Chapter 9 · Light - Reflection and Refraction

NCERT Class 10 Science Chapter 9 Intext Questions (Page 142)

All 4 questions of NCERT Class 10 Science Chapter 9 Intext Questions (Page 142) — Light - Reflection and Refraction — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Define ​the ​principal ​focus ​of a ​concave ​mirror.
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   It is the point on the principal axis where rays parallel to the axis actually meet after reflection from the concave mirror.

   The principal focus of a concave mirror is the point on its principal axis at which rays of light coming parallel to the principal axis actually meet after being reflected from the mirror. It is denoted by $F$, it lies in front of the mirror, and the distance of $F$ from the pole $P$ is the focal length $f$, equal to half the radius of curvature ($f=R/2$).

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2. Q2
   The ​radius ​of ​curvature of ​a spherical mirror is $20$ ​cm. What is ​its focal length?
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   The focal length is $10\text{cm}$, half the radius of curvature; its sign is negative for a concave and positive for a convex mirror.

   The focal length is $10$ cm, because $f=R/2=20/2$. The stem does not say whether the mirror is concave or convex, so only the size is fixed: on the New Cartesian sign convention a concave mirror of this radius has $f=-10$ cm and a convex one has $f=+10$ cm.

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3. Q3
   Name a ​mirror ​that ​can give an erect ​and enlarged image ​of ​an object.
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   A concave mirror, with the object between its pole and focus, gives a virtual, erect and enlarged image.

   A concave mirror, with the object placed between the pole $P$ of the mirror and its principal focus $F$ — that is, nearer to the mirror than one focal length. The image is then formed behind the mirror, and it is virtual, erect and enlarged. A plane mirror can only give an image of the same size and a convex mirror only a diminished one, so neither of those will do.

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4. Q4
   Why do we ​prefer ​a ​convex mirror as a rear-view ​mirror ​in ​vehicles?
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   It always gives an erect, diminished image and a wider field of view than a plane mirror, so the driver sees more traffic behind.

   Because a convex mirror does two things a driver needs at once. First, whatever the distance of the object, it always forms a virtual, erect and diminished image, so the traffic behind is seen the right way up and never inverts. Second, its reflecting surface curves outwards, which gives it a much wider field of view than a plane mirror of the same size, so the driver sees a far larger stretch of the road behind in a small mirror. The price is that the image is diminished, which is why such mirrors warn that objects are closer than they appear.

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