Class 10 Science · Chapter 9

Light - Reflection and Refraction

409 Class 10 Science questions on Light - Reflection and Refraction from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 17 of 21

1. Q321 CBSE 2018
   Draw a labelled ray ​diagram for the formation of image by a convex lens of focal length 15 ​cm when the ​object ​is placed at a distance ​of 25 cm from ​the lens. Determine the size of the image formed, if size of ​the object is ​4 cm.
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   The image is $h^{\prime }=-6$ cm: real, inverted, magnified and 6 cm tall, formed 37.5 cm from the lens.

   Size of the image: $h^{\prime }=-6$ cm, that is, the image is $6$ cm tall and inverted. Working: $\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{15}-\frac{1}{25}=\frac{2}{75}$, so $v=+37.5$ cm; $m=\frac{v}{u}=\frac{37.5}{-25}=-1.5$; $h^{\prime }=m\times h=-1.5\times 4=-6$ cm. Ray diagram (the figure at step 4): the object lies between $F_1$ and $2F_1$; the ray parallel to the principal axis is refracted through $F_2$ and the ray through the optical centre O goes on undeviated. They meet $37.5$ cm behind the lens, beyond $2F_2$, where a real, inverted, magnified image $6$ cm tall is formed.

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2. Q322 CBSE 2018
   You have a ​convex lens of focal length 12 cm. You ​place a candle flame on its principal axis ​at a distance of 60 cm from the lens on its one ​side and place a screen on its other side. Write two characteristics of the image formed on the screen. If you now shift the candle flame towards the lens so that its distance from the lens becomes ​15 cm, then ​state the changes that are observed in these two characteristics of the image formed.
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   At 60 cm the image is real, inverted and diminished; at 15 cm it stays real and inverted but becomes enlarged.

   With the flame at $60$ cm (beyond $2F$): the image on the screen is (i) real and inverted, and (ii) diminished, about one-fourth the size of the flame; it is sharp and bright. [$v=+15$ cm, $m=-\frac{1}{4}$] With the flame moved to $15$ cm (between $F$ and $2F$): (i) the image is still real and inverted (no change), but (ii) it becomes enlarged, about four times the size of the flame, and comparatively less bright. The screen has to be moved to about $60$ cm from the lens to receive it. [$v=+60$ cm, $m=-4$]

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3. Q323 CBSE 2017
   Write ​two different ​uses ​of concave mirrors.
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   Concave mirrors are used as reflectors in torches, search-lights and headlights, and as shaving mirrors.

   Any two of the following; the first two rest on different properties of the mirror. (i) As reflectors in torches, search-lights and vehicle headlights: a bulb placed at the focus gives a powerful parallel beam of light. (ii) As shaving mirrors: the face, held between the pole and the focus, is seen as an enlarged, erect image. Other uses: dentists' mirrors, to see large images of the teeth; and large concave mirrors in solar furnaces, to concentrate sunlight and produce heat.

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4. Q324 CBSE 2017
   Why ​is convex ​mirror used ​as a rear view mirror ​in ​vehicles ​? State ​any one reason.
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   A convex mirror gives a much wider field of view, so the driver sees a large area of traffic behind.

   A convex mirror curves outwards, so it has a much wider field of view than a plane mirror of the same size: the driver can see a large area of the traffic behind the vehicle. (Equally acceptable as the one reason: a convex mirror always forms an erect, though diminished, image of the vehicles behind.)

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5. Q325 CBSE 2017
   Define power of a lens.
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   Power of a lens is the reciprocal of its focal length in metres, $P=\frac{1}{f}$; its unit is the dioptre.

   The power of a lens is the degree of convergence or divergence of light rays achieved by it; it is defined as the reciprocal of its focal length: $P=\frac{1}{f}$, with $f$ in metres. The SI unit of power is the dioptre (D): $1$ D is the power of a lens of focal length $1$ m ($1\text{D}=1{\text{m}}^{-1}$). The power of a convex lens is positive and that of a concave lens is negative.

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6. Q326 CBSE 2017
   The power ​of a ​lens is $+0.5$ D. ​Write its ​focal ​length.
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   $f=+2$ m (+200 cm): the focal length is positive, so the lens is convex.

   Focal length $f=\frac{1}{P}=\frac{1}{+0.5\text{D}}=+2$ m, that is $+200$ cm. The positive sign shows that it is a convex (converging) lens.

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7. Q327 CBSE 2017
   The ​focal ​length ​of ​a lens is ​+25 ​cm. Write the ​power of this lens.
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   $P=+4$ D: the power of the lens is +4 dioptre, so it is convex.

