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Question-1

Why is eye lens of telescope smaller than objective lens?

Solution:
The eye lens of telescope is smaller than its objective lens to sharpen the image which is obtained using telescope.

Question-2

What is short sight? How can it be corrected?

Solution:
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.

Question-3

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

Question-4

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

Solution:
The formula for the power of lens is Power (P) = 1/f.
P = 1/50 = 0.02 dioptre.

Question-5

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

Solution:
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.

Question-6

Define the term "accommodation of the eye".

Solution:
The ability of the eye to focus on near and distant objects, by adjusting its focal length, is called the accommodation of the eye.

Question-7

Define "least distance of distinct vision".

Solution:
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.

Question-8

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

Solution:
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.

Question-9

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?

Solution:
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.

Question-10

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

Solution:
The objective lens of a telescope has a large aperture to have a large light gathering power.

Question-11

Explain clearly the working of a human eye?

Solution:
The light rays coming from the object kept in front of us enter the pupil of the eye and fall on the eye-lens. The eye-lens is a convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision. The light-sensitive cells present in the retina gets activated and generates electrical signals. The retina sends these electrical signals to the brain through the optic nerve and gives rise to the sensation of vision. The mind interprets the inverted image formed on the retina as an erect object.

Question-12

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

Solution:
The amount of light entering the eye is controlled by the pupil. If the intensity of the outside light is low, then the pupil expands to allow more light to enter the eye. If the outside intensity is high, then the pupil contracts so that less light enters the eye. 

Question-13

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

Solution:
Persistence of vision is made use of in cinematography. It is the ability of an eye to continue to see the image of an object for a very short duration even after the removal of the object. We are able to see the movie pictures in a cinema due to the persistence of vision.

The still pictures taken by a movie camera 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 till the image of 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 into one another and give us the feeling of moving images. Thus the principle of persistence of vision is made use of, in the motion picture projection in cinematography.


Question-14

How do we see colours?

Solution:
The retina of our eyes has a large number of light sensitive cells. The cells on the retina are of two shapes:

(i) Rod shaped and
(ii) Cone shaped

The rod shaped cells of our retina respond to the intensity of light. The cone shaped cells of our retina respond to colours. These cells make us see colours and to distinguish between them. Thus cones make colour perception possible.

Question-15

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

Solution:
An eye can focus the images of the distant objects as well as that of the nearby objects on its retina by changing the focal length of its lens. The focal length of the eye-lens is changed by the action of the ciliary muscles. The ciliary muscles can change the thickness of the eye-lens and hence its focal length changes the converging power of the eye-lens.

Question-16

When is a person said to have developed cataract in his eye? How is the vision of a person having cataract restored?

Solution:
Cataract develops when the eye-lens of a person becomes unclear due to the formation of a membrane over it. The vision of a person having cataract can be restored after getting cataract surgery done on the eye-lens having cataract. This defect cannot be corrected by any type of spectacle lenses.

Question-17

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

Solution:
The object should be placed within the focus of the convex lens to use it as a simple microscope.

Question-18

If we need magnification of 375 from a microscope of tube length 15 cm and objective of focal length 0.5 cm, what focal length of eye-piece should we use?

Solution:
Magnification m = 375
Length of the tube L = 15 cm
Focal length of the objective fo = 0.5 cm

We know that
Magnification m =
                 375 =
                       =
                   25 =
                25fe = 50 + 2 fe
                23fe = 50
       fe = 50/23 = 2.17 cm.

Question-19

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.

Solution:
Focal length of objective fo = 10 cm
Focal length of eyepiece fe = 4 cm
(i) In normal adjustment
m = = 2.5

(ii) when the final image of distant object is at D, then
m =
    =
    = 2.9

Question-20

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

Solution:
The objective lens of a telescope has a large aperture, because it can gather more light from the distant object and form a bright image of the distant object.

