Lenses
Short Questions
1. What is a lens?
Answer : A lens is a transparent medium bound by two surfaces, at least one of which is spherical.
2. What is a convex lens?
Answer : A convex lens is thicker at the center and converges light rays to a focal point.
3. What is a concave lens?
Answer : A concave lens is thinner at the center and diverges light rays.
4. What is the principal axis of a lens?
Answer : It is the imaginary line passing through the optical center and centers of curvature.
5. What is the optical center of a lens?
Answer : It is the point on the principal axis where light rays pass without deviation.
6. What is the principal focus of a convex lens?
Answer : It is the point where parallel rays converge after refraction through the lens.
7. What is the principal focus of a concave lens?
Answer : It is the point from which diverging rays appear to originate after refraction.
8. What is the focal length of a lens?
Answer : It is the distance between the optical center and the principal focus.
9. Why is a convex lens called a converging lens?
Answer : It converges parallel light rays to a point on the principal axis.
10. Why is a concave lens called a diverging lens?
Answer : It diverges parallel light rays, making them appear to come from a focal point.
11. What is the lens formula?
Answer : The lens formula is 1/v – 1/u = 1/f, relating object distance (u), image distance (v), and focal length (f).
12. What is the unit of power of a lens?
Answer : The unit of power of a lens is Dioptre (D).
13. How is the power of a lens calculated?
Answer : Power (P) is calculated as P = 1/f, where f is the focal length in meters.
14. What is the power of a lens combination?
Answer : The power of a lens combination is the sum of the powers of individual lenses (P = P1 + P2).
15. What is nearsightedness?
Answer : Nearsightedness (myopia) is a defect where distant objects appear blurry but nearby objects are clear.
16. What lens corrects nearsightedness?
Answer : A concave lens corrects nearsightedness by diverging light before it enters the eye.
17. What is farsightedness?
Answer : Farsightedness (hypermetropia) is a defect where nearby objects appear blurry but distant objects are clear.
18. What lens corrects farsightedness?
Answer : A convex lens corrects farsightedness by converging light before it enters the eye.
19. What is presbyopia?
Answer : Presbyopia is the age-related loss of the eye’s ability to focus on nearby objects.
20. What is persistence of vision?
Answer : It is the phenomenon where an image remains on the retina for 1/16th of a second after the object is removed.
Long Questions
1. Explain the difference between a convex lens and a concave lens in terms of their structure and function.
Answer : A convex lens is thicker at the center, converging light rays to form real or virtual images. A concave lens is thinner at the center, diverging light rays to form only virtual, erect, and smaller images. Their surfaces are parts of spheres, with convex lenses bulging outward and concave lenses curving inward.
2. Describe the Cartesian sign convention for lenses.
Answer : The optical center is the origin, with distances to the right and above the principal axis taken as positive. Distances to the left and below are negative, and the object is placed on the left. The focal length is positive for convex lenses and negative for concave lenses.
3. How does a convex lens form a real image when an object is placed beyond 2F1?
Answer : When an object is beyond 2F1, a convex lens forms a real, inverted, and diminished image between F2 and 2F2. A ray parallel to the principal axis passes through F2, and a ray through the optical center passes undeviated. These rays intersect to form the image, as shown in ray diagrams.
4. Why does a concave lens always form a virtual image?
Answer : A concave lens diverges light rays, making them appear to come from a focal point on the same side as the object. The rays never actually converge, so the image is virtual, erect, and smaller. This occurs regardless of the object’s position, as per the ray diagram rules.
5. How does the human eye adjust to see objects at different distances?
Answer : The eye’s lens changes shape via ciliary muscles to adjust its focal length, a process called accommodation. For distant objects, the lens flattens to increase focal length; for near objects, it rounds to decrease it. This ensures a sharp image forms on the retina.
6. Explain the cause and correction of nearsightedness (myopia).
Answer : Nearsightedness occurs when the eyeball is too long or the lens’s converging power is too high, forming images of distant objects in front of the retina. A concave lens diverges incoming light, allowing the eye’s lens to focus the image on the retina. The lens’s negative power is chosen based on the degree of myopia.
7. Explain the cause and correction of farsightedness (hypermetropia).
Answer : Farsightedness results from a shortened eyeball or reduced lens converging power, causing near object images to form behind the retina. A convex lens converges light before it enters the eye, enabling proper focus on the retina. The lens’s positive power depends on the extent of hypermetropia.
8. What is presbyopia, and how is it corrected?
Answer : Presbyopia is the age-related loss of lens elasticity, reducing the eye’s ability to focus on near objects. Bifocal lenses, with a concave upper part for distant vision and a convex lower part for near vision, correct it. This allows clear vision at varying distances.
9. How does a simple microscope work, and what is its use?
Answer : A simple microscope uses a convex lens with a short focal length to form a magnified, virtual, and erect image. When an object is placed within the focal length, the lens magnifies it, making it appear larger. It is used for tasks like watch repair and examining small objects.
10. Describe the working of a compound microscope.
Answer : A compound microscope uses two convex lenses: an objective with a short focal length and an eyepiece with a longer focal length. The objective forms a real, magnified image, which the eyepiece further magnifies into a virtual image. This allows high magnification for viewing minute objects like cells.
11. Explain the role of lenses in a refracting telescope.
Answer : A refracting telescope uses a large-diameter, long-focal-length objective lens to collect and focus light from distant objects, forming a real image. The eyepiece, with a shorter focal length, magnifies this image for viewing. The lenses’ alignment and adjustable distance enhance magnification and clarity.
12. What is persistence of vision, and how is it demonstrated in daily life?
Answer : Persistence of vision is the retina retaining an image for 1/16th of a second after the object is removed. This is seen when rotating a burning incense stick forms a continuous circle or when a spinning card with a bird and cage appears as one image. It underlies motion perception in films and animations.
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