(i) most relaxed
(ii) most contracted state.
In which of the two cases is the focal length of the eye-lens more?
Answer: In most relaxed state.
(i) Mirrors used by dentists to examine teeth.
(ii) The smallest distance, at which the eye can see objects clearly without strain.
(i) Concave mirror
(ii) Near point or 25 cm.
Answer: Colour blindness is that defect of the eye due to which a person is unable to distinguish certain colours, sometimes even the primary colours.
Answer: No, it will not show dispersion. It will only show deviation.
Answer: The band of seven colours formed on white screen when a beam of white light (or sunlight) is passed through a glass prism is called spectrum of white light.
Answer: The sun can be seen about two minutes before actual sunrise because of atmospheric refraction.
Answer: Least deviated component: Red
Most deviated component: Violet
Answer:For scattering of light, particles are required. Since there are no particle in space, the sky appear dark to astronauts.
Answer: Power of Accommodation: The ability of an eye to focus the distant objects as well as the near by objects on the retina by changing the focal length is called power of accommodation.
Answer: The defect of the eye due to which a person is unable to distinguish between certain colours, is known as colour blindness. Cone-shaped retinal cells are responsible for making a person differentiate between colours. The colour blind persons do not possess cone cells that respond to certain colours.
Answer: After refraction at two parallel faces of a glass slab, a ray of light emerges in a direction parallel to the direction of incidence of white light. As rays of all colours emerge in the same direction, i.e., the direction of the incidence of white light, there is no dispersion. However, there is lateral displacement.
Answer: signal lights are red in colour because the red coloured light having longer wavelength is scattered the least by fog or smoke. Therefore, it can be seen clearly from a distance.
Answer: The splitting of white light into its component colours is called dispersion of light. The band of the coloured components formed due to dispersion of white light is called ‘spectrum’. Seven colours of spectrum are violet, indigo, blue, green, yellow, orange and red also known as ‘VIBGYOR’.
Answer: All colours of light travel at the same speed in a vacuum. When these enter a transparent substance like prism, all slow down by different amounts depending on their wavelength. As these slow down by different amounts, different colours are refracted through different angles which causes dispersion.
Answer: Different wavelengths deviate differently in the prism because the angle of refraction for different colours having different wavelengths is different while passing through the glass prism.
Answer: By using two identical prism, one placed inverted with respect to the other we get a narrow beam of white lgiht incident on one prism emerges out of the second prism as white light.
Answer: The power of accommodation of the eye decreases with ageing. For most people, the near point gradually recedes and the far point comes closer. This defect is called presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Such a person may suffer from myopia and hypermetropia.
This defect is then corrected by using bi-focal lenses of suitable focal lengths. The upper part of the lens is a concave lens which corrects myopia to see the distant objects clearly, while the lower part of the lens has convex lens which corrects the hypermetropia to see the nearby objects clearly.
Answer: In this defect, a person cannot focus on both horizontal as well as vertical lines at the same time. So he can see the objects clearly only in one plane.
This defect is mainly due to the cornea that is not perfectly spherical. As a result, the cornea has different curvatures in different directions of the horizontal and vertical planes. This defect can be corrected by using cylindrical lenses of suitable focal length and suitable axis in the spectacles.
Answer: The ability of the human eye to continue to see the image of an object for very short duration even after the removal of that object is called persistence of vision. In fact the sensation produced by the image of an object lasts for nearly 1/16th of a second on retina.
It is due to persistence of vision that we are able to see movie pictures in a cinema hall. The pictures in the form of a long film are projected on the screen at a rate of about 24 pictures per second. Under these conditions, the image of one picture persists on the retina of the eye till the image of the next picture falls on the screen, and so on. Due to this, the slightly different images of the successive pictures present on the film merge smoothly with one another and give us the feeling of continuity and moving images.
Answer: The phenomenon in which a part of the light incident on a particle is redirected in different directions is called the scattering of light.
