- HUMAN EYE AND ITS CONSTUCTION
- POWER OF ACCOMODATION
- DEFECTS OF VISION AND THEIR CORRECTION
- REFRACTION OF LIGHT THROUGH PRISM
- DISPERSION OF WHITE LIGHT BY GLASS PRISM
- HOW RAINBOW IS CREATED
- ATMOSPHERIC REFRACTION AND ITS APPLICATION
- SCATTERING OF LIGHT AND ITS APPLICATION
If someone asks you which organ of your body helps you to see? Then most of us easily answer that it is eye? But if we asked that why we see by our eye not by our ear, it must be horrible for us till now? In this chapter we discuss about this and also some interesting application of refraction and reflection of light like why rainbow is created, why stars are twinkling, why sky is blue etc.
- The human eye is one of the most valuable and sensitive sense organs.
- It enables us to see the wonderful world and the colours around us. On closing the eyes, we can identify objects to some extent by their smell, taste, sound they make or by touch.
- But it is impossible for anyone to identify colours while our eye is closed. Hence by all sense organ eyes is the most significant one as it enables us to see the beautiful, colourful world around.
Construction of Eye:
- Retina: The light sensitive screen, where the eye lens forms the image of the object is called as Retina.
- Cornea: Light enters the eye through a thin membrane. This membrane is called as Cornea. It forms the transparent bulge on the front surface of the eyeball.
- Eyeball: The spherical organ, which is the house of the all parts of vision, is called as eyeball. The diameter of eyeball is 2.3 cm. (see pic 11.2)
4. Crystalline lens: The transparent material that focuses light rays onto the retina. It is composed of a fibrous Jelly-like structure.
5. Pupil: the opening in the middle of the iris. The pupil regulates and controls the asmount of light entering to the eye.
6. Iris: A dark muscular diaphragm that controls the size of the pupil. It is present behid the cornea.
7.Viterous humour: The colourless fluid that present between the retina and the crystaline lens of the eye.
8.Optic nerve: The nerve that carry electical signals to the brain from the retina.
9. Ciliary muscles: The muscles that controls the size of the crystaline lens of eye.
How we see object by our eye ?
- Our eye is like a camera. When we see any object light rays comes from that object and enters to our eye. Most of the refraction for the light rays entering to our eye occurs at the outer surface of the cornea.
- The crystaline lens merely provides the finer adjustment of focal length required to focus objects at different distance.
- The image of any object at any distsance is formed at the retina of the eye.
- The pupil in the iris, regulates the amount of light entering to the eye.
- A real, inverted and diminished image of the object is formed on the retina.
- The retina is delicate membrane, which have enermous number of light sensitive cells. These light sensitive cells get activated upon illumination and generates electrical signals.
- These signals are sent to the brain by the otical nerves.
- The brain interpretes these signals and finally, processes the information so that we percieve objects as they are. Thus we see an object.
- Damage to or malfunction of any part of the visual system can lead to significant loss of visual functioning. For example, if any of the structuers involved in thte transmission of light like the cornea, pupil, eye lens, aqueous humour and vitereous humour or those responsible for convertion of light in to electrical signals like retina or optic nerve is damaged, will result in visual impairment.
We are not able to see the things in a clearly for some time when you enter from bright light to a room with dim light. Why?
Ans:We know that the pupil of our eye is the door of light rays, those are entering to our eye. This pupil of eye acts like a variable aperture, whose size can be varied with the help of the Iris.
When we are in bright light , the iris contracts the pupil to allow less light to enter the eye. Hence the size of the pupil is small at this time.
But in dim light the Iris exapands the pupil to allow more light to enter the eye. Thus, the pupil opens completely through relaxation of the Iris which, takes sometime. Hence, We are not able to see the things in a clearly for some time when you enter from bright light to a room with dim light
Power of accomodation:
- The ability of the eye lens to adjust its focal length is called accomodation.
- The change in the curvature of eye lens is controlled by the ciliary muscles. The change of the curvatue of eye lens changes the focal length of the crystaline lens.
- When we see a distance object, the ciliary muscles relaxed. Hence the focal length of the crystaline lens increases. Thus we see a distance object.
- But when the object is near our eye the muscles contract. Hence the focal length of the crystaline lens decreases and its thickness increases. Thus we see an object closer to our eye.
- But the focal length of the eye lens cannot be smaller than a certain minimum limit. This minium distance, at which objects can be seen most distinctly without strain, is called the least distsance of distinct vision. It is also called near point. The near point for a young adult is 25 cm.
- Likewise, the farthest point upto which the eye can see objects clearly is called far point of the eye. It is infinity for a normal eye.
- Sometimes the crystaline lens of the eye of some old age persons turns milky or cloudy. This is called cataract.
- This causes partial or complete loss in vision. It is possible to restore vision through surgery.
Why do we have two eyes for vision and not just one?
- Because, two eyes give a wider range of view than one eye.
- In one eye, we have a horizontal field of view about 1500 where in two eyes it becomes 1800. The ability to detect faint objects is, of course, enhanced with two detectors instead of one.
