The Human Eye and the Colourful World
1. The Human Eye
The human eye is a natural optical instrument that helps us see nearby and distant objects. It works like a camera and forms a real and inverted image on the retina.
Structure of the Human Eye
- Cornea: Transparent front part; most refraction occurs here
- Iris: Controls the amount of light entering the eye
- Pupil: Opening through which light enters
- Eye Lens: Convex lens that focuses light on the retina
- Retina: Light-sensitive screen where image is formed
- Optic Nerve: Sends visual signals to the brain
2. Power of Accommodation
The ability of the eye lens to change its focal length to see objects clearly at different distances is called power of accommodation.
- For near objects, the lens becomes thicker
- For distant objects, the lens becomes thinner
Near Point: 25 cm
Far Point: Infinity
3. Defects of Vision and Their Correction
(a) Myopia (Near-sightedness)
- Distant objects appear blurred
- Image forms in front of retina
- Corrected using a concave lens
(b) Hypermetropia (Far-sightedness)
- Near objects appear blurred
- Image forms behind retina
- Corrected using a convex lens
(c) Presbyopia
- Occurs due to aging
- Reduced power of accommodation
- Corrected using bifocal lenses
4. Refraction of Light Through a Prism
When white light passes through a prism, it splits into seven colours. This phenomenon is called dispersion of light.
VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red
5. Scattering of Light
Why is the sky blue?
Blue light has shorter wavelength and scatters more than red light, making the sky appear blue.
Why is the sun red at sunrise and sunset?
During sunrise and sunset, sunlight travels a longer distance and blue light scatters away, leaving red light.
6. Tyndall Effect
The scattering of light by colloidal particles is called the Tyndall Effect.
- Visible light beam in fog or smoke
- Blue colour of the sky
7. Atmospheric Refraction
(a) Twinkling of Stars
Stars appear to twinkle due to continuous refraction of light in the atmosphere.
(b) Advance Sunrise and Delayed Sunset
The sun is visible about 2 minutes earlier at sunrise and 2 minutes later at sunset due to atmospheric refraction.
8. Rainbow Formation
A rainbow is formed due to:
- Refraction of sunlight
- Dispersion of light
- Total internal reflection
Water droplets act like tiny prisms.
MCQs: The Human Eye and the Colourful World
1. Which part of the eye controls the amount of light entering the eye?
a) Retina
b) Iris
c) Cornea
d) Pupil
Answer: b) Iris
2. The image formed on the retina is:
a) Virtual and erect
b) Real and inverted
c) Virtual and inverted
d) Real and erect
Answer: b) Real and inverted
3. The minimum distance of distinct vision for a normal human eye is:
a) 50 cm
b) 100 cm
c) 25 cm
d) Infinity
Answer: c) 25 cm
4. The far point of a normal human eye is:
a) 25 cm
b) 50 cm
c) Infinity
d) Zero
Answer: c) Infinity
5. Which defect of vision can be corrected using a concave lens?
a) Hypermetropia
b) Presbyopia
c) Myopia
d) Cataract
Answer: c) Myopia
6. In myopia, the image is formed:
a) Behind the retina
b) On the retina
c) In front of the retina
d) At infinity
Answer: c) In front of the retina
7. Which lens is used to correct hypermetropia?
a) Concave lens
b) Cylindrical lens
c) Convex lens
d) Bifocal lens
Answer: c) Convex lens
8. Presbyopia occurs due to:
a) Injury to eye
b) Aging
c) Infection
d) Vitamin deficiency
Answer: b) Aging
9. The ability of the eye to focus on near and distant objects is called:
a) Persistence of vision
b) Power of accommodation
c) Dispersion
d) Refraction
Answer: b) Power of accommodation
10. Splitting of white light into seven colours is called:
a) Scattering
b) Reflection
c) Refraction
d) Dispersion
Answer: d) Dispersion
11. Which colour has the shortest wavelength?
a) Red
b) Yellow
c) Blue
d) Violet
Answer: d) Violet
12. Why does the sky appear blue?
a) Due to reflection
b) Due to refraction
c) Due to scattering of blue light
d) Due to dispersion
Answer: c) Due to scattering of blue light
13. The sun appears red at sunrise and sunset because:
a) Blue light scatters away
b) Red light scatters more
c) Atmosphere absorbs blue light
d) Sun emits red light
Answer: a) Blue light scatters away
14. Scattering of light by colloidal particles is called:
a) Dispersion
b) Reflection
c) Tyndall Effect
d) Refraction
Answer: c) Tyndall Effect
15. Which phenomenon is responsible for the twinkling of stars?
a) Reflection
b) Scattering
c) Atmospheric refraction
d) Dispersion
Answer: c) Atmospheric refraction
16. Advance sunrise and delayed sunset occur due to:
a) Reflection of light
b) Atmospheric refraction
c) Scattering
d) Dispersion
Answer: b) Atmospheric refraction
17. Which process is NOT involved in rainbow formation?
a) Refraction
b) Dispersion
c) Total internal reflection
d) Diffraction
Answer: d) Diffraction
18. Water droplets in the atmosphere act like:
a) Mirrors
b) Lenses
c) Prisms
d) Screens
Answer: c) Prisms
19. Which colour scatters the most in the atmosphere?
a) Red
b) Yellow
c) Green
d) Blue
Answer: d) Blue
20. The band of colours obtained from white light is called:
a) Image
b) Spectrum
c) Reflection
d) Shadow
Answer: b) Spectrum
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