THE HUMAN EYE AND THE COLOURFUL WORLD
Class 10 Science – Notes
1. Human Eye
The human eye is a natural optical instrument that enables us to see the objects around us.
2. Important Parts of the Human Eye
Cornea:
It is the transparent front surface of the eye. Most of the refraction of light occurs at the cornea.
Iris:
It controls the size of the pupil.
Pupil:
It is the small opening through which light enters the eye.
Eye Lens:
It is a transparent, flexible lens that focuses light on the retina.
Ciliary Muscles:
They change the shape and focal length of the eye lens.
Retina:
It is a light-sensitive screen on which the image of an object is formed.
Optic Nerve:
It carries signals from the retina to the brain.
3. Power of Accommodation
The ability of the eye lens to change its focal length according to the distance of the object is called power of accommodation.
For a normal eye:
- Near point = 25 cm
- Far point = Infinity (∞)
4. Defects of Vision
(a) Myopia
Myopia is also called short-sightedness.
A person suffering from myopia can see nearby objects clearly but cannot see distant objects clearly.
Cause: The image of a distant object is formed in front of the retina.
Correction: Myopia is corrected using a concave lens.
(b) Hypermetropia
Hypermetropia is also called long-sightedness.
A person suffering from hypermetropia can see distant objects clearly but cannot see nearby objects clearly.
Cause: The image of a nearby object is formed behind the retina.
Correction: Hypermetropia is corrected using a convex lens.
(c) Presbyopia
Presbyopia is an age-related defect of vision.
Cause:
- Weakening of ciliary muscles
- Decrease in flexibility of the eye lens
Correction: It can be corrected using suitable convex lenses or bifocal lenses.
5. Refraction Through a Prism
When light passes through a glass prism, it bends due to refraction.
The angle through which the ray is deviated is called the angle of deviation.
6. Dispersion of White Light
The splitting of white light into its seven constituent colours is called dispersion of light.
The seven colours are:
VIBGYOR
V – Violet
I – Indigo
B – Blue
G – Green
Y – Yellow
O – Orange
R – Red
A rainbow is a natural example of dispersion of sunlight.
7. Atmospheric Refraction
The refraction of light caused by the Earth's atmosphere is called atmospheric refraction.
Examples:
- Twinkling of stars
- Advanced sunrise
- Delayed sunset
8. Twinkling of Stars
Stars appear to twinkle because their light passes through different layers of the atmosphere having different densities.
This causes continuous changes in the apparent position and brightness of the star.
Planets generally do not twinkle because they are much closer and appear as extended sources of light.
9. Advanced Sunrise and Delayed Sunset
Due to atmospheric refraction, the Sun is visible about 2 minutes before actual sunrise and remains visible about 2 minutes after actual sunset.
10. Scattering of Light
The phenomenon in which light is redirected in different directions by small particles is called scattering of light.
Tyndall Effect
The scattering of light by very fine particles suspended in a medium is called the Tyndall effect.
Examples:
- Beam of sunlight visible in a dusty room
- Headlight beam visible in fog
- Sunlight visible through a canopy of dense forest
11. Why is the Sky Blue?
The atmosphere contains very small particles that scatter the shorter wavelengths of light more strongly.
Blue light is scattered more than red light. Therefore, the sky appears blue during the day.
Important Formulas
Lens Formula
1/f = 1/v − 1/u
Magnification
m = h₂/h₁ = v/u
Power of Lens
P = 1/f
Here, f is measured in metres.
Unit of power = Dioptre (D)
Important Exam Questions
- What is accommodation of the eye?
- What is myopia? How is it corrected?
- What is hypermetropia? How is it corrected?
- What is presbyopia?
- What is dispersion of white light?
- Explain the formation of a rainbow.
- Why do stars twinkle?
- Why do planets generally not twinkle?
- Why is the sky blue?
- What is the Tyndall effect?
- Explain atmospheric refraction.
- Why are sunrise and sunset visible earlier and later respectively?
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