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 16 of 21

1. Q301 CBSE 2019
   Draw ​the path of a ray ​of ​light when it enters one of the faces ​of a glass slab at an angle of ​nearly $45^{\circ }$. Label on it

   1. (i)angle of refraction,
   2. (ii)angle of emergence ​and
   3. (iii)lateral displacement.
   Show answer

   The ray bends towards the normal on entering and away on leaving, emerging parallel to the incident ray with $\angle e=\angle i$.

   Diagram (see the figure): the incident ray strikes the upper face of the slab at an angle of incidence of about $45^{\circ }$ and bends towards the normal inside the glass; at the lower face it bends away from the normal as it passes into air. The emergent ray is parallel to the incident ray. (i) Angle of refraction $\angle r$: the angle between the refracted ray inside the slab and the normal at the point of incidence. (ii) Angle of emergence $\angle e$: the angle between the emergent ray and the normal at the point of emergence ($\angle e=\angle i$). (iii) Lateral displacement: the perpendicular distance between the emergent ray and the incident ray produced (drawn dashed).

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2. Q302 CBSE 2019
   List in proper sequence the steps of the ​experiment for determining the ​approximate focal length ​of a given ​concave mirror by ​obtaining ​the image of ​a distant ​object.
   Show answer

   Face the mirror towards a distant object, get a sharp image on a screen, and measure the mirror-to-screen distance as the focal length.

   1. Fix the concave mirror in a holder (stand) and place it near an open window, with its reflecting surface facing a distant object such as a tree or a building. 2. Place a white screen on a stand in front of the mirror, between the mirror and the window, slightly to one side so that it does not block the light from the object. 3. Move the screen back and forth until a sharp, clear image of the distant object (real, inverted and diminished) is obtained on it. 4. Mark the positions of the mirror and the screen on a metre scale and measure the distance between them: this distance is the approximate focal length of the mirror. Repeat and take the average.

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3. Q303 CBSE 2019
   A student ​has to ​trace the path ​of a ray of light passing through a rectangular glass slab ​for four ​different values of angle of incidence.

   1. (a)Write two important precautions for ​this experiment.
   2. (b)List two conclusions the student will draw based ​on his ​experiment.
   Show answer

   Fix the pins vertically and keep $i$ between $30^{\circ }$ and $60^{\circ }$; the emergent ray is parallel to the incident ray but shifted sideways.

   (a) Any two: (1) The pins should be fixed vertically, in a straight line and at least $3$ cm apart, and their feet should be lined up while fixing $P_3$ and $P_4$. (2) The angle of incidence should be between $30^{\circ }$ and $60^{\circ }$. (3) The glass slab must not be moved during the experiment; it should always stay within the outline drawn round it. (b) Any two: (1) The emergent ray is parallel to the incident ray: the angle of emergence equals the angle of incidence, $\angle e=\angle i$. (2) The emergent ray is shifted sideways from the line of the incident ray (lateral displacement). (3) The ray bends towards the normal on entering the glass slab ($\angle r<\angle i$) and away from the normal on leaving it.

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4. Q304 CBSE 2019
   List four precautions ​which ​a ​student ​should observe while determining the ​focal length of ​a given convex lens by obtaining image of a distant object on a screen.
   Show answer

   Hold the lens vertical, get the sharpest image, keep lens, screen and scale in line, and take the mean of at least three readings.

   Precautions (any four): (1) The lens should be held vertical, with its faces parallel to the screen. (2) The position of the lens (or screen) should be adjusted to obtain the clearest, sharpest image on the screen. (3) The base of the lens stand, the screen and the measuring scale should be in a straight line, and the distance measured along the scale. (4) At least three observations should be taken and their mean found. Other correct precautions also earn credit, for example: the object should be very far away, the lens should be clean, and the scale should be read without parallax.

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5. Q305 CBSE 2019
   Out of the five incident rays shown in the following diagram, find any two incident rays that are obeying ​the ​laws of refraction ​of ​light and may be used for ​locating the ​position of the image formed by ​a convex lens. Use these two rays in finding the position, size and nature of the ​image formed when an object is placed between ${\text{F}}_1$ and $2{\text{F}}_1$ of a convex lens.

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   Rays 2 and 3 (any two of 2, 3, 4) obey the laws; the image is beyond $2F_2$, real, inverted and enlarged.

   Any two of rays 2, 3 and 4 (for example, rays 2 and 3). Ray 2 is parallel to the principal axis and after refraction passes through the principal focus $F_2$; ray 3 passes through the optical centre $O$ without deviation; ray 4 passes through the principal focus $F_1$ and emerges parallel to the principal axis. Rays 1 and 5 do not follow these rules. For an object placed between $F_1$ and $2F_1$, these rays meet on the other side of the lens beyond $2F_2$. Position of the image: beyond $2F_2$; size: enlarged (larger than the object); nature: real and inverted.

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6. Q306 CBSE 2019
   Write four ​steps ​in ​proper sequence which should be followed in the determination of focal length of ​a given convex lens by ​obtaining ​a sharp ​image of a distant ​object.
   Show answer

   Face the lens to a distant object, get a sharp image on a screen, and measure the lens-to-screen distance as the focal length.

