Light - Reflection and Refraction : NCERT Intext Questions

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Page 168, Page 171, Page 176, Page 184,

Page 168

Q.1      Define the principal focus of a concave mirror.
Sol.        When rays parallel and close to the principal axis of a concave mirror meet at a point , this point is called "principal focus".

Q.2      The radius of a curvature of a spherical mirror is 20 cm. What is its focal length?
Sol.        $f = {R \over 2} = {{20} \over 2} = 10{\mkern 1mu} cm.$

Q.3      Name a mirror that can give an erect and enlarged imge of an object.
Sol.        Concave mirror.

Q.4      Why do we prefer a convex mirror as a rear-view mirror in vehicles ?
Sol.        We prefer a convex mirror as rear view mirror as it gives wider view and erect images.

Page 171

Q.1      Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Sol.        $f = {R \over 2} = {{32} \over 2} = 16\,cm.$

Q.2      A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located.
Sol.

m = – 3 ; u = – 10 cm
$m = {{ - v} \over u}$
$- 3 = {{ - v} \over { - 10}}$
Therefore, $v = -30\,cm$

Page 176

Q.1       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 ?
Sol.        The ray bends towards the normal as water is optically denser than air.

Q.2      Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass ? The speed of light in vacuum is $3 \times {10^8}$ m ${s^{ - 1}}$.
Sol.        $ref.\,index = {{speed\,of\,light\,in\,vacuum} \over {speed\,of\,light\,in\,air}}$
Therefore, Speed of light in air $= {{3 \times {10^8}} \over {1.5}}m/s. = 2 \times {10^8}m/s.$

Q.3      Find out, from Table , the medium having highest optical density. Also find the medium with lowest optical density. Sol.        Highest optical density is of diamond , lowest optical density is of air.

Q.4      You are given kerosene, turpentine and water. In which of these does the light travel fastest ? Use the information given in Table - Sol.          Light travels faster in a medium having low optical density. Among kerosene, turpentine and water, light travels faster in water.

Q.5      The refractive index of diamond is 2.42. What is the meaning of this statement ?
Sol.         It means that the speed of light in diamond is ${1 \over {2.42}}$ times the speed of light in vacuum.

Page 184

Q.6      Define 1 dioptre of power of a lens.
Sol.        One dioptre is the power of lens whose focal length is 1 m .

Q.7      A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object ? Also, find the power of the lens.
Sol.

v = 50 cm , m = – 1.
Therefore, $u\,\, = {v \over m} = -\,50\,cm\,.$
${1 \over f} = {1 \over v} - {1 \over u}$
$= \,{1 \over {50}}\, - \,({{ - 1} \over {50}})\, = \,{1 \over {25}}$
$P\,\, = \,\,{{100} \over {25}}\,\, = \,\,4D\,.$

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