The blue colour of the sky is due to the scattering of sunlight by the molecules of the atmosphere. The light of shorter wavelength (blue) of the visible spectrum is scattered more than the light of longer wavelength by the atmospheric particles. When we look at the sky, the scattered light enters our eyes which majorly contains blue, colour and hence the sky appears blue.
At the time of sunrise and sunset, when the Sun is near the horizon, sunlight travels a greater distance through the atmosphere to reach us. During this, most of the shorter wavelengths present in it are scattered away from our line of sight by the molecules of air and other fine particles in the atmosphere. So, light reaching us directly from the rising or setting Sun consists mainly of the longer wavelength red colour because of which the Sun appears red.
Answer: At sunrise, light from the sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eye. Hence shorter waves are scattered .. away and longer (red) waves reach our eye. Hence sun appears red.
However, at noon sunlight would travel relatively shorter distance only a little blue and violet colours are reflected and it is nearly the white light which reaches our eye.
Answer: In the bright sunlight, iris causes the pupil to become smaller so that only a small portion of light enters the eye and rods of the retina are also adjusted in the same way. But, when a person enters into a dimly lighted room, each iris takes some time to increase the diameter of the pupil, so that more amount of light can enter the eyes to see the object clearly and rod cells of the retina also take some time to adjust themselves to get the object in the dim light.
Answer: The fine particles in the atmosphere scatter light of shorter wavelength (blue colour) more strongly than the light of longer wavelength (red colour). Therefore, the sky appears blue from the surface of the Earth. For an astronaut, the sky would appear dark because in space there is no atmosphere to scatter light.
Answer: The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour more strongly than red. The scattered blue light enters our eyes. Since we see the blue light from everywhere overhead, the sky appears blue.
Answer: The scattering of light by particles in its path is called Tyndall effect. When a beam of light enters a smoke-filled dark room through a small hole, then its path becomes visible to us. The tiny dust particles present in the air of room scatter the beam of light all around the room. Thus, scattering of light makes the particles visible. 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.
A rainbow is a natural spectrum appearing in the sky Raindrop after a rain. It is produced by dispersion of sunlight by tiny water droplets, present in the atmosphere. The water droplets act like small prisms. When a ray of light falls on water drop (or raindrop) it undergoes refraction and dispersion to form a spectrum. This spectrum undegoes internal reflection (inside the raindrop) and finally refracted again when it comes out of the raindrop. After the dispersion of light and internal reflections, the band of colours reaches observer’s eye in the form of a rainbow.
A rainbow is always formed in the direction opposite to that of the sun.
What is meant by advance sunrise and delayed sunset. Draw a labelled diagram to explain.
The layers of air nearer to earth are denser than those above it. At sunrise and sunset when the sun is below the horizon, the light rays starting from sun are incident on these layers. They pass through successively denser layers and thus get bent more and more towards the normal until they fall upon the eye of the observer O.
To the observer O these rays appear to come from B which is above horizon. It is for this reason that the sun is visible to us a little before it rises above the horizon and so also till a little later it sets below the horizon. The difference of time is about 2 minutes each for early rise and late setting of the Sun.
Answer: Since the atmosphere consists of varying densities the apparent position of the object, as seen through the hot air fluctuates. This wavering of light is an effect of atmospheric refraction.
The twinkling of a star is due to atmospheric refraction of starlight. The atmospheric refraction of light occurs in a medium of gradually changing refractive index.
The planets are much closer to the earth and are thus seen as extended sources. A planet is considered as a collection of large number of point sized sources of light, the total variation in the amount of light entering our eye from all individual point sized sources will average out to zero, thereby nullifying the twinkling effect.
(a) What kind of defect in vision is she suffering from?
(b) What is the focal length of the corrective lens?
(c) What is the nature of the corrective lens?
Answer: Ability of the eye lens to focus nearby as well as distant objects on the retina by changing the curvature or focal length of the eye lens is known as power of accommodation.
lamge distance in the eye is the distance between the eye lens and the retina and it is fixed. As the object approaches from infinity towards the eye, the focal length of the eye lens decreases (or vice-versa) so as to maintain the same image distance.
