Q 1. Why is eye lens of telescope smaller than objective lens?
The eye lens of telescope is smaller than its objective lens to sharpen the image which is obtained using telescope.
Q 2. What is short sight? How can it be corrected?
If a person can see nearer objects clearly but not the far objects, then he is affected by short sight. It can be corrected using a suitable concave lens.
Q 3. Give the condition required to achieve a larger magnification of a small object by a compound microscope ?
To achieve a larger magnification of a small object, both the objective and the eyepiece of a compound microscope should have smaller focal lengths. Magnification of the order of 1000 is possible with very good design of the microscopes, such as in the oil-immersion type of microscopes.
Q 4. A person having a myopic eye used the concave lens of focal length 50cm. What is the power of the lens?
The formula for the power of lens is Power (P) = 1/f.
P = 1/50 = 0.02 dioptre.
Q 5. Which part of the human eye helps in the perception of colours?
The retina of the human eye has a large number of cone-shaped and rod-shaped cells. The rods respond to the intensity of light and the cones to the colours. These help us to perceive colours.
Q 6. Define the term “accommodation of the eye”.
The ability of the eye to focus on near and distant objects, by adjusting its focal length, is called the accommodation of the eye.
Q 7. Why do different components of white light deviate through different angles when passing through a triangular glass prism?
Due to change in refractive index offered by the medium.
Q 8. Give an example of a phenomenon where Tyndall effect can be observed.
When a fine beam of sunlight enters a room containing suspended particles of dust, the path of the beam of light is visible. It is due to the scattering of light (Tyndall effect).
Q 9. What is the cause of dispersion of white light on passing through a prism?
The refractive index of the material of a prism is different for different colours of light as different colours have different speeds in the material of a prism. Also, prism has nonparallel surfaces.
Q 10. Define “least distance of distinct vision”.
The smallest distance, at which the eye can see objects clearly without strain, is called the near point or the least distance of distinct vision. For a young adult with normal vision, it is about 25cm.
Q 11. Differentiate between a microscope and a telescope based on its use.
A microscope is an optical instrument which is used to see the finer details of tiny objects, and a telescope is used for viewing distant objects clearly.
Q 12. Why does the sun appear reddish at sunrise?
At sunrise, the sun looks almost reddish because only red colour ( λb < λr ), which is least scattered is received by our eye and appears to come from the sun. Hence, the appearance of sun at sunrise, near the horizon may look almost reddish.
Q 13. How is the length of the telescope tube related to the distance between the objective and the eyepiece of the telescope? How can the magnification of the telescope be increased?
The length of the telescope tube is equal to the distance between the objective and eyepiece of the telescope. In order to increase the magnification of the telescope, the objective must have a large focal length and the eyepiece a small focal length.
Q 14. Why does the objective lens of an astronomical telescope have a large light gathering power?
The objective lens of a telescope has a large aperture to have a large light gathering power.
Q 15. What is Astigmatism?
Astigmatism is the defect of an eye due to which a person cannot focus on both the horizontal and vertical lines, simultaneously. The reason for this defect is irregularities on the surface of the cornea.
Q 16. Explain the angle of prism.
A triangular glass prism consists of three rectangular lateral surfaces and two triangular bases. Lateral faces are inclined to each other and the angle between two lateral surfaces is called the angle of prism.
Q 17. What is dispersion?
The splitting of white light into its seven colours on passing through a transparent medium (e.g. glass prism) is called dispersion of light.
Q 18. What is spectrum?
When a white light is passed through a transparent medium, the white light splits into seven colours which are made to fall on a white screen. This coloured band is called spectrum.
Q 19. Why is the colour of the clear sky blue?
The scattering of sunlight due to the air molecules in the atmosphere causes the sky to appear blue. Shorter wavelength (Blue colour) is scattered more compared to longer wavelength (Red colour). The scattered blue light enters our eyes.
Q 20. What is phorias?
This is a defect of eye. With this defect, the eye sees two images of one object. This defect is due to lack of balance among the muscles which hold the two eyes in their axes. This lack of balance produces cross-eyes. It is remedied by using Prismatic lenses.
