.
  • The eyeball is too small.
• Correction:
  • Use of a convex lens to focus light on the retina.

(c) Presbyopia

• What is it?
  • Difficulty in seeing nearby objects as age increases.
  • Near point recedes away.
• Cause:
  • Weakening of ciliary muscles.
  • Diminishing flexibility of the eye lens.
• Correction:
  • Use of bifocal lenses (concave for distance, convex for near vision).
  • Use of contact lenses or surgical interventions.

Think It Over

Importance of Eye Donation

• Why donate eyes?
  • Eyes can be donated after death to help blind people see.
  • 35 million people in developing countries are blind, and many can be cured.
  • 4.5 million people, including 60% children under 12, can be cured with corneal transplants.

Guidelines for Eye Donation

• Who can donate?
  • Any age or sex.
  • People with spectacles or cataract surgery.
  • Diabetics, hypertensives, asthma patients, and those without communicable diseases.
• Who cannot donate?
  • People who had AIDS, Hepatitis B/C, rabies, acute leukaemia, tetanus, cholera, meningitis, or encephalitis.
• Procedure:
  • Eyes must be removed within 4-6 hours after death.
  • Contact the nearest eye bank immediately.
  • Eye removal is quick (10-15 minutes) and does not cause disfigurement.
  • Eye bank teams can remove eyes at home or in hospitals.

Role of Eye Banks

• Functions:
  • Collect, evaluate, and distribute donated eyes.
  • Evaluate eyes using strict medical standards.
  • Use unsuitable eyes for research and medical education.
  • Keep donor and recipient identities confidential.

Impact

• Benefits:
  • One pair of eyes can give vision to up to four corneal blind people.

Refraction of Light through a Prism

• Light gets refracted when it passes through a prism, similar to how it does through a rectangular glass slab.
• In a glass slab, the emergent ray is parallel to the incident ray, but in a prism, the emergent ray bends at an angle to the incident ray.

Structure of a Prism

• A triangular glass prism has two triangular bases and three rectangular lateral surfaces.
• The angle between two lateral faces is called the angle of the prism.

Understanding Refraction in a Prism

• Incident Ray (PE): The ray of light entering the prism.
• Refracted Ray (EF): The ray inside the prism.
• Emergent Ray (FS): The ray exiting the prism.
• Refraction Details:
  • At the first surface (AB), light bends towards the normal as it goes from air to glass.
  • At the second surface (AC), light bends away from the normal as it goes from glass to air.
• Angle of Deviation (∠D): The angle between the direction of the incident ray and the emergent ray, unique to the prism.

Conclusion

• The shape of the prism causes the light to bend in a distinct way, creating the angle of deviation. This differs from the refraction in a rectangular glass slab.

Dispersion of White Light by a Glass Prism

• Rainbows display beautiful colors, but how does white sunlight create these colors?
• We need to understand the refraction of light through a prism to answer this.

Explanation

• The prism splits white light into its component colors, forming a spectrum.
• Dispersion: The splitting of light into its colors.
• Different colors bend at different angles when passing through the prism:
  • Red bends the least.
  • Violet bends the most.

Isaac Newton’s Experiment

• Newton used a glass prism to create a spectrum from sunlight.
• By using a second inverted prism, he combined the colors back into white light.
• This proved sunlight is made up of seven colors.

Rainbows

• A natural spectrum appearing after rain, caused by the dispersion of sunlight by water droplets.
• Water droplets act like tiny prisms, refracting and dispersing sunlight.
• Internal reflection inside the droplets also plays a role.
• Rainbows form opposite the Sun’s direction.
• You can also see rainbows near waterfalls or fountains on sunny days, with the Sun behind you.

Key Points

• Dispersion: Splitting of white light into colors by a prism.
• Spectrum: Band of colors formed by dispersion.
• VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
• Rainbows: Natural example of dispersion by water droplets.

Atmospheric Refraction

• Atmospheric refraction is the bending of light by the Earth’s atmosphere.
• It causes phenomena like the flickering of objects seen through hot air and the twinkling of stars.

Flickering of Objects

• Hot air above a fire is less dense and has a lower refractive index than cooler air.
• Light passing through this hot air bends irregularly, causing objects to appear to flicker or waver.

Twinkling of Stars

• Stars twinkle because their light continuously refracts as it passes through different layers of the Earth’s atmosphere.
• The atmosphere’s varying refractive index changes the apparent position of stars, making them seem to move slightly.
• Stars appear brighter or dimmer at different moments due to these small fluctuations.
• Planets don’t twinkle because they are closer and appear as extended sources of light. The variations in light from different points on a planet average out, nullifying the twinkling effect.

Advance Sunrise and Delayed Sunset

• The Sun appears about 2 minutes before actual sunrise and remains visible about 2 minutes after actual sunset due to atmospheric refraction.
• The atmosphere bends sunlight, making the Sun appear higher than it actually is.
• This bending also causes the Sun’s disc to appear flattened at sunrise and sunset.

Key Points

• Atmospheric Refraction: Bending of light by Earth’s atmosphere.
• Twinkling of Stars: Caused by continuous refraction of starlight in the atmosphere.
• No Twinkling of Planets: Planets appear as extended sources, so variations in light cancel out.
• Advance Sunrise and Delayed Sunset: Sun appears earlier and stays longer due to the bending of its light by the atmosphere.

Scattering of Light

• Light interacting with objects causes amazing natural phenomena.
• Examples include the blue sky, the color of sea water, and the red sun during sunrise and sunset.
• Scattering of light by particles is key to understanding these effects.

Tyndall Effect

• The atmosphere is a mix of tiny particles: smoke, water droplets, dust, and air molecules.
• When light hits these particles, it scatters, making the light’s path visible.
• This scattering is called the Tyndall effect.
• Example: Sunlight entering a smoke-filled room or passing through a dense forest shows the Tyndall effect.

Why is the Sky Blue?

• Air molecules and fine particles scatter shorter wavelengths (blue light) more than longer wavelengths (red light).
• Blue light is scattered more, making the sky appear blue.
• Without the atmosphere, the sky would look dark, as seen by passengers at high altitudes.
• Interesting Fact: Red danger signals are used because red light scatters the least and can be seen from a distance.

Color of the Sun at Sunrise and Sunset

• The sun and sky appear red during sunrise and sunset.
• This happens because light passes through thicker layers of the atmosphere, scattering blue light away and leaving red light.
• At noon, the sun appears white as it travels a shorter distance through the atmosphere, scattering less blue light.

Chapter Summary:

• The eye can focus on both near and distant objects by adjusting its focal length. This is called accommodation of the eye.
• The nearest distance at which the eye can see clearly without strain is called the near point or least distance of distinct vision. For young adults, it is about 25 cm.
• Common refractive defects of vision include myopia, hypermetropia, and presbyopia.
  • Myopia (short-sightedness) occurs when distant objects are focused before the retina. It is corrected with a concave lens.
  • Hypermetropia (far-sightedness) occurs when nearby objects are focused beyond the retina. It is corrected with a convex lens.
  • The eye’s accommodation ability decreases with age, leading to presbyopia.
• The splitting of white light into its component colors is called dispersion.
• Scattering of light causes the blue color of the sky and the reddening of the Sun at sunrise and sunset.