Human Eye and Colourful World, class 10 Notes

 

SHARMA CLASSES BHARATPUR

The Human Eye: It is a natural optical instrument which is used to see the objects by human beings.

·         It is like a camera which has a lens and screen system. It is a wonderful gift of nature to the human body. Human eye is nearly spherical in shape of diameter about 2.3 cm. It consists of a convex lens made up of living tissues.

·         We need two eyes because a human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes. Thus, two eyes provide us wider horizontal field of view.
With one eye, the world looks flat, i.e., two dimensional only. With two eyes, the view is three dimensional, i.e., dimension of depth is added to our view.

·         As our two eyes are separated by a few centimetres, each eye observes a slightly different image. Our brain combines the two views into one and we get to know how close or far away the things seen are.

·         It is located in the eye socket.

·         It has two layers that cover it: scleroid and choroid.

·         The function of scleroid: It is the outermost covering that consists of white fibres and its function is to protect all parts of the eye.

·         The function of choroid: It is a grey membrane attached to choroid from inner side. Its function is to darken the eye from inside so that no internal reflection takes place.

 

Structure of the Human Eye

The various parts of eye and their functions :

·         Retina: It is a light sensitive screen inside the eye on which image is formed. It contains rods and cones.

·         Cornea: It is a thin membrane which covers the eye trail. It acts like a lens which refracts the light entering the eye.

·         Aqueous humour: It is fluid which fills the space between cornea and eye lens.

·         Eye lens: It is a convex lens made of transparent and flexible jelly like material. Its curvature can be adjusted with the help of ciliary muscles.

·         Pupil: It is a hole in the middle of iris through which light enters the eye. It appears black because light falling on it goes into the eye and does not come back.

·         Ciliary muscles: These are the muscles which are attached to eye lens and can modify the shape of eye lens which leads to the variation in focal lengths.

·         Iris: It controls the amount of light entering the eye by changing the size of the pupil.

·         Optical nerve: It converts information of the image into a corresponding electric signal and passes it to the brain.

·         Rods and Cones: These are colour sensitive rods and cones shaped cells. Rods are responsible for the vision in dim light while cones are responsible for colour.

• Blind Spot: The junction of the optic nerve and retina, where no rods and cones cells are present is called the blind spot. It is insensitive to light.
• Yellow spot:
  A point on the retina where the most clear image is formed.

The following table lists the main parts of the human eye and their respective functions.

S.No.	Human Eye Part	Functions
1.	Pupil	Opens and closes in order to regulate and control the amount of light.
2.	Iris	Controls light level similar to the aperture of a camera.
3.	Sclera	Protects the outer coat.
4.	Cornea	A thin membrane which provides 67% of the eye’s focusing power.
5.	Crystalline lens	Helps to focus light into the retina.
6.	Conjunctive	Covers the outer surface (visible part) of the eye.
7.	Aqueous humour	protect the exterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure.
8.	Vitreous humour	protect the posterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. It also helps in focusing the image clearly on the retina.
9.	Retina	Captures the light rays focussed by the lens and sends impulses to the brain via the optic nerve.
10.	Optic nerve	Transmits electrical signals to the brain.
11.	Ciliary muscles	Contracts and extends in order to change the lens shape for focusing.

 

Pupil Working:
when we come out of the dark in the bright sunlight, our eyes get closed. And when we entered the hall from the bright light, you won’t be able to see and after some time you would be able to see. Here, the pupil of an eye provides a variable aperture, whose size is controlled by iris.
(a) When the light is bright: Iris contracts the pupil, so that less light enters the eye.
(b) When the light is dim: Iris expands the pupil, so that more light enters the eye.
Pupil opens completely when iris is relaxed.

Persistence of Vision: It is the time for which the sensations of an object continue in the eye. It is about 1/16th of a second.

The image of an object persists on the retina for 1/16 second, even after the removal of the object. The sequence of still pictures taken by a movie camera is projected on a screen at a rate of about 24 images or more per second. The successive impressions of images on the screen appear to merge smoothly into one another to give us the feeling of moving images.