Q 21. Why is the colour of the clear sky blue?
The scattering of sunlight due to the air molecules in the atmosphere causes the sky to appear blue. Shorter wavelength (Blue colour) is scattered more compared to longer wavelength (Red colour). The scattered blue light enters our eyes.
Q 22. What is optic axis?
The straight line passing through the centre of the cornea and the lens is called optic axis of the eye.
Q 23. What is visual axis?
The line passing through centre of the lens and fovea centralize is called visual axis of the eye. When an object is to be seen more minutely, it is brought on the visual axis to get its image on yellow spot which is the most sensitive part of the retina.
Q 24. Define least distance of distinct vision.
Least distance of distinct vision: Minimum distance at which an object can be seen distinctly without any strain from the normal eye, i.e., 25 cm for normal vision.
Q 25. Define Tyndall effect.
Tyndall effect: The phenomenon of scattering of light by colloidal particles gives rise to Tyndall effect.
Tyndall effect can be observed when sunlight passes through a canopy of a dense forest. Here tiny droplets in mist scatters light.
Q 26. Define atmospheric refraction.
Atmospheric refraction: If physical conditions of the refracting medium (air) are not stationary, the apparent position of the object fluctuates.
Q 27. Why are danger signal lights red in colour?
Danger signal lights are red in colour because red colour is least scattered by fog or smoke.
Q 28. Is the position of a star as seen by us in its exact position? Justify your answer.
No, 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.
Q 29. Why do we see a rainbow in the sky only after rainfall?
The rainbow in the sky appears only after rainfall because the suspended water drops behave like prism, and refract, disperse and reflect the light rays internally.
Q 30. On which factor does colour of scattered light depends?
The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light while particle of larger size scatters light of longer wavelengths.
Q 31. On the rainy day, Ram reached his grandfather’s place in village. On the way to the house he saw a beautiful rainbow in the sky. In the night, he saw lots of twinkling stars in the clear sky. He was very excited to see these beautiful natural phenomenon, which he was not able to see in the city, where he lived with his father. Explain the phenomenon on the basis of science. Do you think that pollution in the atmosphere affects the formation of rainbow and twinkling of stars. Do you agree with the fact that pollution free environment will strengthen such natural phenomenon in the cities as well. Elaborate.
- The twinkling of star is due to atmospheric refraction.
- The formation of rainbow is due to dispersion, refraction and internal reflection.
- Yes, pollution in atmosphere affects the formation of rainbow and twinkling of stars.
Advance sunrise and delayed sunset is due to atmospheric refraction.
When the sun is slightly below the horizon, the sunlight coming from the less dense (vacuum) to more dense (air) medium is refracted downwards. So the sun appears to be above the horizon.
Q 33. Explain formation of rainbow.
Rainbow formation: A rainbow is a natural spectrum appearing in the sky after rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. The water droplets act like small prism. They refract and disperse the incident sunlight, then reflect it internally and finally refract it again.
Due to dispersion of light and internal reflection different colours appear.
Q 34. Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.
Refraction of light through prism
PE – Incident ray
EF – Refracted ray
FS – Emergent ray
∠A – Angle of the prism
∠i – Angle of incidence
∠r – Angle of refraction
∠e – Angle of emergence
∠D – Angle of deviation
Refraction of light through a triangular glass prism
- The refraction of light takes place at two surfaces firstly when light enters from air to prism and secondly when light emerges from prism.
- Angle of prism: The angle between the two lateral faces of the prism is called angle of prism.
- Angle of deviation: The angle between incident ray (produced forward) and emergent ray (produced backward).
Q 35. What is the difference in colours of the sun observed during sunrise/sunset and noon? Give explanation for each.
In the morning and evening, the sun lies near the horizon. Sunlight travels through a larger distance in the atmosphere and most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelength. This gives rise to the reddish appearance of the sun.
At noon sun appears white as only a little of blue and voilet colours are scattered.
Q 36. Define the term dispersion of white light. Name the colour of light which bends (i) the most, (ii) the least, while passing through a glass prism. Draw a ray diagram to justify your answer.