Answer: This is due to the ability of the eye lens to adjust its focal length which is known as accommodation. When the cilary muscles are relaxed, the lens becoems thin. Thus, its focal length increases. This enables us to see distant objects clearly. When we are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens becomes thicker. The focal length of the eye lens decreases. This enables us to see nearby objects clearly.
The name of the defect is Presbyopia. Bifocal lens is required to improve the vision.
In bifocal lens, upper portion is of concave or diverging lens to view far off objects and lower part is of convex or converging lens to view nearby objects.
Explain, why a normal eye is not able to see distinctly the objects placed closer than 25 cm. without putting any strain on the eye.
(a) Lens becomes thin
Focal length increases.
(b) Curvature increrases.
Focal length decreases.
This is because focal length of the lens of a normal human eye cannot be decreased below certain limit i.e., 25 cm.
Millions of people of the developing countries are suffering from corneal blindness. This disease can be cured by replacing the defective cornea with the cornea of a donated eye. Your school has organised a campaign in the school and its neighbourhood in order to creat awareness about this fact and motivate people to donate their eyes after death. How can you along with your classmates contribute in this noble cause? State the objectives of organising such campaigns in schools.
1. Cornea: Refraction of the light rays falling on the eye.
2. Iris: To control the size of the pupil.
3. Pupil: To regulate and control the amount of light entering the eye.
4. Retina: To act as a screen to obtain the image of object and generate electrical signals which are sent to the brain via optic nerves.
We can contribute by participating in different ways of motivating people for the noble cause of eye donation such as, street play, banners, poster, door to door campaign, etc.
Objectives of organising such campaigns:
1. To develop the habit of group work
2. To work for a common cause
3. To understand social issues and problems.
Answer: Hypermetropia (Long-sightedness): A person can see distant objects distinctly but cannot see nearby objects so clearly in this case, the image is formed behind the retina. This defect of the eye is hypermetropia.
Causes of hypermetropia: This defect arises because either:
1. the focal length of the eye lens is too long, or
2. the eyeball becomes too short, so that light rays from the nearby object, say at point N, cannot be brought to focus on the retina to give a distinct image.
Hypermetropia can be corrected by using convex lens of suitable focal length in spectacles.
(a) What is myopia? State the two causes of myopia. With the help of labelled ray diagrams show
(i) the eye defect myopia
(ii) correction of myopia using a lens.
(b)Why is the normal eye unable to focus on an object placed within 10 cm from the eye?
(a) Myopia is the defect of the eye vision due to which a person can see the nearly objects clearly but cannot see the far objects so distinctly.
Causes of myopia: Myopia is caused:
• due to the elongation of the eyeball.
• due to decrease in the focal length of the eye lens.
Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.
(a) What is dispersion of white light? State its cause. Draw a ray diagram to show the dispersion of white light by a glass prism.
(b) A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain why it is so.
State the cause of dispersion of white light by a glass prism. How did Newton, using two identical glass prisms, show that white light is made of seven colours? Draw a ray diagram to show the path of a narrow beam of white light, through a combination of two identical prisms arranged together in inverted position with respect to each other, when it is allowed to fall obliquely on one of the faces of the first prism of the combination.
(a) The splitting up of white light into its constituent colours on passing through a refracting medium like a glass prism is called dispersion of light.
The dispersion of white light occurs because 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 as shown below.
(b) A glass slab acts as a combination of two identical glass prisms.
The second prism can be considered to be placed in an inverted position with respect to the first. The first prism splits the white light into its seven colour components. When these colour components fall on the second prism, it recombines them to form white light.
The defect of the vision is hypermetropia.
Given, that u = – 25 cm and y = –75 cm,
The + sign with for the power indicates a convergent lens.
The defect of vision is myopia.
The -ve sign with f indicates the concave lens.
Here, v= -3 m, u = -12 m, f = ?
A concave lens of focal length 4 m should be used.
Given: = -25 cm