Near Point: The nearest point from eye at which the eye can see clearly without strain is called near point. For normal eye it is 25 cm.

Far Point: The farthest point, upto which the eye can see the object clearly is called far point. For normal eye it is infinity.

The range of Vision: Distance between near point and far point of eye is called range of vision.

Power of Accommodation: The ability of the eye lens to adjust its focal length accordingly as the distances is called power of accommodation.

Colour Blindness: A person having defective cone cells is not able to distinguish between the different colours. This defect is known as Colour Blindness.

Defects of Vision and their Correction

Myopia (Short-sightedness): It is a kind of defect in the human eye due to which a person can see near objects clearly but he cannot see the distant objects clearly. Myopia is due to
1. Due to increase in size of eye ball

2.    Due to excessive curvature of cornea

3.    Due to increase in power (or decrease in focal length) of eye lens.

Correction: Since a concave lens has an ability to diverge incoming rays, it is used to correct this defect of vision. The image is allowed to format the retina by using a concave lens of suitable power as shown in the given figure.

Hypermetropia (Long-sightedness): It is a kind of defect in the human eye due to which, a person can see distant objects properly but cannot see the nearby objects clearly. It happens due to

(i) decrease in the power of eye lens i.e., increase in focal length of eye lens.
(ii) shortening of eyeball.

A hypermetropic eye has its least distance of distinct vision greater than 25 cm.

Correction: Since a convex lens has the ability to converge incoming rays, it can be used to correct this defect of vision, as you already have seen in the animation. The ray diagram for the corrective measure for a hypermetropic eye is shown in the given figure.

Presbyopia: It is a kind of defect in human eye which occurs due to ageing. It happens due to the following reasons

(i) decrease in flexibility of eye lens.
(ii) gradual weakening of ciliary muscles.
In this, a person may suffer from both myopia and hypermetropia.

Correction: By using a bifocal lens with appropriate power. Bifocal lenses consist of both concave and convex lens, upper position consists of the concave lens and lower portion consists of a convex lens.

Astigmatism: It is a kind of defect in human eye due to which a person cannot see (focus) simultaneously horizontal and vertical lines both.

Correction: By using a cylindrical lens.

Cataract: Due to the membrane growth over eye lens, the eye lens becomes hazy or even opaque. This leads to a decrease or loss of vision. This problem is called a cataract. It can be corrected only by surgery.

Colour blindness: When a person cannot distinguish between different colours, he is said to be colour blind though his vision may otherwise be normal. Colour blindness is a genetic disorder which occurs by inheritance. So far, there is no cure for colour blindness.

 

Power of the correcting convex lens:
The Lens formula, 1/v − 1/u = 1/f  can be used to calculate the focal length and hence, the power of the myopia correcting lens.
In this case,
Object distance, u = ∞
Image distance, v =
person’s far point Focal length, f =?
Hence, lens formula becomes

In case of a concave lens, the image is formed in front of the lens i.e., on the same side of the
object.
Focal length = -Far point
Now, power of the required lens (P) = 1/ f (in m)

 

Power of the correcting convex lens: Lens formula, 1/v−1/u=1/f can be used to calculate focal length f and hence, power P of the correcting convex lens, where,
Object distance, u = -25 cm, normal near point
Image distance, v = defective near point
Hence, the lens formula is reduced to

1/v+1/25=1/f

 

Refraction of light through a prism, Dispersion of white light by a glass prism, Composition of white light, Recombination of spectrum colours, Rainbow.

 

Refraction of light through a prism: When a ray of light is incident on a rectangular glass slab, after refracting through the slab, it gets displaced laterally. As a result, the emergent ray comes out parallel to the incident ray.
Unlike a rectangular slab, the side of a glass prism are inclined at an angle called the angle of prism.

·         Prism: Prism is a transparent optical object with flat, polished surfaces that refract light. At least two of the flat surfaces must have an angle between them. A prism has two triangular bases and three rectangular face.

