(a) A student suffering from myopia is not able to see distinctly the object placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams explain.(i) Why the student is unable to see distinctly the objects placed beyond 5 cm from his eyes.(ii) The type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.
(a) The two possible reasons due to which this defect (myopia) of vision may have arisen are:
1. The high converging power of the eye lens (because of its short focal length), which causes the formation of the images of objects in front of the retina, or
2. The eye ball is too long, which causes light to focus in front of the retina, instead of directly on the retina.
The ray diagram showing the eye-defect short-sightedness.
The ray diagram showing the correction of short-sightedness by using a lens.
(i) The student is unable to see distinctly the objects placed beyond 5 cm from his eyes because the image of the object is formed in front of the retina and not on it.
(ii) The type of corrective lens used to restore proper vision is Concave or diverging lens. This lens brings the image back to the retina.
(b) Given:
Focal length, $f=-5m$ (minus sign, since lens is concave)
To find: Power of the lens, $P$.
Solution:
We know that, power of the lens is given as-
$P=\frac {1}{f}$
Putting the value of $f$ in the formula, we get-
$P=\frac {1}{-5}$
$P=-0.2D$
Thus, the power of the lens as per the new Cartesian sign convention is -0.2D.
Related Articles
- A student uses spectacles of focal length – 2·5 m. (a) Name the defect of vision he is suffering from.(b) Which lens is used for the correction of this defect?(c) List two main causes of developing this defect.(d) Compute the power of this lens.
- 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?
- (a) What is short-sightedness? State the two causes of short-sightedness (or myopia). With the help of ray diagrams, show:(i) the eye-defect short-sightedness.(ii) correction of short-sightedness by using a lens.(b) A person having short-sight cannot see objects clearly beyond a distance of 1.5 m. What would be the nature and power of the corrective lens to restore proper vision?
- Explain with the help of labelled ray diagram, the defect of vision called myopia and how it is corrected by a lens.
- Explain with the help of labelled ray-diagram, the defect of vision called hypermetropia, and how it is corrected by a lens.
- Find the focal length of a lens of power -2.0 D. What type of lens is this?
- (a) What type of lens is used as a magnifying glass?(b) What type of lens is used to correct the defect of vision called myopia? (c) A convex lens has a focal length of 10.5 cm. Where should an object be placed in front of this convex lens so as to obtain an image which is real, inverted and same size as the object?
- Name the defect of vision which can be corrected by a converging lens. Show clearly by a ray diagram how the lens corrects the defect.
- Name the defect of vision which can be corrected by a perging lens. Show clearly by a ray diagram how the lens corrects the defect.
- (a) A person cannot read newspaper placed nearer than 50 cm from his eyes. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.(b) We see advertisements for eye donation on television or in newspapers. Write the importance of such advertisements.
- An object is 2 m from a lens which forms an erect image one-fourth (exactly) the size of the object. Determine the focal length of the lens. What type of lens is this?
- Rishi went to a palmist to show his palm. The palmist used a special lens for this purpose.(i) State the nature of the lens and the reason for its use.(ii) Where should the palmist place/hold the lens so as to have a real and magnified image of an object?(iii) If the focal length of this lens is 10 cm and the lens is held at a distance of 5 cm from the palm, use the lens formula to find the position and size of the image.
- A person suffering from the eye-defect myopia (short-sightedness) can see clearly only up to a distance of 2 metres. What is the nature and power of lens required to rectify this defect?
- (a) An object is placed 10 cm from a lens of focal length 5 cm. Draw the ray diagrams to show the formation of image if the lens is (i) converging, and (ii) perging.(b) State one practical use each of convex mirror, concave mirror, convex lens and concave lens.
- An object is placed at a distance of 60 cm from a concave lens of focal length 30 cm.(i) Use the lens formula to find the distance of the image from the lens.(ii) List four characteristics of the image (nature, position, size, erect/inverted) formed by the lens in this case.(iii) Draw ray diagram to justify your answer of the part (ii).
Kickstart Your Career
Get certified by completing the course
Get Started
Data Structure
Networking
RDBMS
Operating System
Java
MS Excel
iOS
HTML
CSS
Android
Python
C Programming
C++
C#
MongoDB
MySQL
Javascript
PHP
Physics
Chemistry
Biology
Mathematics
English
Economics
Psychology
Social Studies
Fashion Studies
Legal Studies
To Continue Learning Please Login