Question-21

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

Solution:
There are mainly four common defects of vision that can be corrected by the use of suitable eye glasses or spectacles. These are

(i) Myopia or near-sightedness,
(ii) Hypermetropia or far-sightedness,
(iii) Presbyopia, and
(iv) Astigmatism.

Question-22

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

Solution:
Refraction of light through the eye lens is the basic principle for the visualization of pictures and motion.

Question-23

What are coaxial lenses and where are they used?

Solution:
Coaxial lenses are lenses having common principal axes. A compound microscope, in its simple form, consists of coaxial lens (two lenses having common principal axes) mounted on the outer ends of two hollow metallic tubes. One of the two lenses acts as the objective and the other as the eyepiece.

Question-24

Draw a diagram of a simple microscope.

Solution:

Question-25

How does the eye adjust its focal length?

Solution:
The eye adjust its focal length by means of its ciliary’s muscles.

Question-26

Explain presbyopia and astigmatism.

Solution:
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.


Question-27

A person uses convex lens spectacles. What vision defect does he have? Draw a diagram

(i) to show the defective eye
(ii) to show the correction with the lens.

Solution:
When a person uses convex lens to cure his long sight (hypermetropia).


Long Sight


Question-28

How is the terrestrial telescope different from the astronomical telescope?

Solution:
The terrestrial telescopes have one more additional convex lens (erecting lens), placed between the objective and the eyepiece, to make the final image erect. This erecting lens is not required in the astronomical telescope as the celestial objects that are viewed using a astronomical telescope are spherical in shape, so it does not matter whether the final image is inverted or erect.

Question-29

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

Solution:
A compound microscope uses a very short focal length objective lens to form a greatly enlarged image. This image is then viewed with a short focal length eyepiece used as a simple magnifier. The image should be formed at infinity to minimize eyestrain.

A telescope is an instrument designed for the observation of remote objects. The term usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation and for other signal types.


Question-30

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

Solution:
Magnification = 6

Focal length, f = = 4.17 cm
(Taking D as the least distance of distinct vision).


Question-31

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.

Solution:
focal length of the object, f0 = 1.0 cm
Eyepiece of focal length, fe = 4.0 cm
Tube–length of the microscope, L = 20 cm
Magnification of compound microscope, m =

                                                             =
                                                             = 5 × 25

                                                             = 125

Magnification of compound microscope is, m = 125.


Question-32

A telescope has an objective of focal length 140.0 cm and an eyepiece of focal length 5.0 cm. Find
(i) the magnification of the telescope for viewing distant objects for normal adjustment
(ii) separation between the objective lens and the eyepiece.

Solution:
Objective of focal length, f0 = 140.0 cm.
Eyepiece if focal length, fe = 5.0 cm.
(i) The magnification of the telescope, m =
                                                           =
                                                           = 28.
(ii) Separation between the objective lens and the eyepiece,
                     
                     L = f0 + fe
                       = 140 + 5

                       = 145 cm = 1.45 m.

Question-33

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?

Solution:
(i) Focal length of lens for distance viewing f1 =
                                                                  = -0.182 m
                                                                  = -18.2 cm

(ii) Focal length for near-vision f2 =
                                                 = 0.667 m = 66.7 cm.

Question-34

What is Astigmatism?

Solution:
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.

Question-35

Explain the angle of prism.

Solution:
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.

Question-36

What is dispersion?

Solution:
The splitting of white light into its seven colours on passing through a transparent medium (e.g. glass prism) is called dispersion of light.

Question-37

What is spectrum?

Solution:
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.

Question-38

Why is the colour of the clear sky blue?

Solution:
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.

Question-39

What is phorias?

Solution:
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.

Question-40

Write the names of colours got from the word VIBGYOR.

Solution:
V - Violet

I - Indigo

B - Blue

G - Green

Y - Yellow

O - Orange

R – Red.


Question-41

Why is the colour of the clear sky blue?

Solution:
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.

Question-42

What is optic axis?

Solution:
The straight line passing through the centre of the cornea and the lens is called optic axis of the eye.

Question-43

What is visual axis?

Solution:
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.




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