Here you get the CBSE Class 10 Science chapter 10, Light- Reflection and Refraction: NCERT Exemplar Problems and Solutions (Part-II). This part of the chapter includes solutions for Question No. 20 to 29 from the NCERT Exemplar Problems for Class 10 Science Chapter: Light- Reflection and Refraction. These questions include only the Short Answer Type Questions framed from various important topics in the chapter. Each question is provided with a detailed explanation.
NCERT Exemplar problems are a very good resource for preparing the critical questions like Higher Order Thinking Skill (HOTS) questions. All these questions are very important to prepare for CBSE Class 10 Science Board Examination 2017-2018 as well as other competitive exams.
Find below the NCERT Exemplar problems and their solutions for Class 10 Science Chapter, Light- Reflection and Refraction:
Short Answer type Question
Question. 20 Identify the device used as a spherical mirror or lens in following cases. When the image formed is virtual and erect in each case.
(a) Object is placed between device and its focus, image formed is enlarged and behind it.
(b) Object is placed between the focus and device, image formed is enlarged and on the same side as the of the object
(c) Object is placed between infinity and device, image formed is diminished and between focus and optical centre on the same side as that of the object.
(d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.
(a) Concave mirror is used.
Question. 21 Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.
As light rays enter from any other medium to glass, it deviates from its path through certain angle (). When this light ray exits from the other face of glass slab to the medium from which it is coming, then the it again deviate with the same angle but in opposite sense.
Hence, a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself.
Question. 22 A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like, kerosene or turpentine? Support your answer with reason.
A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water due to phenomenon of refraction of light.
As we know, different materials have different refractive index. So, the amount of refraction for different media will be different. Hence, pencil will appear to be bent more or less depending on the refractive index of the medium.
Question. 23 How is the refractive index of a medium related to the speed of light? Obtain an expression for refractive index of a medium with respect to another in terms of speed of light in these two media?
Question. 24 Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.
Let, refractive index of diamond with respect to air = nDA = absolute refractive index of diamond,
refractive index of glass with respect to air = nGA = absolute refractive index of glass,
refractive index of diamond with respect to glass = nDG
NDG = 1.6 and nGA = 1.5
Question. 25 A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
The statement is correct as convex lens of focal length 20 cm can produce magnified virtual and real image both.
The object should be placed between focus F and optical centre O (i.e., a distance less than 20 cm from the lens) for magnified, virtual and erect image.
The object should be placed between F and 2F (i.e., at a distance between 20 cm to 40 cm) for real, inverted and enlarged image.
Question. 26 Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?
She must move the screen towards the pole lens to obtain a sharp image of the building.
It is because window pane is lying beyond 2F and convex lens forms its image, on the other side between F and 2F.
When she tries to focus the building visible to her outside the window (which is at infinity) then, the image of the building is formed at the focus.
The approximate focal length of this lens is 15 cm.
Question. 27 How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm, respectively. Which lens will you use to obtain more convergent light?
The power (P) of a lens is related to its focal length (f) by following relation:
Lens of higher power or smaller focal length will converge the light rays more. So, the lens of focal length 20 cm will be used.
Question. 28 Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel to each other, whatever may be angle of incidence. Show the same with the help of diagram.
When two plane mirrors are placed at right angle with each other, then the incident ray and reflected ray will always be parallel to each other, irrespective of the angle of incidence.
Question. 29 Draw a ray diagram showing the path of rays of light when it enters with oblique incidence (i) from air into water; (ii) from water into air.
(i) Ray diagram showing the path of rays of light when it enters with oblique incidence from air into water
(ii) Ray diagram showing the path of rays of light when it enters with oblique incidence from water into air.
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