Dispersion of white light by a glass prism
Dispersion: The splitting of light into its component colours is called dispersion.
The red light bends the least while violet bends the most.
Spectrum: The band of the coloured components of a light beam is called spectrum. i.e., VIBGYOR
Q 37. Explain twinkling of stars.
Twinkling of stars:
- The twinkling of stars is due to atmospheric refraction of starlight.
- When starlight enters the earth’s atmosphere, it suffers refraction continuously. Since the physical conditions of the earth’s atmosphere are not stationary the stars appear twinkling.
Q 38. List four common refraction defects of vision. Suggest the way of correcting these defects.
Defects of vision:
(i) Cataract: Crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract.
It is possible to restore vision through cataract surgery.
(ii) Myopia: (Near sightedness)
A person with myopia can see nearby objects clearly but cannot see distant objects clearly.
Cause:
- Due to excessive curvature of the eye lens.
- Elongation of the eyeball.
Correction:
Concave lens of suitable power.
(a) Far point of myopic eye
(b) Myopic eye
(c) Correction for myopia
a), (b) The myopic eye, and (c) correction for myopia with a concave lens
(iii) Hypermetropia (far-sightedness)
A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
Cause:
- The focal length of the eye lens is too long.
- The eyeball has become too small.
Correction:
Convex lens of suitable power.
Q 40. What is meant by spectrum of white light? How can we recombine the components of white light after a prism has separated them? Draw a diagram to illustrate it
The coloured pattern VIBGYOR formed by a prism by splitting the incident white light is called a spectrum. By having two prisms, inverted to each other, one can recombine the light to get white light again..
Q 41. Explain why do the planets not twinkle but the stars twinkle.
Planets being of larger size can be taken as a collection of large number of point-sized objects/sources of light, which nullify the twinkling effect of each other
Due to varying conditions of atmosphere, starlight undergoes multiple refraction and its path varying slightly while passing through the atmosphere. Therefore, the apparent position of star fluctuates and amount of light entering the eye changing continuously. The star sometimes appear brighter and some other time, it appears fainter. This causes twinkling of star.
Q 42. What is myopia (near-sightedness)? Draw a ray diagram to show how it can be corrected using a lens.
Myopia is the inability of an eye in viewing long distant objects. The image in this case is formed before the retina. For every myopic eye, there exists a far point beyond which clear image cannot be seen.
The short-sightedness is corrected by using a concave lens which diverges and shifts the image to the retina.
Q 43. What is hypermetropia (far-sighted-ness)? Draw a ray diagram to show how this defect can be corrected using a lens.
Hypermetropia is the inability of an eye in viewing the nearby objects. The image in this case is formed beyond the retina. For a hypermetropic eye, there exists a near point. It is corrected by using a convex lens, which converges and shifts the image to the retina.
Q 44. (a) What are the values of (i) near point and (ii) far point of vision of a normal adult person?
(b) A student has difficulty in reading the blackboard while sitting in the last row. What could be his defect of vision? Draw a ray diagram to illustrate this defect of vision.
(a) (i) 25 cm
(ii) Infinity (∞)
(b) Short-sightedness. In this case, the image falls before the retina. It can be corrected by using a concave lens.
Q 45. What is meant by the term ‘power of accommodation’ of human eye? How does it help a person to see nearby as well as distant objects clearly.
The ability of eye lens to adjust its focal length to form the sharp image of the object at varying distances on the retina is called its power of accommodation. When we see the nearby object, the ciliary muscles contract, it increases the thickness of eye lens. The eye lens then becomes thicker. As a result, the focal length of eye lens decreases in such a way that the clear sharp image of nearby object is formed on the retina. Thus, the object is seen clearly to us.
When we see the distant object, these muscles become relaxed, thus the eye lens becomes thinner, and consequently focal length of the lens increases. Therefore, the parallel rays coming from the distant object are focused on the retina and object is seen clearly to us. Thus, the accommodation power of an eye helps a person to see nearby as well as distant objects clearly.