physics - the human eye and the colourful world

Class 10 - Physics

The Human Eye and The Colourful World

Q&A
Question:

Explain presbyopia and astigmatism.

Answer:

Presbyopia is part of the natural aging process of the eye, and can be easily corrected. Technically, presbyopia is the loss of the eye’s ability to change its focus to see objects that are near. It is not a disease. It’s as natural as wrinkles, and it affects everybody at some point in life. Presbyopia generally starts to appear around age 40.
Astimagtism: The cornea is the transparent layer over the colored part of the eye. It bends (refracts) light rays and helps focus the light onto the retina in the back of the eye so people can see. When the cornea is oblong shaped, it causes light rays to focus on two different points on the retina, instead of just one. As a result, people with significant astigmatism may have distorted or blurry vision.



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Questions and Answers


The Human Eye and The Colourful World - Questions
1.

How does the eye adjust its focal length?


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2.

Give the condition required to achieve a larger magnification of a small object by a compound microscope ?


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3.

How is the terrestrial telescope different from the astronomical telescope?


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4.

What is visual axis?


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5.

How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.


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6.

Differentiate between a microscope and a telescope based on its use.


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7.

Which of the two, objective or eye-piece, of a telescope has a large aperture? Give reason for your answer.


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8.

Explain clearly the working of a human eye?


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9.

What is visual axis?


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10.

Why do stars twinkle?


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11.

What property of the eye is the principle of motion, pictures?


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12.

Is the position of a star as seen by us its true position? Justify your answer.


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13.

What is meant by power of accommodation of the eye?


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14.

How does the eye adjust itself to deal with light of varying intensity?


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15.

Write the names of colours got from the word VIBGYOR.


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16.

Explain presbyopia and astigmatism.


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17.

What is dispersion?


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18.

Why does the objective lens of an astronomical telescope have a large light gathering power?


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19.

What are coaxial lenses and where are they used?


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20.

How will you distinguish between a compound microscope and a telescope.


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21.

Why is the colour of the clear sky blue?


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22.

What is short sight? How can it be corrected?


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23.

Which part of human eye helps in the perception of colours?


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24.

What are the common defects of vision that can be corrected by the use of suitable eyeglasses or spectacles?


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25.

What is Astigmatism?


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26.

Define the term “accommodation of the eye”.


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27.

Explain the angle of prism.


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28.

A lens used as a simple magnifier gives magnification of 6. What is its focal length?


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29.

What is the far point and near point of the human eye with normal vision ?


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30.

What is spectrum?


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31.

Where should an object be placed in front of a convex lens so as to use it as a simple microscope?


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32.

Define “least distance of distinct vision”.


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33.

Why does the sky appear dark of blue to an astronaut?


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34.

What is optic axis?


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35.

Why is eye lens of telescope smaller than objective lens?


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36.

A person having a myopic eye used the concave lens of focal length 50cm. What is the power of the lens?


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37.

Why does the sun appear reddish early in the morning?


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38.

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?


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39.

If focal length of the objective and eye-piece of a telescope are 10 cm and 4 cm respectively, what will be its magnifying power in
(i) normal adjustments,
(ii) adjustment where image of distance object is formed at D? Given D = 25 cm.


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40.

A person needs a lens of power – 4.5 D for correction of her vision.
(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?


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41.

A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from ? How can it be corrected ?


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42.

Which phenomenon of vision is made use of in cinematography? Explain briefly how it is used.


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43.

How does an eye focus the objects lying at various distances?


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44.

Draw a diagram of a simple microscope.


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45.

A compound microscope has an objective of focal length 1.0 cm and an eyepiece of focal length 4.0 cm. If the length of the tube is 20 cm, find the magnification of the compound microscope.


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46.

What is phorias?


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47.

A 52-year old near-sighted person wears eye-glass with a power of –5.5 dioptres for distance viewing. His doctor prescribes a correction of +1.5 dioptres in the near-vision section of his bi-focals. This is measured relative to the main part of the lens (i) What is the focal length of his distance-viewing part of the lens? (ii) What is the focal length of the near-vision section of the lens?


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48.

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?


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49.

A person needs a lens of power −5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?


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50.

Explain why the planets do not twinkle ?


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