# Very Short Answer Questions [1 marks] 1. Define lens. Ans. A piece of optical material bounded by one or two spherical surfaces is called lens. 2. Why is a convex lens called converging lens? Ans. It is because a parallel beam of light on passing through it converges at a point. 3. Define optical centre of a lens. Ans. A point within the lens, where a line drawn through its aperture intersects the principal axis. 4. Define principal focus for a convex lens. Ans. A point on the principal axis of a lens, where the rays coming parallel to the principal axis after refraction meet is called principal focus of the convex lens. 5. Define principal focus of a concave lens? Ans. A point on the principal axis of a lens, where the rays coming parallel to the principal axis after refraction, appear to meet is called principal focus of the concave lens. 6. A virtual, erect and enlarged image is formed by a lens. Is the lens convex or concave? Ans. The lens is convex lens. 7. Where will you place an electric lamp from a convex lens, so that a parallel beam of light comes out of it ? Ans. The lamp should be placed at the principal focus of the lens. 8. A small object is placed in front of convex lens, when a real, inverted and enlarged image is formed on the other side of lens. Where is the object placed? Ans. The object is between F and 2F of the convex lens. 9. Printed letters appear diminished when viewed through a lens. What is the nature of lens? Ans. The lens is a concave lens. 10. A thin spherical lens has a focal length – 20 cm. What is the nature of lens? Ans. The lens is a concave lens. 11. A thin spherical lens has radius of curvature 50 cm. What is its focal length? Ans. F = R/2 = 25 cm 12. What do you understand by the term power of lens? Ans. The reciprocal of focal length of a lens in metres, is called power of lens. 13. Name the unit in which power of lens is measured. Ans. The power of lens is measured in dioptres. 14. The power of a lens is +1.5 D. Is the lens convex or concave? Ans. It is a convex lens. [Note: Power of a convex lens is positive] 15. The power of a lens is –2 D. Is the lens converging or diverging? Ans. It is a diverging lens. [Note: Power of a concave lens is negative] 16. P1 and P2 are the power of two thin lenses. What is the power P when both lenses are placed in contact with one another? Ans. P = P1 + P2. 17. What do you understand by the term refractive index? Ans. The ratio between the sine of angle of incidence (in air) to the sine of angle of refraction (in a denser medium) is called refractive index. 18. Refractive index of glass is 1.5. What do you understand from the statement? Ans. It means that ratio of sine of angle of incidence (in air) to sine of angle of refraction (in glass) is 1.5. 19. At what angle a ray of light should strike the surface of glass, so that it does not suffer any refraction? Ans. It should strike normally i.e., angle of incidence should be zero. 20. How is refractive index related to velocity of light in a medium and velocity of light in vacuum? Ans.  Refractive index = Velocity of light in medium / Velocity of light in vacuum. 21. What do you understand from the term lateral displacement? Ans. Lateral displacement is perpendicular shift in the path of the incident ray, while emerging out from an optical slab. 22. Will the lateral displacement increase/decrease, if glass block is made thicker?
Ans. The lateral displacement will increase. 23. How is the focal length of a spherical mirror related to its radius of curvature? Ans. Focal length is half of the radius of curvature. 24. Why is a convex mirror is called diverging mirror? Ans. It is because, a parallel beam of light after reflection from it, diverges outward. 25. Why is a concave mirror is called converging mirror? Ans. It is because, a parallel beam of light after reflection from it, converges at one point. 26. Is the image formed in a convex mirror always virtual? Ans. Yes, image formed by convex mirror is always virtual. 27. Where is the image formed in a concave mirror, when the object is between infinity and centre of curvature? Ans. The image is formed between the principal focus and centre of curvature, in front of the concave mirror. 28. Where is the image formed in a convex mirror, when the object is anywhere in front of it? Ans. The image is formed between the pole and principal focus, behind the convex mirror. 29. No matter how far you stand from a spherical mirror, your image appears always erect. What is the kind of spherical mirror? Ans. The spherical mirror is a convex mirror. 30. A spherical mirror has focal length of – 40 cm. What is the kind of mirror? Ans. The spherical mirror is a concave mirror. 31. What is the focal length of a plane mirror? Ans. Focal length of a plane mirror is infinity (∞). 32. How does the frequency of a beam of ultraviolet light change when it does from air into glass? Ans. Frequency of ultraviolet light does not change. 33. What is the focal length of a plane mirror? Ans. The focal length of a plane mirror is infinite. 34. When light undergoes refraction at the surface of separation of two media, what happens to its wavelength? Ans. Wavelength changes. 35. When light undergoes refraction, what happens to its frequency? Ans. Frequency does not change. 36. How does a focal length of convex lens change if monochromatic red light is used instead of monochromatic blue light? Ans. Because red light and blue light both has different refractive indices. 37. If the power of a lens is + 5 dioptre, what is its focal length? Ans. Focal length of lens = 1/5 m = 0.2 m = 200 mm 38. Draw a ray diagram to show reflection of an incident ray parallel to principal axis by a convex mirror.  39. What is the minimum distance between an object and its real image in case of a concave mirror ? Ans. Zero, when object is at centre of curvature, its real image is also formed at the same point. 40. What are the two factors on which the lateral displacement of an emergent ray from a glass slab depends ? Ans. (i) Angle of incidence (ii) Thickness of glass slab 41. Between which two points related to a concave mirror should an object be placed to obtain on a screen an image twice the size of the object? Ans. Between centre of curvature and the principal focus.