The Human Eye:
- The human eye is the most valuable and sensitive organ in our human body.
- In human eye the image is formed in the retina. Through cornea light enters the eye which forms the transparent bulge on the front of eyeball. Iris is the structure which is present behind the cornea and it is dark muscular diaphragm which controls the size of the pupil.
- The light entering the eye is controlled and regulated by the pupil.
- Retina consist of delicate membrane which is made form number of light sensitive cells.
- The inverted image will be formed on the retina due to eye lens.
- The cells of retina generate electrical signal and this signals through optic nerve are transmitted to brain where they get interpreted.
- The following figure shows the neat labelled diagram of human eye.
Fig. neat labelled diagram of human eye
Power of accommodation:
- As the curvature of the eye lens changes its focal length also changes.
- When the muscles get relaxed which causes to lens become thin and hence it’s focal length increases.
- When we see nearby objects then ciliary muscle get contracted which causes increase in curvature of the lens and hence lens become thicker and finally it’s focal length decreases.
- Thus, accommodation of eye is nothing but the ability of our eye lens to adjust focal length.
- The minimum distance up to which we can see the objects clearly without any strain in the eye is called as least distance of distinct vision or the near point of the eye.
- The near point of the eye for young adult with normal vision is about 25cm.
- And the maximum distance up to which we can see the objects clearly is called as far point of the eye.
- And for our normal eye the far point is at infinity.
Defects of vision and their correction:
The following are the three common refractive defects of vision.
1.) Myopia or near sightedness:
- The person suffering with myopia can see nearby objects clearly but he can’t see the distant objects clearly.
- Such person has far point near than infinity and hence he may see hoti few meters only.
- In myopic eye the image is formed in front of the retina as shown in the figure.
- The myopia causes due to following reasons:
- Due to excessive curvature of the eye lens
- Due to elongation of the eye balls.
- Myopia can be corrected by using concave lens of suitable power and focal length.
Fig. myopic eye
Fig. correction in myopic eye using concave lens
2.) Hypermetropia or far sightedness:
- The person suffering from hypermetropia can see distant objects clearly but cannot see nearby objects clearly.
- The near point for such person goes beyond the normal near point.
- The hypermetropia causes because of the following reasons:
- Due to increase in focal length of the eye lens
- Due to small size of eyeballs.
- Hypermetropia can be corrected by using a convex lens of proper power and focal length.
Fig. Hypermetropic eye
Fig. correction in hypermetropic eye using convex lens
3.) Presbyopia:
- Due to aging also the power of accommodation of our eye goes on decreasing and hence near point goes farther away. And we can’t see the nearby objects clearly and distinctly without proper glasses. Such person is suffering from the defect called as presbyopia.
- It causes due to weakening of the ciliary muscle and diminishing flexibility of the eye lens.
- The person suffering from both hypermetropia and myopia have to use bifocal lens of proper focal length.
Dispersion of white light by a glass prism:
- When the white light is passed through the prism it splits into band of colours. The sequence of the colours is VIBGYOUR.
- The splitting of white light into its constituent colours when passed through prism is called as dispersion of light.
- Isaac Newton was the first person who used glass prism to obtain spectrum of light.
- He passed the white light through glass prism and obtained spectrum of light containing seven colours.
- Again, he passed this spectrum of light through second identical prism to split these seven colours again, but he was observed that when two identical prisms are placed inverted to each other as shown in fig. above then after emerging light beam from second prism does not contains spectrum of light and it becomes wholly the white light.
- From this experiment he concludes that sunlight is made up from seven colours which are VIBGYOR.
Formation of rainbow:
- After rainfall the sky is fully clear and clouds are in the opposite direction to that of sun, so that sunlight is bright.
- After raining the atmosphere is filled with small water droplets, and this each drop of water acts as a tiny prism.
- When the sunlight is passed through this raindrop at right angle then it splits into spectrum of seven colours which we called as rainbow.
- The rainbow is always formed at opposite side of the sun.
Fig. formation of rainbow
Atmospheric refraction:
- The air near to fire is hotter than the cooler air above it. The refractive index of hotter air is slightly less than the cooler air. And hence the object seen through the hot air seem like to fluctuating. This is due to the atmospheric Refraction.
- The twinkling of stars is also due to the atmospheric refraction.
Twinkling of stars:
- The position of the star as seen by us is its apparent position which changes continuously, due to which we see twinkling of stars.
- This is due to the refraction of starlight through earth’s atmosphere. The light coming from star undergoes numbers of refraction continuously through earth’s atmosphere due the variation of refractive index in layers of atmosphere.
- Due to which the starlight get bends towards normal & the position of star is different from its actual position.
- When viewed from the horizon the position of star appears slightly higher than the actual position.
- This apparent position of the star does not remain constant it changes continuously due to changes in the condition of atmosphere.
- The star is at large distant apart from our eye which acts as a point source of light. The light coming from star reaching to our eyes gets flicker due to which it appears brighter sometime and fainter sometime.
- And this is called as twinkling of stars.
- Also, the sun is visible to us 2 minutes before the actual sunrise and 2 minutes later the sunset only because of the atmospheric Refraction.
Scattering of light:
- Light coming from the sun come across various dust particles, tiny particles, smoke in the air and get scattered from that.
- The blue colour of the sky, colour of water in deep sea, the reddish-brown colour of sun at sunset and sunrise all these phenomena are only due to the scattering of light.
Tyndall effect:
- When the light gets scattered from the colloidal particles then the phenomenon occurred is called as Tyndall effect.
For example:
- When sunlight enters the room through small hole and passes through the smoky fine beam. Due to which the air particles will be visible to us. This happens only due to the Tyndall effect.
Blue colour of the sky:
- The blue colour of the sky is due to scattering of light by molecules of air and dust particles whose size is very small as compared to wavelength of light.
- The wavelength of blue & violet light is small & their scattering is more when the light is passed through them than other colours.
- According to Rayleigh’s scattering,
Scattering of light α 1/ λ4
- Hence, less the wavelength of light more is the scattering.
- The order is, λv<λb….<λrand hence the violet & blue colours are scattered more in atmosphere but violet light is not more sensitive to our eyes and we see blue colour mostly, so the sky appears blue.
Colour of the sun at sunset and sunrise:
- The sun appears reddish at the time of sunset or sunrise while appears white at noon.
- At the time of sunset or sunrise, the sun is very near to horizon. So it needs to travel longer distance through atmosphere to reach the observer on earth.
- During this, the blue and violet light are scattered more and away from the path of light & hence, cannot reach the observer directly. The light reaching to observer is only red light. And hence sun appears reddish at the time of sunset or sunrise.
- At noon the sun is at overhead, so the light coming from the sun travels shorter distance in atmosphere.
- And at that time only a small amount of blue and violet coloured light is scattered, due to which sun appears white at noon.
The diagram below explains the above discussed topic