   1. Fix the convex lens in a lens holder and place it on a table near an open window, facing a distant object such as a tree or a building. 2. Place a white screen in a stand on the other side of the lens, with the object, the lens and the screen in a straight line. 3. Move the screen (or the lens) back and forth until a sharp, inverted and diminished image of the distant object is formed on the screen. 4. Measure the distance between the lens and the screen with a metre scale; this distance is the focal length of the lens. (Repeat with other distant objects and take the mean.)

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7. Q307 CBSE 2018
   If the image formed by a ​spherical ​mirror for all positions of ​the object placed in front of it is ​always ​erect and diminished, what type of mirror is it ? Draw a labelled ray ​diagram to support your answer.
   Show answer

   The mirror is convex: it always forms a virtual, erect, diminished image between P and F.

   The mirror is a convex mirror. Ray diagram (see the figure): an object AB stands in front of the convex mirror, with the pole P on the mirror and the principal focus F and centre of curvature C behind it. A ray from A parallel to the principal axis is reflected so that it appears to come from F; a ray from A directed towards C is reflected back along its own path. The reflected rays diverge, and their backward extensions (dashed) meet behind the mirror at A′, between P and F. A′B′ is the image: virtual, erect and diminished. Arrows mark the direction of every ray.

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8. Q308 CBSE 2018
   An ​object is ​kept 40 cm in front of a ​concave mirror of ​focal length ​20 cm. Find ​the position and nature of the image. Draw a ray diagram to show the formation of image in ​this case.
   Show answer

   $v=-40$ cm: the image forms at the centre of curvature $C$, real, inverted and the same size as the object.

   Position: $v=-40$ cm, so the image is formed $40$ cm in front of the mirror, at the centre of curvature $C$ (the same place as the object). Nature: real and inverted, and of the same size as the object ($m=-\frac{v}{u}=-1$). Ray diagram: a ray from the top of the object parallel to the axis is reflected through $F$, and a ray through $F$ is reflected parallel to the axis; they meet below $C$, where the inverted image forms (see the figure).

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9. Q309 CBSE 2018
   State the laws ​of ​refraction of light. Explain the term ‘absolute refractive index of ​a ​medium’ ​and write an expression to relate it with the ​speed of light ​in vacuum.
   Show answer

   The incident ray, refracted ray and normal lie in one plane; $\frac{\sin i}{\sin r}$ is constant; absolute refractive index $n_m=\frac{c}{v}$.

   First law: the incident ray, the refracted ray and the normal to the interface of the two media at the point of incidence all lie in the same plane. Second law (Snell's law): the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for light of a given colour and for the given pair of media, $\frac{\sin i}{\sin r}=\text{constant}$. Absolute refractive index of a medium: the refractive index of the medium with respect to vacuum. It is the ratio of the speed of light in vacuum to the speed of light in the medium: $n_m=\frac{\text{speed of light in vacuum}}{\text{speed of light in the medium}}=\frac{c}{v}$.

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10. Q310 CBSE 2018
    We wish to obtain an equal sized inverted image of a ​candle flame on a ​screen kept at distance of 4 m from the candle ​flame.

    1. (a)​Name the type of lens that should be used.
    2. (b)What should be the focal length of ​the lens and at ​what distance from the candle flame the lens be placed.
    3. (c)Draw a labelled diagram to ​show the image formation in this case.
    Show answer

    A convex lens of focal length 1 m, placed 2 m from the flame, so flame and image are each at 2F.

    (a) A convex (converging) lens. (b) For an equal-sized, real, inverted image the candle flame must be at $2F_1$, and the image is then formed at $2F_2$. So the flame-to-screen distance is $2f+2f=4f=4$ m, which gives the focal length $f=1$ m ($100$ cm). The lens must be placed $2$ m from the candle flame, midway between the flame and the screen. (c) Ray diagram: the flame AB stands at $2F_1$, $2$ m in front of the convex lens. A ray from B parallel to the principal axis is refracted through $F_2$, and a ray from B through the optical centre O goes straight on. They meet at B′ on the screen at $2F_2$, $2$ m behind the lens, forming a real, inverted image A′B′ of the same size as the flame (see the figure).

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11. Q311 CBSE 2018
    What is meant by power of a ​lens ? Write its SI unit. ​A ​student uses a lens of focal length ​40 ​cm ​and another of $-20$ cm. ​Write the nature and power of each lens.
    Show answer

    Power is the reciprocal of the focal length in metres, measured in dioptre; the lenses are convex, $+2.5$ D, and concave, $-5$ D.

    Power of a lens: the degree of convergence or divergence of light rays produced by the lens, that is, its ability to converge or diverge the light passing through it. It is the reciprocal of the focal length in metres, $P=\frac{1}{f}$. SI unit: dioptre (D); $1\text{D}=1{\text{m}}^{-1}$. Lens of focal length $40$ cm: $P=\frac{1}{+0.40\text{m}}=+2.5$ D; it is a converging (convex) lens. Lens of focal length $-20$ cm: $P=\frac{1}{-0.20\text{m}}=-5$ D; it is a diverging (concave) lens.