- Some animals, usually prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view.
- But our two eyes are positioned on the front of our heads, and it thus reduces our field of view in favour of what is called stereopsis.
- Shut one eye and the world looks flat – two-dimensional. Keep both eyes open and the world takes on the third dimension of depth.
- Because our eyes are separated by a few centimetres, each eye sees a slightly different image. Our brain combines the two images into one, using the extra information to tell us how
close or far away things are.
Defects of vision and their correction:
- Sometimes, the eye may gradually lose its power of accommodation. In such conditions, the person cannot see the objects distinctly and comfortably. The vision becomes blurred due to the refractive defects of the eye. This is called the defects of vision.
- There are three types of refractive defects or defects of vision. These are
- Myopia or near sightedness
- Hypermetrophia or far sightedness
- Myophia or near sightedness:
- The refractive defects in which, the near objects are visible clearly but the far objects are not visible clearly or sometimes not, is called as Myophia or near sightdness.
What happens in myophia:
- A person having myophia has the far point nearer than infinity. Means a person having myophia cannot see a object at infinity. He may able to see upto some distance.
- In a myophic eye , the image of a distance object is formed infront of the retina and not at the retina itself.
Why it happens/ causes:
This defect may arise due to
- Excessive curvature of the eye lens
- Elongation of the eye ball
Example of Myophia:
A student, who can see the letters of book clearly but fails to see the letters written in a blackboard far from him, we may say that the student has myophia.
Correction of myophia:
- In myophia the focal length of the eye lens decreases and power increases, hence image of a distance object is created before retina. So, for its correction, the focal length of the eye lens should be increased and power should be decreased.
- Hence a concave lens of suitable focal length or suitable power will bring the image back on to the retina and thus the defect is corrected.
2. Hypermetrophia or far sightedness:
- The refractive defects in which, the far objects are visible clearly but the near objects are not visible clearly or sometimes not, is called as Hypermetrophia or Far sightdness.
What happens in myophia:
i. A person having hypermetrophia has the near point farther than normal near point i.e 25cm. Means a person having hypermetrophia cannot see an object near 25 cm. He has to keep a reading material much beyond 25 cm from the eye for comfortable reading.
ii. In a hypermetrophic eye , the image of a near object is formed behind the retina and not at the retina itself.
Why it happens/ causes:
This defect may arise due to
- The focal length of the eye lens is too long
- The eyeball has become too small
Example of Hypermetrophia:
A student, who cannot see the letters of book clearly but can see the letters written in a blackboard far from him, we may say that the student has hypermetrophia.
Correction of Hypermetrophia:
- In hypermetrophia the focal length of the eye lens increases and power decreases, hence image of a near object is created after retina. So, for its correction, the focal length of the eye lens should be decreased and power should be increased.
- Hence a convex lens of suitable focal length or suitable power will bring the image back on to the retina and thus the defect is corrected by a suitable eye-glass having converging or convex lens.
- 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.
Bio focal lens:
- 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.
- Today, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
Refraction of light through Prism:
- A traingular glass prism has two trangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other.
- The angle between its two lateral faces is called the angle of the prism. Let’s discuss about the refraction through a prism.
- In pic 11.5, PE the incident ray, EF is the refracted
- ray and FS is the emergent ray. At the surface AB of the prism the ray of light is entering from air (rear medium) to the glass (denser medium). Hence, the light ray bends towards the normal.
- At second surface AC, the light ray moves from glass (denser medium) to Air (rear medium). Hence the light ray bent away from the normal.’
- We can notice that, the emergent ray bends from the incident ray by making some angle. This angle is called as angle of deviation, which is not same as the refraction through a reactangular glass slab. Because in glass slab the emergent ray and the incident ray are parallel.
- But in prism angle of deviation is formed because of the peculiar shape of it.
- The angle of deviation is denoted as ⦟D.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM:
- When white light falls on a prism, the prism spilt the incident white light into a band of colour. This phenomenon by which the incident white light spilts into its component colour is called Dispersion.
- The band of colour contains seven different colours. These are Violet, Idigo, Blue, Green, Yellow, Orange, Red from bottom to top. These band of coloured components of
light beam is called Spectrum. The acronym of sequence of the spectrum is “VIBGYOR”. (see pic 11.6)
- Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most. Thus the rays of each colour emerge along different paths and thus become distinct. It is the band of distinct colours that we see in a spectrum.
- Example of dispersion: formation of rainbow
- Sir Issac Newton was the first to use a glass prism to study the spectrum of sunlight. He tried to split the colours of the spectrum of
- white light. further by using another similar prism, he incidenced the found spectrum on it. However, he could not get any more colours. He then placed a second identical prism in an inverted position with respect to the first prism, to pass through the second prism. He found a beam of white light emerging from the other side of the second prism. This observation gave Newton the idea that the sunlight is made up of seven colours.
- Any light gives spectrum similar to sunlight is often refered as white light.
