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Question. (A) Explain the following term used in relation to defects in vision and correction provided by them:
(iii) Bifocal lenses
(B) Why is the normal eye unable to focus on an object placed within 10 cm from the eye?
(C) If due to some disease or injury, the cornea of an eye is clouded, then the vision is impaired and the person may become blind. This type of blindness may be cured by replacing the defective cornea with the cornea of the donated eye. Suggest some measures for motivating people to donate their eyes after death.
Ans. (A) (i) Myopia: In this defect a person can see nearby objects but not far away objects. It is corrected by using concave lens
(ii) Astigmatism: In this defect the eye cannot focus objects both horizontally and vertically at the same time. It is corrected by using cylindrical lenses.
(iii) Bifocal lenses: These are lenses which have both concave and convex structure. These are used when a person is suffering from presbyopia and myopia.
(iv) Far-sightedness: When a person can see faraway objects but not nearby objects. It is called far-sightedness. This defect is corrected by using convex lenses.
(B) To see an object placed at a distance of less than 25cm, the ciliary muscles have to make the focal length still smaller, Which cannot make the focal length so small. Hence, it is not possible to see a distance lens then 25 cm.
(C) Make people aware that donating an eye after death will give vision to two people and reduce their sufferings.
Question. How does the refractive index of earth’s atmosphere vary with height?
Ans. The hotter air in the atmosphere is lighter than the cooler air and has a refractive index slightly less than that of the cooler air. The refractive index of the upper hotter atmosphere is therefore less than the cooler layers of atmosphere below it
Question. Define power of a lens.
Ans. Power (P), of a lens is the reciprocal of its focal
length. Power = 1/f , where f is the focal length of the lens.
Question. Why does the emergent ray in a prism bend at an angle to the direction of the incident ray?
Ans. The emergent ray in a prism bends at an angle to the direction of the incident ray as both the refracting surfaces in a prism are inclined to each other.
CLASS X HUMAN EYE AND THE COLOURFUL WORLD WORKSHEET 4 SECTION A CONCEPTUAL QUESTIONS
Imagine for a moment that the earth has no atmosphere. What would be the colour of sky in such case? Give reason for your answer.
Newton placed two triangular prisms in inverted position with respect to each other. He is then let white light pass through them. What was his observation on screen? Draw diagram of this experiment.
The power of a lens is +1.5D. Name the type of defect of vision that can be corrected using this lens. Draw a ray diagram to illustrate this defect of vision.
What is meant by least distance of distinct vision? Mention its value.
Explain how is the focal length of eye lens is changed by the ciliary muscles when object distance when object distance from it changes?
Explain the formation of rainbow in the sky with the help of diagram. State the phenomena involved in proper sequence.
A person is unable to see the objects beyond 2m clearly. Name the defect of vision he is suffering from. State two causes due to which this defect may arise. Draw ray diagrams to show the defect of vision and correction of this defect using an appropriate lens.
Why is normal eye not able to see clearly the objects placed closer than 25cm?
What is atmospheric refraction? What is its cause?
How is the amount of light entering the eye controlled?
The time difference between actual sun set and the apparent sun set is 2 minutes. Explain this fact with the help of diagram
How does the size of particles affect the scattering of light?
Red lights are used as warning signals at road crossing. Why?
Name the following parts of human eye:
(i) A thin membrane through which light enters
(ii) The part of eye sensitive to light.
List the factors on which the angle of deviation through a prism depends.
Why do stars twinkle but not the planets?
“Stars seem higher than they actually are” – explain why?
Draw a neat diagram to show refraction of light ray through a prism. Mark angle of incidence, angle of emergence and angle of deviation.
Why does power to see clearly near objects as well as far off objects diminishes with age? Name the defect that is likely to arise in eye such condition.
Explain the structure and function of Human eye.
I )What is Tyndall effect?
ii) Draw a figure which shows the arrangement for observing the phenomenon
of scattering of light in laboratory? iii) What colours would you observe in the experiment? Why?
Section B Numerical problems
A person with a myopic eye cannot see beyond 1.2m distinctly. What should be the power of corrective lens used to restore his proper vision?
The near point of Hypermetropic eye is 75cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25cm from the eye?
The near point of Hypermetropic eye is 100cm from the eye. What is the power of the lens which he requires to read clearly a book held at 25cm from the eye?
The far point of a person is 2m. Find the power and focal length of the lens required to rectify this problem.
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