CBSE Class 10 Physics The Humane Eyes Notes

IMPORTANT CONCEPTS:
Human eye can be treated as an optical instrument. Light rays coming from the object to be seen enter the eye through Cornea and fall on the eye lens through the pupil of the eye.

 Eye lens being double convex lens forms a real inverted and smaller image of the object on the retina.

The retina contains numerous light sensitive cells which are activated by the light falling on the retina and generate electrical signals which are sent to brain via optic nerve and the brain processes this information and we perceive objects as they are.

The human eye has the following parts:-

(a) Cornea:-The transparent spherical membrane covering the front of the eye.

(b) Iris:-The coloured diaphragm between the cornea and lens.

(c) Pupil:-The small hole in the iris.

(d) Eye lens:-Its is a transparent lens made of jelly like material.

(e) Ciliary muscles:-These muscles hold the lens in position.
(f) Retina:-The back surface of the eye.

(g) Blind spot:-The point at which the optic nerve leaves the eye. An image formed at this point is not sent to
the brain.

(h) Aqueous humour:-A clear liquid region between the cornea and the lens.

(i) Vitreous humour:-The space between eye lens and retina is is filled with another liquid called Vitreous
humour.

Persistence of vision: The image of an object seen persists on the retina for 1/16 second even after the removal of the object. This continuance of sensation of eye for some timed is called persistence of vision.

Colour blindness: It is said to occur when a person cannot distinguish between colours though his vision may otherwise be normal.

Accomodation:-The ability of the eye to focus both near and distant objects, by adjusting the focal length, is called the accommodation of the eye or the ability of the ciliary muscles to change the focal length of the eye lens is called accommodation.

Far Point of the Eye: It is the farthest point at which the object can be seen clearly. For a normal eye, the far point lies at infinity.

Near point of the Eye: It is the closest point at which an object can be seen clearly. For normal eye, the near point lies at 25 cm from the eye(least distance of distinct vision)

Defects of vision:

Myopia(or) Short sightedness: Human eye can see clearly the objects lying at short distances from it. but not the far off objects

Causes of Myopia:

1. Increase in the length of the eye ball as if distance of the retina from the eye has increased.
2. Decrease in focal length of eye lens when the eye is fully relaxed.

Remedy: To correct a myopic eye, the person has to wear spectacle with a concave lens of suitable focal length. (i.e.) the focal length of concave lens is equal to the distance of the far point of the myopic eye. 

Hypermetropia (or) Long sightedness: It is that defect of a human eye by virtue of which it can see clearly the objects lying at large distances from it but the nearby objects cannot be seen clearly.

Causes of Hypermetropia: 

(1) Decrease in length of eye ball as if distance of retina from the eye lens has decreased 

(2) Increase in the focal length of the eye lens when the eye is fully relaxed.

Remedy: To correct a hypermetropic eye, the person has to wear spectacle with a convex lens of suitable focal length. 

The focal length of the convex lens is given by f=(x d) / (x-d)

where x is the distance of near point of defective eye

d is the distance of near point of normal eye (25 cm) 

Presbyopia: In this defect old person cannot read and write comfortably.

Remedy: An old person has to use spectacles with a convex lens of suitable focal length

When a person suffers from both myopia and hypermetropia his spectacles have bi-focal lenses (i.e) both concave and convex lenses.


Astigmatism

The defect by which the person is notable to differentiate horizontal and vertical position, is called astigmatism. It can be rectified by using cylindrical lenses.


DISPERSION:

It is the phenomena of splitting of white light into its constituent seven colours on passing through a Glass prism.

Cause of Dispersion: 

Every colour has its own characteristic wave length or frequency. Different colours move with same speed in air/vacuum. But their speeds in refracting media like glass are different. Therefore, refractive index of the medium for different colours is different. As a result, different colours undergo different deviations on passing through the prism. Hence, different colours emerge from the prism along different directions.

Visible spectrum: The band of seven colours obtained due to the dispersion of white light is called a visible spectrum (VIBGYOR).


Application of dispersion:

RAINBOW: It is a concentric coloured circular arc in the sky when the sun rays fall on rain drops during or after a shower. To watch  a rainbow observer must stand with his back towards the sun.

The formation of rainbow is due to the dispersion of white light from the sun and the phenomenon of total internal reflection of light from the water droplets suspended in the air after a shower

Total internal reflection: When light travels from denser to rarer medium incident on the interface separating two media at an angle larger than a particular angle called critical angle (which is a constant for a given pair of media) will be totally reflected back into the denser medium

ATMOSPHERIC REFRACTION:

• Atmosphere layers having different temperature hence optical density of different layers are different. As we go upwards, the atmosphere becomes rarified. So the Refractive Index decreases as we go upwards. So, the light from the sun or any other star undergoes refraction.

Example: Twinkling of stars, early sun rise and delayed sun set.

SCATTERING OF LIGHT:

As the sun light travels through the earth atmosphere, it gets scattered by a large number of molecules.

Scattering of light takes place when the size of the scattering molecule is very very small when compared to the wavelength of light.

Intensity of scattered light (Is) varies inversely to the 4th power of the wave length (λ) of incident light.
Is µ 1/λ4

Blue colour of sky is due to the scattering of sun light. Since the intensity of scattered light varies inversely as a 4th power of wave length, the blue colour (shorter wave length) is scattered much more strongly. Hence the sky appears blue.

White colour of Clouds 

The clouds are at lower height. They are seen due to the scattering of light from the lower part of atmosphere which contains large number of dust particles and water drop lets whose size is very large when compared to wave length of different colours in sun light. So all the wave lengths are scattered equally and hence it appears white.

The sun looks reddish at the time of sun rise and sun set

At the time sun rise and sun set the sun is near the horizon. So the rays from the sun have to travel large part of atmosphere. Since λ blue < λ red and the intensity of scattered light is proportional to 1/λ4 most of the blue light is scattered away. Only red colours which is least scattered enter our eye and hence the sun looks red.

Danger signals are Red

It is because wave length of red colour is large and intensity of scattered light µ 1/λ4 the red light gets least scattered and can be seen from maximum distance.

Sky appears dark to an Astronaut

It is because at such huge heights there is nothing to scatter sun light. Therefore, the sky appears dark.

Useful study link from:  http://jsuniltutorial.weebly.com/the-human-eye-and-the-colourful-world.html

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