Why Rainbow forms:
- A rainbow is a natural spectrum appearing in the sky after a rain shower (Pic. 11.8). It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere due to the rain.
- A rainbow is always formed in a direction opposite to that of the Sun.
- The water droplets act like small prisms. They refract and disperse the incident sunlight on them and then reflect it internally, and finally refract it again when it comes out of the raindrop (Pic. 11.9).
- Due to the dispersion of light and internal reflection, different colours reach the observer’s eye. And we call it Rainbow.
- We can also see rainbow on a sunny day when you look at the sky through a waterfall or through a water fountain, with the Sun behind us.
- we observe, the apparent random wavering or flickering of objects seen through a turbulent stream of hot air rising above a fire or a radiator.
- It occurs because the air just above the fire becomes hotter than the air further up. The hotter air is lighter (less dense) than the cooler air above it, and has a refractive index slightly less than that of the cooler air.
- Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the object, as seen through the hot air, fluctuates.
- This wavering is thus an effect of atmospheric refraction (refraction of light by the earth’s atmosphere) on a small scale in our local environment. The twinkling of stars is a similar phenomenon on a much larger scale.
Twinkling of stars:
- The twinkling of a star is due to atmosphericrefraction of starlight. The starlight, on entering the earth’s atmosphere,
- undergoes refraction continuously before it reaches our eye. Because the refractive index of air gradually increases when we move towards the ground. Hence the light from the star i.e star light, gradually moves towards the normal. For which 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.(see pic 11.10)
- Further, 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.
- Since 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 at some other time, fainter, which is the twinkling effect.
Why don’t the planets twinkle?
- The planets are much closer to the earth than the stars, 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.
Advance sunrise and delayed sunset:
- By actual sunrise and sunset we mean the actual crossing of the horizon by the Sun. Likewise, by actual sunset, we mean the actual crossing of the horizon by the sun.
- The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset. It is due to the atmospheric refraction.
- The refractive index of air gradually increases when we move towards the ground. Hence the lower air label is more denser than the higher label air. (see pic 11.11)
- The light rays, those come from the sun suffers refraction due to the varying refractive index of the air at different labels.
- Hence, when these refracted rays fall on our eye, we see the apparent position of the sun. Hence, The time difference between actual sunset and the apparent sunset is about 2 minutes. The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon.
SCATTERING OF LIGHT:
- The phenomenon of light by which a light ray deviated from its straight line path, when small particles of considerable size like dust, gas particles etc comes in its path, is called as scattering of light.
- Ex: sky seems to be blue, colour of water in deep sea, Tyndall effect, the reddening of the sun at sunrise and the sunset etc.
- Tyndall Effect:
- The phenomenon of scattering of light by colloidal particles like fog, smoke, tiny water droplets, suspended particles of dust and molecules of air etc is called Tyndall effect. For the Tyndall effect the path of the light beams becomes visible.
- Ex: a) this phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible. b) Tyndall effect can also be observed when sunlight
passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.(see pic 11.12)
- The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include 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.
- Among the 7 colours of white light, which colour is scattered, depends on the size of the particle present in the scattering medium.
- Very fine or small particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths like red light.
- If the size of the scattering particles is large enough, then, the scattered light may even appear white.
- Why is the colour of the clear Sky Blue?
- The earth’s atmosphere is composed of heterogeneous mixture of minute particles like smoke, tiny water droplets, suspended particles of dust and molecules of air.
- These molecules of air and other fine particles in have size smaller than the wavelength of visible light. Hence, These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
- The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter
wavelengths) more strongly than red. The scattered blue light enters our eyes hence the sky seems blue.
A) What happens if earth has no atmosphere?
Ans: If earth has no atmosphere, than there is no particle in earth. For which the scattering of light is not prominent. Hence we cannot see the paths of light according to the Tyndall effect. So the sky appears dark.
B) Why does the sky appear dark instead of blue to an astronaut?
Ans: The astronauts generally fly on very high altitudes. There is no atmosphere at very high altitudes. Hence, the scattering of light is not prominent at such heights. Hence the sky appears dark instead of blue to the astronaut.
C) Why ‘danger’ signal lights are red in colour?
Ans: We know that small or tiny particles scatter blue light which has a smaller wavelength. But the red is least scattered by fog or smoke as it has the longer wavelength. Therefore, it can be seen in the same colour at a distance, being not scattered. Hence the person, who approaches to the danger places can see the light from distance and be aware. Hence Red light is used as danger signals.
- Colour of the Sun at Sunrise and Sunset:
- During the Sunset and Sunrise the Sun is at the horizon. Hence the light rays, those come from the Sun travels through the different layers of the atmosphere and travel a longer path as compare to the rays those come from the afternoon Sun, as it is over our head.
- Hence, most of the smaller wavelength blue lights are scattered by the tiny particles of the atmosphere.
- The light rays having a longer wavelength are scattered very low. Hence the longer wavelength red light falls on our eye. For which, the colour of the Sun looks bright Red during the Sunset and Sunrise. ( see pic 11. )