·         Bending of light: Light changes its speed when it moves from one medium to another. For example, from air (rarer medium) into the glass (denser medium of the prism). This speed change causes the light to be refracted and to enter the new medium at a different angle. The degree of bending of the light’s path depends on the angle that the incident ray of light makes with the surface, and on the ratio between the refractive index of the two media.

Angle of incidence (i): It is the angle between the incident ray and the normal at the point of incidence.
Angle of emergence (e): It is the angle between the emergent ray and the normal at the point of emergence.
prism (A): It is the angle between the two refracting faces of the prism.
Angle of deviation (D): The amount of overall refraction caused by the passage of a light ray through a prism is often expressed in terms of the angle of deviation.
The angle of deviation is the angle made between the incident ray of light entering the first face of the prism and the refracted ray that emerges from the second face of the prism. Because of the different indices of refraction for the different wavelengths of visible light, the angle of deviation varies with wavelength.

·         Prism diagram with different rays and angles:

Relation of all the four angles ∠A, ∠D, ∠i and ∠e.  ∠A +∠D = ∠i + ∠e

·         Angle of deviation depends upon the:
(a) angle of prism.
(b) nature of material of the prism.
(c) angle of incidence.

·         When angle of incidence increases, angle of deviation decreases, till it becomes minimum at a particular angle of incidence.

·         The refracted ray becomes parallel to the base of the prism under the minimum deviation position.

·         On increasing the angle of incidence in a prism experiment the emergent angle will decrease Beyond 90 degree.

 

Dispersion of white light by a glass prism: The phenomenon of splitting of white light into its seven constituent colours when it passes through a glass prism is called dispersion of white light.

 The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red. The sequence of colours remembers as VIBGYOR. The band of seven colours is called the spectrum.

The different component colour of light bends at a different angle with respect to the incident angle. The violet light bends the least while the red bends most.

 

Cause of dispersion : Every colour has its own characteristic wavelength/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.

Composition of white light: White light consists of seven colours i.e., violet, indigo, blue, green, yellow, orange and red.

Monochromatic light: Light consisting of single colour or wavelength is called monochromatic light, example; sodium light.

Polychromatic light: Light consisting of more than two colours or wavelengths is called polychromatic light, example; white light.

Recombination of white light: Newton found that when an inverted prism is placed in the path of dispersed light then after passing through the prism, they recombine to form white light.

Issac Newton: He was the first, who obtained spectrum of sunlight by using glass prism. He tried to split the spectrum of white light more by using another similar prism, but he could not get any more colours.
He repeated the experiment using second prism in inverted position with respect to the first prism. It allowed all the colours of spectrum to pass through second prism. He found white light emerges on the other side of second prism.

He concluded that Sun is made up of seven visible colour VIBGYOR.

 

Rainbow: It is the spectrum of sunlight in nature. It is formed due to the dispersion of sunlight by the tiny water droplet, present in the atmosphere.

Formation of the rainbow: The water droplets act like small prism. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.
Red colour appears on top and violet at the bottom of rainbow.
A rainbow is always formed in a direction opposite to that of Sun.
At ‘A’ – Refraction and dispersion take place.
At ‘B’ – Internal reflection takes place.
At ‘C’ – Refraction and dispersion take place.

Atmospheric Refraction: The refraction of light caused by the Earth’s atmosphere (having air layers of varying optical densities) is called Atmospheric Refraction.

 

Appearance of Star Position: It is due to atmospheric refraction of star light.
The temperature and density of different layer of atmosphere keeps varying. Hence, we have different medium.
Distant star act as point source of light. When the starlight enter the Earth’s atmosphere, it undergoes refraction continuously, due to changing refractive index i.e. from Rarer to denser. It bends towards the normal.
Due to this, the apparent position of the star is different from actual position. The star appear higher than its actual position.

Twinkling of Star: It is also due to atmospheric refraction.

