- If an object is placed in front of the eye then an image of the object is formed on the retina of the eye.
- Eye lens: It is a bi-convex lens of variable focal length made of a transparent and flexible substance. The average refractive index for different parts of the lens is approximately 1.45.
- Accommodation of eye: The ability of the eye to alter its focal length and hence to focus the images of objects at various distances on the retina is called accommodation of the eye.
- The minimum distance from the eye up to which objects can be seen clearly and easily is known as the least distance of distinct vision.
- The point at the minimum distance at which an object can be seen easily and clearly by the eye is called the near point of the eye. In case of normal eye it is about 25 cm from the eye.
- The distance between the near and far points of the eye is called the range of vision. Within this range, wherever the object is situated, our eyes can see it by accommodation.
- The pupil of our eyes automatically contracts in light of high intensity and expands in light of low intensity. This ability of our eyes is called adaptation of eye.
- When an object produces its image on the retina of the eye, the impression thus produced does not disappear immediately after the object is removed. The impression persists for \frac1{10} second in our brain and it is known as persistence of vision.
- When we see an object, each eye forms an image of the object, so that two sets of impressions reach the brain simultaneously and the brain correlates these to get a single impression of the object. As a result, a three-dimensional view of the object can be seen. This kind of vision of three-dimensional impression of an object by two eyes is known as binocular vision.
- Different kinds of defects of vision:
| Name | Characteristics | Remedies |
| Hypermetropia or long-sightedness | If the eye can see the distant objects clearly but cannot see the near objects clearly then this kind of defect of the eye is called hypermetropia. | A person suffering from hypermetropia should use positive power lens i.e, a convex lens. |
| Myopia or short-sightedness | If the eye can see the near object clearly but cannot see the distant objects clearly then this kind of defect is called myopia. | A person suffering from myopia should use a negative power lens i.e, a concave lens. |
| Presbyopia | With age, ciliary muscles of the eye loses its elasticity and a normal eye gradually loses its power of accommodation. As a result the near point recedes away from the eye than normal distance of distinct vision and this kind of defect is called presbyopia | For the correctness of this kind of defect spectacles made of a convex lens of suitable focal length should be used. |
| Astigmatism | It is an optical defect, in which, vision becomes blurred. It is due to the inability of the eye to focus a point object into a sharp focused image on the retina. | For the correction this kind of defect cylindrical lens should be used. |
- The instrument by which we can see a magnified image of very small objects is called a microscope. There are two types of microscopes: (i) Simple microscope or magnifying glass and (ii) Compound microscope.
- (i) To observe planets, stars etc. in the sky a telescope is used. Here, the final image becomes virtual and inverted with respect to the object. (ii) To see the distant objects on the earth’s surface a terrestrial telescope is used. In this case, the image becomes virtual but erect with respect to the object. (iii) In Galilean telescope the objective lens is convex but the eyepiece lens is concave.
- The power for separating the images of two objects lying close to each other is called the resolving power of an optical instrument.
Useful Relations for Solving Numerical Problems
- Angular magnification = (angle subtended by the image at the eye)/(angle subtended by the object at the eye)
- Angular magnification in case of compound microscope,
m=\frac{L}{F_0}.\frac{D}{f_e}
[where L = length of the tube= v + f_e, D = the least distance of distinct vision, f_0 = focal length of the objective lens, f_e = focal length of the eyepiece]
- In case of focusing at infinity, angular magnification of a telescope,
m =\frac {f_0}{f_e} = diameter of the objective/diameter of the eye-piece and length of the telescope tube, L = f_0+f_e
In case of focusing for clear vision, angular magnification, m = \frac {f_0}{f_e} (\frac{f_e}{D} + 1)
and length of the telescope tube, L = \frac {Df_e}{D+f_e}
MULTIPLE CHOICE QUESTIONS
Structure of Human Eye
- Normal eye cannot see the object nearby from a distance of 25 cm, as
- focal length of the eye is 25 cm
- distance between the eye lens and the retina is 25 cm
- the eye is unable to adjust the distance between the lens and the retina below a certain limit
- the eye is unable to adjust the focal length of the eye lens below a certain limit.
- Ability of the eye to see objects at all distances is called
- binocular vision
- myopia
- hypermetropia
- accommodation
- If there had been one eye of the man then
- image of the object would have been inverted
- visible region would have decreased
- image would have not been seen three dimensional
- 2 and 3 both
- Image formed on retina of eye is proportional to
- size of object
- area of object
- size of object/size of image
- size of image/size of object
- The numerical aperture for a human eye is of the order of
- 1
- 0.1
- 0.01
- 0.001
Defects of Vision and Their Remedies
- What type of lens should be used in spectacles for the remedy of myopia?
