THE HUMAN EYE AND THE COLORFUL WORLD
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.
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.
Power of accommodation:
It is the ability of the eye to observe distinctly the object situated at widely different distances from the eye on account of change in focal length of eye.
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: Following are the common defects of vision (or) refractive defects of eye:
1. Myopia(or) Short sightedness:
It is that defect of human eye by virtue of which it can see clearly the objects lying at short distances from it. But the far off objects cannot be seen by the Myopic Eye.
Possible 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.
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.
Question: The far point of a myopic eye is at 50 cm. Calculate the power of the lens to correct the vision.
Ans: The focal length of the concave lens to be used f=-50cm. Therefore P=1/f (metre) = 1/-0.5 = -2D
2. 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.
Possible 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)
Questions 2. The near point of Hypermetropic eye is 75 cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25 cms from the eye?
Ans: Focal length of convex lens to be used is given by f=xd/x-d.
Here x= 75 cm and d=25 cm. Therefore, f=75 x 25 / 75-25 = 37.5 cm.
Hence power p=1/f (meter) = 1/.375 = 2.66D
3. Presbyopia: It is that defect of human eye due to which an old person cannot read and write comfortably.
Remedy: To correct this, an old person has to use spectacles with a convex lens of suitable focal length
o When a person suffers from both myopia and hypermetropia his spectacles have bi-focal lenses (i.e) both concave and convex lenses.
It is the phenomena of splitting of white light into its constituent seven colours on passing through a Glass prism.
Cause of Dispersion:
Different wavelengths of light having different colours moves with different speed in a medium, though they travel with a same velocity in air or vacuum. And hence the different colours deviate through different angles on passing through the glass prism. That is why they split on coming out of 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 shines on rain drops during or after a shower.
Essential conditions for observing a rainbow is that the 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
Different layers of atmosphere are at different temperature. Due to this the optical density of different layers are different. As you 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.
• 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 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.
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