Haryana State Board HBSE 10th Class Science Notes Chapter 11 Human Eye and Colourful World Notes.
Haryana Board 10th Class Science Notes Chapter 11 Human Eye and Colourful World
The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and its colours.
Working of eyes:
- The light rays coming from the object enter the eye through cornea.
- The muscular diaphragm called iris controls the amount of light that enters the eye.
- There is a hole in the iris called the pupil of eye.
- After passing through the pupil, the light rays are incident on the convex eye-lens.
- Ciliary muscles hold the eye lens. They change the thickness of the eye lens while focusing which helps in proper viewing of the objects.
- The screen on which the image is formed in the eye is called retina. Retina consists of a large number of light sensitive cells.
- The retina sends these signals to the brain through optic nerve.
Power of Accommodation
Accommodation power of an eye:
The ability of the eye lens to adjust its focal length as per requirement so that objects can be seen clearly is called accommodation power of an eye.
Near point of an eye:
The minimum distance at which the objects can be seen clearly without contracting the eye lens i.e. without any strain is called the least distance of the distinct vision or near point of an eye. For a young adult having normal vision, the near point of the eye is 25 cm.
Far point of an eye :
The farthest distance up to which the eye can see objects clearly is called far point of an eye. The farthest point of a person with normal vision lies at an infinite distance. Thus, a person with normal vision can see objects clearly from 25 cm to infinite distance.
In old aged people when the eye lens becomes milky and cloudy, the vision becomes hazy or even opaque due to the formation of a membrane over the lens. This condition is called cataracts.
Defects of Vision and Their Correction
Types of defect of vision:
- Near sightedness or Myopia
- Far sightedness or Hypermetropia
Near-sightedness or Myopia:
When the lens is unable to become thin, the light rays converge more than they should. So, the image gets formed before the retina rather than on it. Hence, distant objects cannot be seen clearly. This defect is known as near-sightedness or myopia.
If eye lens does not become thick as per the requirement, then the rays coming from nearby objects gets less converged and hence are focused behind the retina. Hence, nearby objects cannot be seen clearly. This type of defect is known as far-sightedness or hypermetropia.
As a person grows older, the power of accommodation of an eye usually decreases.
→ The near point of aged people recedes and they find it difficult to see nearby objects clearly without spectacles. Such a defect is called presbyopia.
Refraction of Light through a Prism and Dispersion of White Light by a Glass Prism
A prism is a portion of a transparent medium bounded by two plane faces inclined to each other at a certain angle.
Dispersion of light:
Splitting of white light into its seven constituent colours on passing through a transparent medium like a glass prism is called dispersion of light. The band of colours formed after dispersion is known as spectrum.
On the screen, we get a band of seven colours in the following order from bottom to top:
Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR).
A rainbow is a natural spectrum visible in the sky after rain shower. Rainbow is formed when the water droplets present in the atmosphere disperse the sunlight falling on them.
Atmospheric refraction is the deviation of light or other electromagnetic waves from a straight line as it passes through the atmosphere due to the variation in air density. Phenomena such as twinkling of stars, early sunrise and delayed sunset occur due to this effect.
Scattering of light:
The deflection of light by minute particles and molecules in all the directions is known as scattering of light. The colour of scattered light depends upon the size of scattering particles.
The scattering of light in the nature due to small particles present in the atmosphere is called Tyndall effect.