NCERT Near Point Science Chapter Class 10
Near Point
The minimum distance which objects can be seen comfortably and distinctly without any strain known as least distance for distinct vision also called at near point.
Far point
The farthest point up to which the eye can see objects clearly is known as far point.
Persistence of vision
The image formed on retina stays on it for 1/16 of a second even after removal of the object. This continuation of sensation of vision is known as persistence of vision.
Defect Of vision
- Myopia
- Hypermetropia
- Astigamation
Myopia
The person suffering from myopia cannot see the distant objects but can see the nearby objects clearly.
Cause:
- Excessive curvature of eye lens.
- Elongation of eye ball
In case of myopia, the image of distant object is formed in formed in front of the retina.
Myopia can be corrected by using concave lens of suitable focal length.
Hypermeteropla
The person suffering from hypermeteropla can see the distant objects clearly but not the nearly objects.
Causes
- Focal length becomes too large
- Shorting of eye ball
In case of hypermeteropia , the image of near by object is formed behind the retina.
Hypermetropia can be corrected by convex lens of suitable power.
Refraction through prism
Prism
Prism is a refractive medium bounded by 2 triangular surface and 3 rectangular surfaces.
Angle of prism
The angle between 3 refracting surface is known as angle of prism.
Angle of deviation
The angle between the incident ray produced backward.
i + e = A+D
Dispersion
The splitting of white light into its constituent colour by a refractive medium.
The basic cause of dispersion is the difference of speed of different colour of light.
Note : Speed of red light is maximum
Note : Speed of violet light is minimum
Rainbow Formation
First the sunlight is dispersed into different colours.
This dispersed light is then internally reflected by water droplet.
The ray of light then comes out refracted.
Atmospheric Refraction
The refraction occur through the layers of atmosphere of earth is known as atmospheric refraction.
Question : The apparent position of star created due to atmospheric refraction of light.The path of the star light fluctuate due to the change in the physical condition of earth’s atmosphere like temperature etc.
The amount of star light reaching our eyes also change. So, same times the star appear brighter or fainter.
Questions: Why planets do not twinkles ?
Answer :Planets are closes to earth and can be considered as extended sources of reflected light.
Planets can also be considered as a large no of point sources of light collected together.
Hence, the overall brightness always remains the same.
Therefore planet do not twinkle.
Questions:Why red light is used as danger light ?
Answer :Red colour light having the largest wavelength is scattered the least. This red colour light is also visible to a larger distance in fog and mist.
Reddish appearance of sun during sun rise and sunset
During sunrise or sunset, the sun is in the horizon. The sunlight has to travel largest distance through layer atmosphere. As red light has longest wavelength it is scattered least and reaches the observer. But the blue light is scattered and cannot reach.
