Explain presbyopia and astigmatism.
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.
Why does the objective lens of an astronomical telescope have a large light gathering power?
Give the condition required to achieve a larger magnification of a small object by a compound microscope ?
A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Which of the two, objective or eye-piece, of a telescope has a large aperture? Give reason for your answer.
A person having a myopic eye used the concave lens of focal length 50cm. What is the power of the lens?
Where should an object be placed in front of a convex lens so as to use it as a simple microscope?
What are the common defects of vision that can be corrected by the use of suitable eyeglasses or spectacles?
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.
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?
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?
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?
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?
Which phenomenon of vision is made use of in cinematography? Explain briefly how it is used.
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.
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.
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?
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.
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?
A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw ray diagram for the correction of this defect.