- concave lens whose focal length is equal to the distance of the far point of the defective eye from the lens
- convex lens whose focal length is equal to the distance of the far point of the defective eye from the lens
- concave lens whose focal length is equal to the distance of the near point of the defective eye from the lens
- convex lens whose focal length is equal to the distance of the near point of the defective eye from the lens
- What type of lens should be used in spectacles for the remedy of hypermetropia?
- concave lens whose focal length = \frac{25d}{25-d} ; where d =distance of the far point of the defective eye from the lens
- concave lens whose focal length = \frac{25d}{25-d} where d= distance of the far point of the defective eye from the lens
- convex lens whose focal length = \frac{25d}{25-d} where d = least distance of distinct vision of the defective eye
- concave lens whose focal length = \frac{25d}{25-d} where d = least distance of distinct vision of the defective eye
- A person who can see things most clearly at a distance of 10cm, requires spectacles to be able to see clearly things at a distance of 30cm. What should be the focal length of the spectacles?
- 15 cm (concave)
- 15 cm (convex)
- 10 cm
- zero
- The least distance of distinct vision of a man is 45 cm. He uses a lens of focal length 15 cm for reading. The magnification which he gets
- 4
- 3
- 2
- 1
Microscope
- 10 The image formed by the objective of a compound microscope is
- virtual and magnified
- virtual and diminished
- real and diminished
- real and magnified
- To get a large magnification from a compound microscope,
- the focal length of the objective should be large while the focal length of the eyepiece should be small.
- the focal length of the objective should be small while focal length of the eyepiece should be large
- both of the focal lengths of the objective and the eye- piece should be large
- both of the focal lengths of the objective and the eyepiece should be small
- The length of the tube of a compound microscope is 21.5 cm. The focal lengths of the objective and the eyepiece are 1.6 cm and 2.1cm respectively. If the final image is situated at infinity, then the distance of the object from the objective
- 3 cm
- 1.7 cm
- 6 cm
- 4.8 cm
- 13 The angular magnification of a simple microscope can be increased by
- increasing the focal length of the lens
- increasing the size of the object
- increasing the aperture of the lens
- increasing the power of the lens
- 14 The length of the tube of a microscope ls 14 cm and its magnifying power for normal eye is 25. The focal length of the eyeplece is 5 cm. The distance of the object from the objective is
- 2.4 cm
- 2.5 cm
- 3.6 cm
- 1.8 cm
- Magnifying power of a compound microscope is 100. If the magnification produced by the eyepiece is 5, then the magnification by the objective will be
- 40
- 30
- 20
- 10
Telescope
- A Galileo’s telescope has an objective of focal length 100 cm and magnifying power 50. The distance between the two lenses in normal adjustment is
- 106 cm
- 92 cm
- 98 cm
- 102 cm
- In a Galileo’s telescope, the inverted image formed by its objective serves as a virtual object for its eyeplece. If the eyepiece has to form an inverted and magnified Image of the virtual object, the eyepiece has to be a concave lens and it must be so placed that the virtual object falls
- within F
- between F and 2F
- at 2P
- beyond 2f
- In an astronomical telescope, if the focal lengths of the objective and the eyepiece are f_0 and f_e respectively, then the magnification of this instrument is almost
- f_0 + f_e
- f_0\times f_e
- \frac{f_0}{f_e}
- \frac12(f_0 + f_e)
- In case of normal focusing of an astronomical telescope, the final image is formed at
- focus of the eyepiece
- least distance of distinct vision
- focus of the objective
- infinity
- 20 An astronomical telescope has two lenses of power 0.5 D and 20 D. The magnifying power of the telescope is
- 8
- 20
- 30
- 40
- The angular magnification of an astronomical telescope will be maximum, if the focal lengths of the objective and the eyepiece are respectively
- 1 m and 5 cm
- 3 m and 4 cm
- 2m and 6 cm
- 4m and 3 cm
VERY SHORT ANSWER TYPE QUESTIONS
Structure of Human Eye
- Give a practical application of persistence of vision. [cinema]
