Light- Reflection and Refraction NCERT Solution
1. Define the principal focus of a concave mirror.
Answer: The principal focus of a concave mirror is a point on its principal axis to which all the light rays are parallel and close to the axis converge after reflection from the concave mirror.
2. Name a mirror that can give an erect and enlarged image of an object.
Answer: Concave
3. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer: Convex mirrors are commonly used as rear-view mirrors in vehicles because they give an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
4. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Answer: It bends towards the normal. This is because it travels from an optically rarer to , optically denser medium.
5. You are given kerosene, turpentine and water. In which of these does the light travel fastest?
Answer: Speed of light in a medium is inversely proportional to refractive index of the medium,as refractive index of water is minimum, so light travels fastest in water.
6. The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer: The refractive index of diamond is 2.42 this means that the speed of light in diamond will reduce by a factor of 2.42 as compared to its speed in air.
7. Define 1 dioptre of power of a lens.
Answer: 1 Dioptre is defined as the power of a lens of focal length 1 meter.
8. Name the type of mirror used in the following situations. (a) Headlights of a car. (b) Side/rear-view mirror of a vehicle. (c) Solar furnace. Support your answer with reason.
Answer: (a) Concave mirror is used in the headlights of a car. This is because concave mirrors can produce powerful parallel beam of light when the light source is placed at their principal focus.
(b) Convex mirrors are commonly used as rear-view mirrors in vehicles because they give an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
(c) Concave mirrors are convergent mirrors. That is why they are used to construct solar furnaces. Concave mirrors converge the light incident on them at a single point known as principal focus. Hence, they can be used to produce a large amount of heat at that point.
9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations
Answer: The convex lens will form complete image of an object, even if its one half is covered with black paper. It can be understood by the following two cases.
Case I: When the upper half of the lens is covered
In this case, a ray of light coming from the object will be refracted by the lower half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the following figure.
Answer: The magnification produced by a plane mirror is +1 means that the image formed is virtual, erect, and of the same size as that of an object.
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