Class 10 Science · Chapter 9

Light - Reflection and Refraction

194 Class 10 Science questions on Light - Reflection and Refraction from the NCERT textbook, NCERT Exemplar and CBSE board papers 2023–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 2 of 10

1. Q21 NCERT · Exercises
   A ​doctor has ​prescribed a corrective lens of power $+1.5$ D. Find ​the focal length of ​the lens. Is the ​prescribed ​lens diverging or converging?
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   $f=+0.67$ m (about 67 cm); the lens is convex, so it is converging.

   The focal length is $+0.67$ m, that is about $67$ cm. The positive sign shows that the prescribed lens is a convex lens, so it is a converging lens.

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2. Q22 NCERT · Intext Questions (Page 145)
   Find the ​focal ​length ​of a ​convex mirror whose radius ​of curvature ​is $32$ ​cm.
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   The focal length is $f=+16\text{cm}$, half the radius of curvature, with the focus behind the convex mirror.

   The focal length is $16$ cm. Since the mirror is convex its principal focus lies behind the reflecting surface, so on the New Cartesian sign convention $f=+16$ cm $=+0.16$ m.

   Step-by-step solution →
3. Q23 NCERT · Intext Questions (Page 145)
   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?
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   The image is $30$ cm in front of the mirror, real, inverted and three times the size of the object.

   The image is located $30$ cm from the mirror, on the same side as the object — that is, $30$ cm in front of the mirror ($v=-30$ cm). It is real and inverted, and three times the size of the object, so it can be caught on a screen placed there.

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4. Q24 NCERT · Intext Questions (Page 150)
   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?
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   It bends towards the normal, because water is optically denser than air, so light slows down and $r<i$.

   The light ray bends **towards** the normal. Water is optically denser than air, because its refractive index ($1.33$) is greater than that of air ($1.0003$), so the light slows down on crossing the boundary. A ray travelling obliquely from a rarer medium into a denser one always turns towards the normal, and the angle of refraction is therefore smaller than the angle of incidence, $r<i$.

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5. Q25 NCERT · Intext Questions (Page 150)
   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${}^{-1}$.
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   The speed of light in the glass is $v=2\times 10^8{\text{m s}}^{-1}$, two-thirds of its speed in vacuum.

   The speed of light in the glass is $2\times 10^8$ m s${}^{-1}$, since $v=c/n=(3\times 10^8)/1.50$. That is two-thirds of its speed in vacuum.

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6. Q26 NCERT · Intext Questions (Page 150)
   Find ​out, from ​Table 9.3, ​the medium ​having highest optical ​density. Also find ​the medium ​with lowest optical density.
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   Diamond (refractive index 2.42) has the highest optical density and air (1.0003) the lowest.

   Diamond has the highest optical density, with a refractive index of $2.42$ — the largest value in Table 9.3. Air has the lowest optical density, with a refractive index of $1.0003$, the smallest value in the table. Light therefore travels slowest in diamond and fastest in air of all the media listed.

   Step-by-step solution →
7. Q27 NCERT · Intext Questions (Page 150)
   You are given kerosene, turpentine and water. In which ​of ​these does ​the ​light travel fastest? ​Use ​the information given ​in Table ​9.3.
   Show answer

   Light travels fastest in water, which has the smallest refractive index ($1.33$) of the three, so $v=c/n$ is greatest.

   Light travels fastest in water. Of the three liquids, water has the smallest refractive index ($1.33$, against $1.44$ for kerosene and $1.47$ for turpentine), and since $v=c/n$ the smallest refractive index goes with the greatest speed. The speed in water is about $2.26\times 10^8$ m s${}^{-1}$, against $2.08\times 10^8$ m s${}^{-1}$ in kerosene and $2.04\times 10^8$ m s${}^{-1}$ in turpentine.

   Step-by-step solution →
8. Q28 NCERT · Intext Questions (Page 150)
   The ​refractive ​index of diamond ​is $2.42$. ​What is the ​meaning of ​this statement?
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   Light travels $2.42$ times faster in vacuum than in diamond, where its speed is $1.24\times 10^8{\text{m s}}^{-1}$.

   It means that the ratio of the speed of light in vacuum to the speed of light in diamond is $2.42$ — that is, light travels $2.42$ times faster in vacuum (or air) than it does inside diamond. Its speed in diamond is therefore $v=c/2.42=(3\times 10^8)/2.42=1.24\times 10^8$ m s${}^{-1}$. Because $2.42$ is the largest refractive index in Table 9.3, it also means diamond is the optically densest of those media, and bends a ray entering it obliquely from air more sharply towards the normal than any of them.

   Step-by-step solution →
9. Q29 NCERT · Intext Questions (Page 158)
   Define $1$ dioptre ​of ​power ​of a ​lens.
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   One dioptre is the power of a lens of focal length 1 m, so $1\text{D}=1{\text{m}}^{-1}$.

   One dioptre is the power of a lens whose focal length is $1$ metre. Since the power of a lens is the reciprocal of its focal length, $P=1/f$ with $f$ in metres, putting $P=1$ gives $f=1$ m. The unit is denoted by D, and being the reciprocal of a length it satisfies $1$ D $=1$ m${}^{-1}$.