- The impression of a three-dimensional image created by our two eyes is called ________ [Fill in the blank]. [binocular vision]
- For a normal eye the least distance of distinct vision is ________ [Fill in the blank]. [25 cm]
- The far point of a normal eye is ________ [Fill in the blank]. [at infinity]
- Vision of normal eye ranges from ________ to ________ [Fill in the blanks]. [25 cm, infinity]
Defects of Vision and Their Remedies
- How can the defect of astigmatism be corrected? [by using spectacles with cylindrical or spherocylindrical sense]
- The minimum distance of distinct vision for a person is 1 m. What eye defect does he suffer from? [long-sightedness]
- ________ is the eye-defect which old people usually suffer from [Fill in the blank]. [presbyopia]
- Cylindrical lens are used as a remedy for ________ [Fill in the blank]. [astigmatism]
- For long-sightedness, ________ lens should be used [Fill in the blank]. [convex]
- For short-sightedness, ________ lens should be used [Fill in the blank]. [concave]
- A person uses spectacles of power + 2 D. His defect of vision is called ________[Fill in the blank]. [long sight or hypermetropia]
Microscope
- If the least distance of distinct vision is D and focal length of the lens is f then what is the equation for magnification in a simple microscope? [ 1+ \frac{D}{F} ]
- If the length of the tube of a compound microscope is increased, the magnification increases-Is this statement true or false? [true]
- If the focal length of a microscope is small, magnification is ________[Fill in the blank]. [large]
- To increase magnification of a compound microscope, the objective and the eyepiece of ________ focal lengths and the microscope tube of ________ length are to be taken [Fill in the blanks]. [small, large]
Telescope
- In what type of telescope is the final image erect? [in terrestrial telescope and Galilean telescope]
- What is used for the objective of a reflecting telescope? [a concave mirror]
- Write the names of two ordinary types of the telescope. [astronomical telescope and terrestrial telescope]
- Is the length of the instrument to be changed if the focal length of the objective of an astronomical telescope is increased? [Yes, the length will incease]
- In an astronomical telescope, the focal length and the aperture of the obiective are ________ and the focal length and the aperture of the eyepiece are taken ________ as compared to those of the objective [Fill in the blanks]. [large, small]
- The final image is an astronomical telescope is ________ and ________ with respect to the object [Fill in the blanks]. [virtual, inverted
- The final image in a terrestrial telescope is ________ with ________ respect to the object [Fill in the blanks]. [virtual, erect]
- In Galileo’s telescope, the objective is ________ lens but the eyepiece is ________ lens [Fill in the blanks]. [convex, concave]
SHORT ANSWER TYPE QUESTIONS-I
- Why is it difficult to thread a needle by seeing through one eye?
- What is the advantage of two eyes instead of a single one?
- If the light is suddenly extinguished at night in a lighted room explain why we do not see anything in the room momentarily.
- When we suddenly come out into the sunlit area from our rooms why are we blinded by light momentarily?
- How can an astronomical telescope be converted to a terrestrial telescope?
- Write two points of difference between astronomical telescope and binocular.
- Write two points of dissimilarity between an astronomical telescope and a Galilean telescope.
- Why do we consider angular magnification instead of linear magnification in case of optical instruments?
SHORT ANSWER TYPE QUESTIONS-II
- Why do we prefer reflecting type telescope for astronomical purposes?
- What is the difference between the ‘angular magnification’ and ‘magnification’ produced by a lens?
- A telescope increases the brightness of a point source (such as a distant star) but not the brightness of the background compared towhat is seen by the unaided eye. Thus it facilitates observation by improving the contrast between a star and its background. Explain the statement carefully.
- A hypermetropic person prefers to remove his spectacles while looking at the sky. Explain why.
- Why must both the objective and the eyepiece of a compound microscope have short focal lengths?
