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प्रश्न
By closing the eyes and gently pressing them by your palms, you may see some specks of brilliant light. How do you get this sensation while there is no light entering your eyes?
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उत्तर १
If we look at a bright object and then close our eyes, the sensation of light persists for a short period of time. This is known as the persistence image or the after image. It lasts for one-tenth of a second. Therefore, by closing your eyes and gently pressing them with your palms, you see some specks of brilliant light.
उत्तर २
Phosphenes are the sense of seeing dazzling light after gently touching the eyes with your palms. Mechanical activation of retinal cells causes this behaviour, even when no external light is present. This is how it happens:
- Mechanical Stimulation: Closing your eyes and lightly pressing them with your palms applies pressure to the eyeballs.
- Stimulation of Retinal Cells: Pressure on the eyeballs stimulates retinal cells, including photoreceptors (rods and cones).
- Activation of the Optic Nerve: Stimulated retinal cells transmit messages to the brain via the optic nerve.
- Perception of Light: The brain interprets these signals as light, resulting in the impression of phosphenes, or dazzling light.
Mechanical stimulation mimics the impact of light on the retina, creating a sense of light even when no external light enters the eyes. Other ways to create phosphenes include rubbing your eyes or being exposed to strong magnetic fields.
Notes
Students can refer to the provided solutions based on their preferred marks.
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संबंधित प्रश्न
A student suffering from myopia is not able to see distinctly the objects 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 m 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
What are the two most common defects of vision (or defects of eye)? How are they corrected?
The near-point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect? (Assume that the near-point of the normal eye is 25 cm).
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.
Myopia and hypermetropia can be corrected by:
Assertion: Concave mirrors are used as reflectors in torches, vehicle head lights and in search lights.
Reason: When an object is placed beyond the center of curvature of a concave mirror, the image formed is real and inverted.
A person is unable to see clearly a poster fixed on a distant wall. He however sees it clearly when standing at a distance of about 2 m from the wall.
- Draw ray diagram to show the formation of image by his eye lens when he is far away from the wall.
- List two possible reasons of this defect of vision.
- Draw ray diagram to show the correction of this defect using appropriate lens.
Match the following:
| Column - I | Column - II |
| 1. Retina | a. Path way of light |
| 2. Pupil | b. Far point comes closer |
| 3. Ciliary muscles | c. near point moves away |
| 4. Myopia | d. Screen of the eye |
| 5. Hypermetropia | e. Power of accommodation |
A teacher drew the diagram of the heart on the blackboard and told the students to copy it in their notebooks. Mahesh couldn't see the diagram clearly as it appeared blurred to him.
- Name the defect of the eye Mahesh is suffering from.
- Where is the image formed in this defect?
- Mahesh consults an eye doctor and is prescribed suitable lenses to correct the defect. Which type of lens do his spectacles have?
