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CBSE NOTES CLASS 10 SCIENCE CHAPTER 11

HUMAN EYE AND THE COLOURFUL WORLD

Human Eye

Human eye acts like a camera, which makes real inverted image on the retina. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.

Cornea

A thin membrane, through which light enters the eye. It forms the transparent bulge on the front surface of the eyeball.

Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.

Eye lens

It is a crystalline lens that provides the finer adjustment of focal length required to focus objects at different distances on the retina.

Iris is a dark muscular diaphragm behind the cornea that controls the size of the pupil.

The pupil is a small hole which regulates and controls the amount of light entering the eye.

Retina

The lens system forms real inverted image of the objects on retina. It acts like a screen. Retina contains millions of cone and rod cells which of are sensitive to color and intensities of light respectively.

The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.

Vitreous Humour

The vitreous humour is a clear gel-like substance that occupies the space behind the lens and in front of the retina at the back of the eye. It supports the lens and helps maintain the shape of the entire vitreous chamber and posterior cavity.

Acquous Humour

The aqueous humour is a transparent, watery fluid similar to plasma, but containing low protein concentrations. It is secreted from the ciliary epithelium. It maintains the intraocular pressure and keeps the eye ball taut.

Power of Accommodation of Eye

The eye is able to change the shape (curvature) and the focal length of the eye lens with the help of the ciliary muscles. This property of the eye is called accommodation.

The change in the curvature of the eye lens changes its focal length.

When the ciliary muscles are relaxed, the eye lens becomes thin and its focal length increases. This enables us to see distant objects clearly.

When the ciliary muscles contract the eye lens becomes thick and its focal length decreases. This enables us to see nearby objects clearly

Hence, the human eye lens is able to adjust its focal length to view both distant and nearby objects on the retina.

For example, when the muscle is relaxed, the focal length is about 2.5 cm and objects at infinity are in sharp focus on the retina. When the object is brought closer to the eye, in order to maintain the same image-lens distance ($\cong$ 2.5 cm), the focal length of the eye lens becomes shorter by the action of the ciliary muscles.

Near Point of Eye

The closest distance for which the lens can focus light on the retina is called the least distance of distinct vision, or the near point. The standard value of near point for normal vision is 25 cm (D).

Far Point of Eye

Farthest distance for which the lens can focus the light on the retina is called far point or largest distance of distinct vision (Infinity for normal eye).

With the increase in age, the near point shifts away and the far point shifts closer to the eye due to decreasing effectiveness of the ciliary muscle and the loss of flexibility of the lens.

Eye Defects

(i) Myopia or Short-Sightedness

The defect: It is a defect of eye due to which a person can see nearby objects clearly but cannot see far away objects clearly.

Myopia is also known as near-sightedness.

A person with this defect has the far point nearer than infinity.

In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.

Reasons: This defect may arise due to,

• excessive curvature of the eye lens, or

• elongation of the eyeball.

Correction: This defect can be corrected by using a concave lens of suitable power.

A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.

(ii) Hypermetropia or Long-Sightedness

The defect: Hypermetropia is also known as far-sightedness.

A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point (25 cm).

Reasons: This defect arises either because,

• the focal length of the eye lens is too long, or

• the eyeball has become too small.

Correction: This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.

(iii) Presbyopia

The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.

It can be corrected using bi-focal lenses.

• Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses.

A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.

• These days, it is also possible to correct the refractive defects with contact lenses or through surgical interventions.

(iv) Astigmatism

In this defect, a person cannot focus on horizontal and vertical lines at the same distance at the same time. This defect can be removed by using suitable cylindrical lenses.

(v) Colour Blindness

In this defect, a person is unable to distinguish between few colours. The reason of this defect is the absence of cone cells sensitive for these colours. This defect cannot be removed.

(vi) Cataract

In this defect an opaque white membrane is developed on cornea due to which person lost power of vision partially or completely. This defect can be removed by removing this membrane through surgery.

REFRACTION OF LIGHT THROUGH A PRISM

Prism is uniform transparent medium bounded between two refracting surfaces, inclined at an angle.

When the incident ray PE falls on the air-glass interface AB, it gets refracted as EF after bending towards the normal NN’.

At the second surface AC, the light ray is entering from glass to air. Hence it bends away from normal MM’and emerges as FR.

Angle of Deviation

The angle subtended between the direction of incident light ray and emergent light ray from a prism is called angle of deviation (D).

Dispersion of White Light by a Glass Prism

The splitting of white light into its constituent colours in the sequence of VIBGYOR, on passing through a prism, is called dispersion of light.

The pattern of colour components of light is called the spectrum of light.

The red light bends the least, while the violet light bends the most (refractive index nv > nR)

Does the prism itself create colour in some way or does it only separate the colours already present in white light?

Newton’s Experiment on Prisms

When two similar prisms are put inverted with respect to each other, the emergent beam is white light.

That means, the colours were split by the first prism and the second prism combined them back.

The colours are actually light with different wavelengths.

Red light has the longest and violet the shortest wavelength.

Thick lenses could be assumed as made of many prisms; therefore, thick lenses show chromatic aberration due to dispersion of light.

Critical Angle

The angle of incidence in a denser medium for which the angle of refraction in rarer medium becomes 90°, is called critical angle (C or ic).

Critical angle increases with temperature.

Critical angle for diamond = 24°,

Critical angle for glass = 42°,

Critical angle for water = 48°

Refractive index of denser medium

Total Internal Reflection (TIR)

When a light ray travelling from a denser medium towards a rarer medium is incident at the interface at an angle of incidence greater than critical angle, then light rays reflected back in to the denser medium. This phenomenon is called TIR.

