the human eye and the colourful world

main components of the eye

main components of the eye

cornea

iris

pupil

ciliary muscles

eye lens

retina

optic nerve

construction of the eye

the front part of the eye is called cornea. it is transparent and bulging outwards (convex). the light enters the eye through cornea. the cornea converges most of the light rays

behind the cornea is a flat, coloured ring shaped membrane called pupil

in the middle of the pupil is a hole called iris, which is where light enters from. it is black as no light is reflected from it

behind the pupil is the eye lens, which is a convex lens made of transparent jelly material. it is flexible and can change its shape (and converging power) with the help of the attached ciliary muscles on each end. it converges the rest of the light rays onto the retina

the back part of the eye has the screen on which the image is formed, retina

blind spot

it is a small area of the retina insensitive to light where the optic nerve leaves the eye

function of the pupil

the iris controls the amount of light entering the eyes but adjusting its size

during the day/ in bright areas, the iris contracts the pupil (makes it small) to reduce the amount of light entering

during the night/ in dark areas, the iris relaxes the pupil (makes it big) to increase the amount of light entering

rods and cones

the retina consists of a large number of light sensitive cells. they are of two types;

rods: they are rod-shaped cells which are sensitive to dim light. they are not color sensitive. night animals like owls have more rods to help them see

cones: they are cone-shaped cells which are sensitive to bright light. they are color sensitive

power of accommodation

the ability of an eye to focus on distant as well as nearby objects on the retina by changing the focal length of the lens is called accommodation

in a normal eye, the far point is infinity and close point is 25 cm

nature of light coming from objects before entering the eye

the light rays coming from an object are initially diverging and then slowly become parallel

the rays of light coming from infinity are parallel to one another when they reach the eye and hence need a large focal length/ thin lens

the rays of light coming from a nearby object are diverging and hence need a short focal length/ thick lens

range of vision

the farthest point from the eye at which an object can be seen clearly is called the far point

the nearest point up to which the eye can clearly see an object without any strain is called the near point or the least distance of distinct vision

myopia

myopia is the defect of vision due to which a person cannot see distant objects clearly. the far point of an eye suffering from myopia is less than infinity. it may be due to the following;

due to high converging power of lens (short focal length)

due to the eye-ball being too long

it is corrected by using spectacles containing concave lenses which slightly diverge the parallel beams of light, which appear to be coming from the eye’s actual far point

hypermetropia

hypermetropia is the defect of vision due to which a person cannot see nearby objects clearly. the near point of a hypermetropic eye is more than 25 cm away. it may be due to the following reasons;

due to low converging power of the eye lens (large focal length)

due to eye ball being too short

is it corrected by using spectacles containing convex lenses. it slightly converges the parallel beams of light and makes it appear to be coming from the actual near point of the eye

presbyopia

presbyopia is the defect due to which old people cannot see nearby objects clearly. it is due to the loss of power of accommodation which comes with age

it is corrected by using convex lenses

cataract

the medical condition in which the lens if the eye of a person becomes gradually cloudy resulting in blurred vision is called cateract

glass prism

a glass prism is a transparent object made of glass having two triangular ends and three rectangular ends

in a glass prism, the emergent ray is not parallel to the incident ray

when a ray of light enters a prism, the light gets deviated. it bends towards the normal upon entering and away from the normal upon exiting. the angle between the incident and emergent ray is called the angle of deviation

dispersion of light

when white light is passed through a prism, it splits to form a band of seven colors on a white screen, called spectrum (VIBGYOR). the splitting of the white light is called dispersion of light

it occurs because different colors of white light travel in different speeds and have different wavelengths in a prism

the violet color is deviated the maximum while the red color is deviated the least. hence, violet has the least speed while red has the most speed

recombination of spectrum

the seven colors of the spectrum can be recombined to give back white light when passed through an inverted prism

this proves that the refraction produced by the second prism is equal and opposite to that produced by the first prism

the rainbow

the rainbow is an arch of seven colors visible in the sky which is produced by the dispersion of sunlight by the raindrops in the atmosphere

each raindrop acts as a tiny prism splitting the sunlight into a spectrum

the rainbow

the rainbow is an arch of seven colors visible in the sky which is produced by the dispersion of sunlight by the raindrops in the atmosphere

each raindrop acts as a tiny prism splitting the sunlight into a spectrum

atmospheric refraction

the atmosphere has air layers with different densities. thus, when light rays pass through, they undergo through refraction. this is called atmospheric refraction

twinkling of stars

the twinkling of stars is due to the atmospheric refraction of star’s light. the continuously changing atmosphere refracts the light from stars from one moment to the next

the star-light entering our eyes increases and decreases continuously due to atmospheric refraction

planets do not twinkle as the continuously changing atmosphere is unable to cause variations in light coming from a large body

stars seeming higher than they actually are

due to atmospheric refraction, stars seem to be up higher in the sky than they actually are

rays of light undergo continuous refraction when travelling through air of different densities. this is why by the time it reaches our eyes, the stars look higher than they are

advance sunrise and delayed sunset

we can see the sun 2 minutes before sunrise and 2 minutes after sunset due to atmospheric refraction

the actual sunrise takes place when the sun is just above the horizon. however, sunlight goes through continuous refraction and hence the sun appears to be higher than it is

similarly, the actual sunset takes place when the sun is just below the horizon. however, sunlight goes through continuous refraction and the sun appears to be higher than it is

scattering of light

light is scattered when it falls on various types of suspended particles. depending on the size of the particles, the scattering can be if white light or of colored lights. several phenomena like the blue color of sky and the red color of sun at sunset/ sunrise can be explained by this

tyndall effect

the scattering of light by particles in its path is called tyndall effect

when white light consisting of seven colors is passed through a clear liquid having small suspended particles in it, then the blue lights having shorter wavelengths are scattered more than the red lights having longer wavelengths

why the sky is blue

the scattering of blue light of the white sunlight by air molecules present in the atmosphere causes the blue color of the sky

when sunlight passes through the atmosphere, the longer wavelength lights (red lights) do not get scattered much by the air molecules and pass straight through. the shorter wavelength lights (blue lights) get scattered all around the sky which enter our eyes making the sky look blue

if the earth had no atmosphere, there would be no particles to scatter sunlight and hence the sky would look dark and black

why danger signal lights are red in color

this is because the red colored lights have longer wavelengths and are least scattered by fog or smoke particles. this ensures that the signals can be seen even from large distances