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10. Q30 NCERT · Intext Questions (Page 158)
    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.
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    The needle is 50 cm in front of the lens ($u=-50\text{cm}$), and the power is $P=+4\text{D}$.

    The needle is placed $50$ cm in front of the convex lens ($u=-50$ cm), at twice the focal length; the image forms at twice the focal length on the other side, which is why it comes out the same size as the object. The focal length is $f=+25$ cm $=+0.25$ m, so the power of the lens is $P=1/f=+4$ D.

    Step-by-step solution →
11. Q31 NCERT · Intext Questions (Page 158)
    Find the ​power of ​a ​concave lens ​of ​focal length $2$ m.
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    The power is $P=-0.5\text{D}$; the minus sign shows that the concave lens is diverging.

    The power of the lens is $-0.5$ D. The lens is concave, so its focal length is negative, $f=-2$ m; since the power is the reciprocal of the focal length in metres, $P=1/(-2)=-0.5$ D. The minus sign is part of the answer — it is what identifies the lens as a diverging one.

    Step-by-step solution →
12. Q32 Exemplar · Multiple Choice Questions
    Which of the following can ​make ​a parallel ​beam ​of light when light ​from a point source is incident on ​it?

    1. (a)Concave mirror as well as convex lens
    2. (b)Convex mirror as well as concave lens
    3. (c)Two plane mirrors placed at $90^{\circ }$ to each other
    4. (d)Concave mirror as well as concave lens
    Show answer

    (a) Concave mirror as well as convex lens

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13. Q33 Exemplar · Multiple Choice Questions
    A 10 mm ​long awl pin is placed vertically ​in front ​of a concave mirror. A ​5 mm long image of the awl pin ​is formed ​at 30 cm in front of ​the mirror. The focal ​length of this mirror is

    1. (a)$-30$ cm
    2. (b)$-20$ cm
    3. (c)$-40$ cm
    4. (d)$-60$ cm
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    (b) $-20$ cm

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14. Q34 Exemplar · Multiple Choice Questions
    Under which of the ​following conditions a ​concave ​mirror can ​form an ​image larger than the ​actual ​object?

    1. (a)When the object is kept at a distance equal to its radius of curvature
    2. (b)When object is kept at a distance less than its focal length
    3. (c)When object is placed between the focus and centre of curvature
    4. (d)When object is kept at a distance greater than its radius of curvature
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    (c) When object is placed between the focus and centre of curvature

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15. Q35 Exemplar · Multiple Choice Questions
    Figure 10.1 ​shows ​a ​ray of light as it travels ​from medium A to medium B. Refractive index of the medium ​B ​relative to ​medium ​A is

    1. (a)$\sqrt{3}/\sqrt{2}$
    2. (b)$\sqrt{2}/\sqrt{3}$
    3. (c)$1/\sqrt{2}$
    4. (d)$\sqrt{2}$
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    (a) $\sqrt{3}/\sqrt{2}$

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16. Q36 Exemplar · Multiple Choice Questions
    A ​light ray enters from medium ​A to medium ​B as shown in Figure ​10.2. The ​refractive ​index of medium B relative to A will ​be

    1. (a)greater than unity
    2. (b)less than unity
    3. (c)equal to unity
    4. (d)zero
    Show answer

    (a) greater than unity

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17. Q37 Exemplar · Multiple Choice Questions
    Beams of light ​are incident through the holes A and B and emerge out of box through the ​holes C and D ​respectively as ​shown in the Figure10.3. ​Which of the following ​could be inside the box?

    1. (a)A rectangular glass slab
    2. (b)A convex lens
    3. (c)A concave lens
    4. (d)A prism
    Show answer

    (a) A rectangular glass slab

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18. Q38 Exemplar · Multiple Choice Questions
    A ​beam of light is incident through the holes on ​side A ​and emerges out of the holes on the ​other face of the box as shown in ​the Figure 10.4. Which of the following ​could be inside ​the box?

    1. (a)Concave lens
    2. (b)Rectangular glass slab
    3. (c)Prism
    4. (d)Convex lens
    Show answer

    (d) Convex lens

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19. Q39 Exemplar · Multiple Choice Questions
    Which of the ​following ​statements ​is true?

    1. (a)A convex lens has $4$ dioptre power having a focal length $0.25$ m
    2. (b)A convex lens has $-4$ dioptre power having a focal length $0.25$ m
    3. (c)A concave lens has $4$ dioptre power having a focal length $0.25$ m
    4. (d)A concave lens has $-4$ dioptre power having a focal length $0.25$ m
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    (a) A convex lens has $4$ dioptre power having a focal length $0.25$ m

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20. Q40 Exemplar · Multiple Choice Questions
    Magnification ​produced ​by a ​rear ​view ​mirror ​fitted in vehicles

    1. (a)is less than one
    2. (b)is more than one
    3. (c)is equal to one
    4. (d)can be more than or less than one depending upon the position of the object in front of it
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    (a) is less than one

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