Distant star act like a point source of light. As the beam of starlight keeps deviating from its path, the apparent position of star keeps on changing because physical condition of earth’s atmosphere is not stationary.
Hence, the amount of light enters our eyes fluctuate sometimes bright and sometime dim. This is the “Twinkling effect of star”.

Planets do not twinkle:

Planets do not twinkle because they appear larger in size than the stars as they are relatively closer to Earth. Planets can be considered as a collection of a large number of point-size sources of light.
The different parts of these planets produce either brighter or dimmer effect in such a way that the average of brighter and dimmer effects is zero. Hence, planets not twinkle. Therefore the total amount of light entering our eyes from all individual point source will nullify the twinkling effect.

 

 

The duration of day becomes approximately 4 minutes shorter if there is no atmosphere on earth:

Actual sun rise happens when it is below the horizon in the morning. The rays of light from the sun below the horizon reach our eyes because of refraction of light. Similarly, the sun can be seen about few minutes after the actual sun set. Thus the duration of, day time will increase by 4 minutes.
This is due to atmospheric refraction. Because of this sun is visible about 2 minutes earlier than actual sunrise and about 2 minutes after the actual sun set.

Apparent flattering of the Sun’s disc at sunset and sunrise is due to atmospheric refraction.

Scattering of light: According to Rayleigh’ Law of Scattering, the amount of scattered light ∝ 1/λ (λ = wavelength)

Scattering of light decreases with increase in wavelength.

 

Tyndall Effect: When a beam of light strikes, the minute particle of earth’s atmosphere, suspended particles of dust and molecule of air the path of beam become visible. The phenomenon of scattering of light by the colloidal particle gives rise to Tyndall Effect.
It can be observed when sunlight passes through a canopy of a dense forest.
The colour of the scattered light depends on the size of the scattering particles.

Colour of Sunrise and Sunset: While sunset and sunrise, the colour of the sun and its surroundihg appear red. During sunset and sunrise, the sun is near to horizon, and therefore, the sunlight has to travel larger distance in atmosphere. Due to this, most of the blue light (shorter wavelength) is scattered away by the particles. The light of longer wavelength (red colour) reaches our eye. This is why sun appear red in colour.

danger signal or sign is made of red colour
Red colour scatteres the most when strikes the small particle of fog and smoke because it has the maximum wavelength (visible spectrum). Hence, from large distance also, we can see the red colour clearly.

 

Sky appear dark instead of blue to an astronaut

During sunrise, the light rays coming from the Sun have to travel a greater distance in the earth’s atmosphere before reaching our eyes. In this journey, the shorter wavelengths of light are scattered out and only longer wavelengths are able to reach our eyes.  Since blue colour has a shorter wavelength and red colour has a longer wavelength, the red colour is scattered the least and is able to reach our eyes after the atmospheric scattering of light. Therefore, the Sun appears reddish early in the morning.

 

At noon sun appears white: At noon, the sun is overhead and sunlight would travel shorter distance relatively through the atmosphere. Hence, at noon, the sun appear white as only little of the blue and violet colours are scattered.

Eye donation :

By donating our eyes after we die, one pair of our eyes can give vision to two corneal blind people. Eye donors may belong to any sex or any age group. People suffering from diabetes, hypertension, asthma or any other non- communicable diseases can donate eyes. People who have been using spectacles or those operated for cataract can also donate eyes.

 

Q. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

Given:
Object distance, u = -d = -25 cm
Image distance, v = -1 m = -100 cm
Focal length, f = ?
Using the lens formula,

 

Q. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

Ans. Object distance, u = ∞

Image distance, v = -80 cm (at far point)
Focal length = f
According to the lens formula,

We know,

Power,

A concave lens of power -1.25 D is required by the person to correct his defect.

 

Q. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Ans. The power P of a lens of focal length f is given by the relation

(i) Power of the lens used for correcting distant vision = -5.5 D

Focal length of the required lens,

The focal length of the lens for correcting distant vision is -0.181 m.

(ii) Power of the lens used for correcting near vision = +1.5 D

Focal length of the required lens,

The focal length of the lens for correcting near vision is 0.667 m.