The refractive index is maximum for violet colour of light and minimum for red colour of light. i.e., nv > nR therefore critical angle is maximum for red colour of light and minimum for violet colour of light, i.e., Cv < CR

Total internal reflection occurs if angle of incidence in denser medium exceeds critical angle.

Mirage is an optical illusion observed in deserts and roads on a hot day when the air near the ground is holler and hence rarer than the air above. When the layers of air close to the ground have varying temperature with hottest layers near the ground, light from a distant tree may undergo total internal reflection, and the apparent image of the tree may create an illusion to the observer that the tree is near a pool of water.

Brilliance of Diamond

The brilliance of Diamonds is due to the total internal reflection of light inside them. The critical angle for diamond-air interface (≅ 24.4°) is very small, therefore once light enters a diamond, it is very likely to undergo total internal reflection inside it. Diamond cutting enhances sparkle. By cutting the diamond suitably, multiple total internal reflections can be made to occur.

Rainbow Formation

This is a phenomenon due to combined effect of dispersion, refraction and reflection of sunlight by spherical water droplets of rain.

The sun should be shining in one part of the sky (say west) while it is raining in the opposite part of the sky (say east).

Rainbow can only be seen when the back of the observer is towards the sun.

Mechanism - Sunlight is first refracted as it enters a raindrop, which causes the different wavelengths (colours) of white light to separate.

Longer wavelength of light (red) are bent the least while the shorter wavelength (violet) are bent the most.

These component rays strike the inner surface of the water drop and get internally reflected if the angle between the refracted ray and normal to the drop surface is greater than the critical angle (48o, in this case).

The reflected light is refracted again as it comes out of the drop as shown in the figure.

The violet light emerges at an angle of 40º related to the incoming sunlight and red light emerges at an angle of 42o. For other colours, angles lie in between these two values.

If the light undergoes only one internal reflection, the rainbow formed is called, primary rainbow. The red light from drop 1 and violet light from drop 2 reach the observers eye. The violet from drop 1 and red light from drop 2 are directed at level above or below the observer. Thus the observer sees a rainbow with red colour on the top and violet on the bottom.

On the other hand if there is double internal reflection and the rainbow is seen as violet on top of red, it is called secondary rainbow. It is a 4 step process.

Apparent Depth

The bottom of glass filled by water is raised (apparent depth is less than real depth)

Atmospheric Refraction & Apperent Height of Stars

The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index.

Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.

Twinkling of Stars

The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index. This apparent position of the star is not stationary, but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary. Also the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers – the star sometimes appears brighter and some times, fainter, which is the twinkling effect.

Why don’t the planets twinkle?

The planets are much closer to the earth, and are thus seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect.

Early Sunrise and Late Sunset

The sun is visible a little before the actual sunrise and until a little after the actual sunset due to refraction of light through the atmosphere.

By actual sunrise we mean the actual crossing of the horizon by the sun. Due to this, the apparent shift in the direction of the sun is by about half a degree and the corresponding time difference between actual sunset and apparent sunset is about 2 minutes.

The apparent flattening (oval shape) of the sun at sunset and sunrise is also due to the same phenomenon.

Scattering of Light

As sunlight travels through the earth’s atmosphere, it gets scattered (changes its direction) by the atmospheric particles.

Light of shorter wavelengths is scattered much more than light of longer wavelengths.

Hence, the bluish colour predominates in a clear sky, since blue has a shorter wavelength than red and is scattered much more strongly.

In fact, violet gets scattered even more than blue, having a shorter wavelength, but since our eyes are more sensitive to blue than violet, we see the sky blue.

Large particles like dust and water droplets present in the atmosphere behave differently. The scattering depends on relative size of the wavelength of light λ, and the scatterer (of typical size, say, a).

• Why rain clouds are darker?

• The clouds which have droplets of water with a >> λ are generally white.

Why sun appears red at sun rise and at sun set?

At sunset or sunrise, the sun’s rays have to pass through a larger distance in the atmosphere and most of the blue and other shorter wavelengths are removed by scattering. The least scattered light reaching our eyes, therefore, the sun looks reddish.

Why the sky appears blue?

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. This scattered blue light reaches our eyes from whichever direction we look at.

Tyndall Effect

The earth’s atmosphere is a heterogeneous mixture of minute particles of smoke, tiny water droplets, suspended particles of dust and molecules of air. When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect.

Examples of Tyndall effect

• When a fine beam of sunlight enters a smoke-filled room through a small hole, scattering of light makes the particles visible.

• When sunlight passes through a canopy of a dense forest, tiny water droplets in the mist, scatter light.

The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough, then, the scattered light may even appear white.

Activity to Demonstrate Tyndall Effect

Place a strong source (S) of white light at the focus of a converging lens (L1). This lens provides a parallel beam of light.

Allow the light beam to pass through a transparent glass tank (T) containing clear water.

Allow the beam of light to pass through a circular hole (c) made in a cardboard. Obtain a sharp image of the circular hole on a screen (MN) using a second converging lens (L2).

Dissolve about 200 g of sodium thiosulphate (hypo) in about 2L of clean water taken in the tank. Add about 1 to 2 mL of concentrated sulphuric acid to the water. The fine microscopic sulphur particles precipitate in about 2 to 3 minutes.

We observe at first the orange red spot and then bright crimson red colour on the screen.

Discussion

As the size of sulphur particles increases in colloid, the wave length of the light dispersed also increases.

As the sulphur particles begin to form, the blue light is seen from the three sides of the glass tank and the orange red spot is observed on the screen. In the beginning only light with very small wave lengths is dispersed (mainly in the blue region), due to small size of the colloidal particles.

As the size of colloidal particles increases, the yellow and orange light also gets dispersed. As a result we see a bright crimson red spot on the screen.