PROBLEM SET-l
Defects of Vision and Their Remedies
- A shortsighted man can read the writings of a book keeping it at a distance of 15 cm from his eyes. To read the book placed at a distance of 60 cm from his eyes what type of spectacles should he use? What will be the power of the spectacles? [concave lens, – 5 m^{-1} ]
- A man uses spectacles of a concave lens for his defective eye and can read a book placed at a distance of 25 cm from him. The focal length of the lens of the spectacles is 50 cm. At what distance should the book be kept if he wants to read it without spectacles? [at a distance of 16.67 cm from his eyes]
- By using the spectacles of a concave lens of power – 5 m^{-1} a man can read a book keeping it at a distance of 30 cm from his eyes. Without spectacles what is his least distance of distinct vision? What is the nature of defect in his eyes? [12 cm; short-sight
- A boy can see the objects distinctly only in between 15 cm and 200 cm from his eyes. What should be the power of the lens (in – m^{-1} unit) he will use to see the objects at infinity distinctly? What will the least distance of distinct vision of his eyes with spectacles? [16.22 cm ]
- The near point of a man is at a distance of 200 cm from his eyes. What spectacles should he use in order to read a print 25 cm away from his eyes? [convex lens, 3.5 m^{-1} ]
- The far point of a myopic (short-sighted) person is 80 cm in front of the eye. What is the power of the lens required to enable him to see very distant objects clearly. [ -1.25 m^{-1} ]
- The near point of a hypermetropic (long-sighted) person is 75 cm from the eye. What is the power of the lens required to enable the person to read clearly a book held at 25 cm from the eye? [ m^{-1} ]
- The least distance of distinct vision of a man is 1 m. What type of eye defect does he suffer from? What is the power of the lens of his spectacles? [long-sight; 3 m^{-1} ]
- A long-sighted man can see distinctly any object situated at a distance of 2.5 m from him. If the man wants to read a book kept at a distance of 25 cm from his eyes, what type of lens will he use in the spectacles. [convex lens, 27.78 cm ]
Microscope
- The focal length of the lens of a simple microscope is 5 cm. If the lens is held near the eyes the image is formed at 25 cm i.e., at the least distance of distinct vision. What is the magnifying power of the instrument? [6]
- Two lenses having power + 15 m^{-1} and + 5 m^{-1} are placed in contact with each other in an ordinary microscope. If the image is formed at a distance of 25 cm, what is the magnifying power of the instrument? [6]
- Two convex lenses of focal length 1.5 cm and 9 cm are arranged to form a microscope. A small object is placed 1.8 cm from the objective. If the image appears to be 37.5 cm from the eyepiece, what will be the distance between the objective and the eyepiece? [16.26 cm]
- The focal lengths of the objective and the eyepiece of a compound microscope are 4 mm and 25 mm respectively. The length of the tube of the instrument is 16 cm. If the final image is formed at infinity and the least distance of distinct vision is 25 cm, determine the magnifying power of the instrument. [327.5]
- The focal lengths of the objective and the eyepiece of a compound microscope are 2 cm and 10 cm respectively. At what distance from the objective must an object be placed so that the final image will be formed at infinity? The distance between the two lenses is 16 cm. [3 cm]
- The focal lengths of the objective and the eyepiece of a compound microcope are 0.5 cm and 1 cm respectively. The distance between the lenses is 16 cm. If the Anal image is formed at the least distance of distinct vision i.e., at 25 cm, determine the magnifying power of the instrument. [765]
Telescope
- The length of the tube of an astronomical telescope is 80 cm and the magnifying power of the instrument is 19. Calculate the focal lengths of the objective and the eyepiece. [ f_0= 76 cm, f_e = 4 cm ]
- The magnifying power of an astronomical telescope for normal vision is 10. The focal length of the objective is 20 cm. What will be the magnifying power for focusing the final image formed at a distance of 25 cm from the eyepiece? [18.8]
PROBLEM SET-Il
Microscope
- The focal lengths of the objective and the eyepiece of a compound microscope are I cm and 5 cm respectively. If an object is placed at a distance of 1.1 cm from the objective the final image is formed at a distance of 25 cm from the eyes. Determine the magnification and the distance between the lenses. [ 60, 15.17 cm ]
- Thin converging lenses of focal lengths 2 cm and 10 cm are used respectively as the objective and the eyepiece of a microscope. The distance between the optical centre of the lenses is 26 cm. If an object is placed at a distance of 2.25 cm from the objective, find the position and magnification of the final image. [40 cm from the eyepiece, 40 ]
- If in compound microscope m_1 and m_2 be the linear magnification of the objective lens and eye lens respectively, then what is the magnifying power of the microscope? [ m_1m_2 ]
Telescope
- The focal lengths of the objective and the eyepiece of an astronomical telescope are 20 cm and 1 cm. If the instrument is focused for infinity and focused for distinct vision what will be the magnifying powers? What will be the length of the tube of the instrument (m = 20, L = 21 cm, m = 20.8, L = 20.96 cm ]
- To observe terrestrial bodies the angular magnification of a telescope is 5 . The distance between the objective and the eyepiece is 36 cm and the final image is formed at infinity. Determine the focal lengths of the objective lens and eyepiece lens. [30 cm, 6 cm]
- A simple telescope, consisting of an objective of focal length 60 cm and a single eye lens of focal length 5 cm is focused on a distant object in such a way that parallel rays come out from the eye lens. If the object subtends an angle 2° at the objective, what is the angular width of the image? [24°]