   $P=\frac{1}{f}=\frac{1}{+0.25\text{m}}=+4$ D. The power of the lens is $+4$ dioptre; the positive sign shows that it is a convex (converging) lens.

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8. Q328 CBSE 2017
   What ​is ​power ​of ​a lens ​? Define its ​S.I. ​unit.
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   Power is the reciprocal of focal length in metres; one dioptre is the power of a lens of focal length 1 m.

   Power of a lens is the degree of convergence or divergence of light rays achieved by the lens. It is the reciprocal of the focal length of the lens: $P=\frac{1}{f}$, with $f$ in metres. The power of a convex lens is positive and that of a concave lens is negative. SI unit: the dioptre (D). One dioptre is the power of a lens whose focal length is 1 metre, so $1\text{D}=1{\text{m}}^{-1}$.

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9. Q329 CBSE 2017
   An object is ​placed at a ​distance of 30 cm in ​front ​of a convex mirror of ​focal ​length 15 cm. Write four characteristics of the ​image formed by the mirror.
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   The image is virtual, erect, diminished to one-third the size, and formed 10 cm behind the mirror between P and F.

   With $u=-30$ cm and $f=+15$ cm, the mirror formula gives $\frac{1}{v}=\frac{1}{15}+\frac{1}{30}=\frac{1}{10}$, so $v=+10$ cm, and $m=-\frac{v}{u}=+\frac{1}{3}$. The four characteristics of the image are: (1) Virtual: it is formed behind the mirror and cannot be obtained on a screen. (2) Erect: $m$ is positive. (3) Diminished: it is one-third the size of the object. (4) Position: 10 cm behind the mirror, between the pole $P$ and the principal focus $F$.

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10. Q330 CBSE 2017
    An object is ​placed ​at a ​distance ​of ​40 ​cm in front of ​a ​convex mirror of radius of curvature 40 cm. List four characteristics of the image formed by the mirror.
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    $v\approx +13.3$ cm and $m=+\frac{1}{3}$: the image is virtual, erect, diminished and behind the mirror.

    With $u=-40$ cm and $f=R/2=+20$ cm, the mirror formula gives $\frac{1}{v}=\frac{1}{20}+\frac{1}{40}=\frac{3}{40}$, so $v=+\frac{40}{3}$ cm $\approx +13.3$ cm, and $m=-\frac{v}{u}=+\frac{1}{3}$. The four characteristics of the image are: (1) Virtual: it is formed behind the mirror and cannot be obtained on a screen. (2) Erect: $m$ is positive. (3) Diminished: it is one-third the size of the object. (4) Position: $13.3$ cm behind the mirror, between the pole and the principal focus.

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11. Q331 CBSE 2017
    An ​object is placed at a distance ​of 12 cm in front ​of ​a concave mirror of radius ​of ​curvature 30 cm. List four characteristics of the image formed by the mirror.
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    The image is virtual, erect, enlarged five times and formed 60 cm behind the mirror.

    Here $f=\frac{R}{2}=-15$ cm and $u=-12$ cm, so $\frac{1}{v}=-\frac{1}{15}+\frac{1}{12}=\frac{1}{60}$, giving $v=+60$ cm, and $m=-\frac{v}{u}=+5$. The object lies between the pole and the focus, and the four characteristics of the image are: (1) Virtual: it is formed behind the mirror and cannot be obtained on a screen. (2) Erect: $m$ is positive. (3) Enlarged: five times the size of the object. (4) Position: 60 cm behind the mirror.

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12. Q332 CBSE 2017
    Redraw the following ​diagram ​on your ​answer-sheet and show ​the path of the reflected ray. Also ​mark the angle of ​incidence ($\angle i$) and the angle of reflection ​($\angle r$) on the diagram.

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    The reflected ray passes through $F$; the normal joins the point of incidence to $C$, and $\angle i=\angle r$.

    The mirror is concave (its hatching, on the right, marks the back). Draw the reflected ray from the point where the incident ray meets the mirror through the principal focus F, and continue it beyond F. Join the point of incidence to C: this radius is the normal to the mirror there. The angle of incidence $\angle i$ is the angle between the incident ray and the normal, and the angle of reflection $\angle r$ is the angle between the normal and the reflected ray, with $\angle i=\angle r$.

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13. Q333 CBSE 2017
    An object is placed at a ​distance of 30 cm from ​a concave lens ​of focal length 15 ​cm. List four characteristics ​(nature, ​position, etc.) of ​the image formed by the ​lens.
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    The image is virtual, erect, diminished to one-third the size, and formed 10 cm from the lens on the object's side.

    (1) Nature: virtual, formed on the same side of the lens as the object ($v=-10$ cm). (2) Orientation: erect, since $m=+\frac{1}{3}$ is positive. (3) Size: diminished, one third the size of the object. (4) Position: $10$ cm from the optical centre on the object's side, between the optical centre $O$ and the focus $F_1$.

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14. Q334 CBSE 2017
    An object is placed ​at a distance of $15$ cm ​from a convex lens of focal length $20$ cm. List ​four characteristics (nature, position, etc.) ​of the image formed ​by the lens.
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    The image is virtual, erect, magnified four times and formed 60 cm from the lens on the object's side.

    (1) Nature: virtual — it cannot be obtained on a screen. (2) Orientation: erect. (3) Size: magnified, $4$ times the size of the object ($m=+4$). (4) Position: on the same side of the lens as the object, $60$ cm from the lens ($v=-60$ cm), beyond $2F_1$.

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15. Q335 CBSE 2017
    An ​object ​is placed at a distance of 15 ​cm from a concave lens of focal length 30 ​cm. List four characteristics (nature, ​position, ​etc.) of the image formed by the lens.
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    The image is virtual, erect, diminished to two-thirds, and formed 10 cm from the lens on the object's side.

    Four characteristics of the image: Nature: virtual. Orientation: erect. Size: diminished, two-thirds of the size of the object ($m=+\frac{2}{3}$). Position: $10$ cm from the lens on the same side as the object ($v=-10$ cm), between the optical centre and the focus $F_1$.

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16. Q336 CBSE 2017
    An ​object ​is ​placed at a distance of 8 cm in front of ​a concave mirror of focal ​length 10 cm. Write any four ​characteristics of ​the ​image formed.
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    The image is virtual, erect, enlarged five times and formed 40 cm behind the mirror.

    With $u=-8$ cm and $f=-10$ cm, the mirror formula gives $v=+40$ cm, and $m=-\frac{v}{u}=+5$. Four characteristics of the image: (1) It is virtual: it cannot be obtained on a screen. (2) It is erect. (3) It is enlarged, five times the size of the object. (4) It is formed behind the mirror, 40 cm from the pole.

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17. Q337 CBSE 2017
    Redraw the ​following ​diagram on your answer-sheet and show the ​path of the reflected ray. Also mark the angle of incidence ​($\angle i$) ​and ​the angle of ​reflection ($\angle r$) ​on the diagram.

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    The mirror is concave (its hatching, on the right, marks the silvered back). The incident ray passes through the principal focus $F$, so after reflection it travels parallel to the principal axis: draw it from the point of incidence $M$ back to the left, parallel to the axis. Join $M$ to $C$: this radius is the normal at $M$. The angle of incidence $\angle i$ is the angle between the incident ray and the normal $CM$, and the angle of reflection $\angle r$ is the angle between the normal $CM$ and the reflected ray, with $\angle i=\angle r$.

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18. Q338 CBSE 2017
    Redraw the following diagram on ​your answer-sheet ​and ​show the path ​of the reflected ray. Also mark the ​angle of incidence ($\angle i$) and the ​angle of reflection ($\angle r$) on the ​diagram.

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    The mirror is convex (F and C lie behind it), and the incident ray is directed towards F. After reflection the ray travels back parallel to the principal axis. The normal at the point of incidence is the line joining C to that point, produced in front of the mirror; $\angle i$ is the angle between the incident ray and this normal, $\angle r$ the angle between the normal and the reflected ray, and $\angle i=\angle r$ (see the figure).

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19. Q339 CBSE 2017
    Write the relationship ​between absolute refractive index of ​a medium and the speed of ​light ​in vacuum. ​If the speed of light in vacuum is $3\times 10^8$ m/s, ​find the absolute ​refractive index of the medium ​in which light travels with a speed of $1.4\times 10^8$ m/s.
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    $n_m=\frac{c}{v}$; the absolute refractive index of the medium is about 2.14, with no unit.

    The absolute refractive index of a medium is the speed of light in vacuum divided by the speed of light in the medium: $n_m=\frac{c}{v}$. $n_m=\frac{3\times 10^8\text{m/s}}{1.4\times 10^8\text{m/s}}=\frac{15}{7}\approx 2.14$ (no unit).

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20. Q340 CBSE 2017
    State any four characteristics ​of ​the ​images formed ​by ​convex ​mirrors.
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    The image is virtual, erect, diminished and formed behind the mirror between the pole and the principal focus.

    The images formed by a convex mirror are: (i) Virtual: formed behind the mirror, so they cannot be obtained on a screen. (ii) Erect. (iii) Diminished: smaller than the object. (iv) Formed behind the mirror, between the pole P and the principal focus F (at F itself, highly diminished and point-sized, when the object is at infinity).

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