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12. Q312 CBSE 2018
    A 5 cm tall object ​is placed ​at ​a distance of 30 cm ​from a ​convex mirror of focal length ​15 cm. Find the position, ​size and nature of the image formed.
    Show answer

    $v=+10$ cm: a virtual, erect, diminished image $\frac{5}{3}\approx 1.67$ cm tall, 10 cm behind the mirror.

    Position: $v=+10$ cm, i.e. the image is formed $10$ cm behind the mirror (between the pole and the focus). Size: $m=-\frac{v}{u}=-\frac{10}{-30}=+\frac{1}{3}$, so $h^{\prime }=\frac{1}{3}\times 5=\frac{5}{3}\approx 1.67$ cm, one-third the height of the object. Nature: virtual, erect and diminished.

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13. Q313 CBSE 2018
    (a) What are spherical lenses ? What are two types of these ​lenses ? Explain in brief ​the meanings ​of the following terms :

    1. (i)Principal axis
    2. (ii)Optical ​centre
    3. (iii)Aperture (b) A burning candle and a screen are placed 60 cm apart. When a spherical lens is placed ​exactly in the middle of ​the two, a distinct image of the flame is ​obtained on the screen. What is the nature and focal length of the lens ? Also state the nature and size of the image formed.
    Show answer

    The lens is convex with $f=+15$ cm; the image is real, inverted and the same size as the flame ($m=-1$).

    (a) Spherical lens: a piece of transparent material (such as glass) bound by two surfaces, of which one or both are spherical. Two types: (1) convex or converging lens - thicker at the middle than at the edges; (2) concave or diverging lens - thinner at the middle than at the edges. (i) Principal axis: the imaginary straight line passing through the two centres of curvature ($C_1$ and $C_2$) of the lens; it passes through the optical centre. (ii) Optical centre: the central point O of the lens; a ray of light through it passes without any deviation. (iii) Aperture: the effective diameter of the circular outline of the spherical lens. (b) Nature of the lens: the image is obtained on a screen, so it is real, and only a convex lens forms a real image; the lens is convex (converging). Focal length: the lens is midway, so $u=-30$ cm and $v=+30$ cm; $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{30}+\frac{1}{30}=\frac{1}{15}$, so $f=+15$ cm (the candle is at $2F$: $4f=60$ cm). Image: real and inverted, and of the same size as the flame ($m=\frac{v}{u}=-1$).

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14. Q314 CBSE 2018
    1. (a)State ​the ​laws of refraction of light.
    2. (b)What is meant by absolute refractive ​index of a medium.
    3. (c)The absolute refractive ​index of diamond is 2.42. If ​the absolute refractive ​index ​of glass is 1.5 and ​the speed of light in glass is $2\times 10^8$ m/s, calculate the speed of light in diamond.
    Show answer

    In diamond, light travels at $1.24\times 10^8$ m/s, found from $c=n_g{v}_g=3\times 10^8$ m/s in glass and $v=\frac{c}{n}$.

    (a) Laws of refraction of light: (i) The incident ray, the refracted ray and the normal to the interface of the two transparent media at the point of incidence all lie in the same plane. (ii) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for light of a given colour and for the given pair of media: $\frac{\sin i}{\sin r}=\text{constant}$ (Snell's law of refraction). (b) The absolute refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium: $n_m=\frac{c}{v}$. (c) For glass, $c=n_g\times v_g=1.5\times 2\times 10^8=3\times 10^8$ m/s. For diamond, $v_d=\frac{c}{n_d}=\frac{3\times 10^8}{2.42}\approx 1.24\times 10^8$ m/s.

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15. Q315 CBSE 2018
    1. (a)State the laws of reflection of light.
    2. (b)List any two characteristics of the ​images formed by plane mirrors.
    3. (c)An object is placed in ​front ​of a spherical mirror of focal length $-20$ cm at a distance of ​30 cm. At ​what distance from the mirror a screen be placed ​in order to obtain a sharp image of the object ​on ​it ?
    Show answer

    The screen must be 60 cm in front of the concave mirror ($v=-60$ cm); the image is real, inverted and twice as large.

    (a) Laws of reflection: (i) The angle of incidence is equal to the angle of reflection. (ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray all lie in the same plane. (b) Any two: the image formed by a plane mirror is virtual and erect; it is of the same size as the object; it is as far behind the mirror as the object is in front of it; it is laterally inverted. (c) The mirror is concave ($f=-20$ cm) and $u=-30$ cm. By the mirror formula, $\frac{1}{v}=\frac{1}{f}-\frac{1}{u}=-\frac{1}{20}+\frac{1}{30}=-\frac{1}{60}$, so $v=-60$ cm. The screen should be placed $60$ cm from the mirror, on the same side as the object (in front of the mirror). The image on it is real, inverted and twice the size of the object ($m=-2$).

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16. Q316 CBSE 2018
    (a) In ​the context of spherical mirrors, ​define the following terms :

    1. (i)Pole
    2. (ii)​Centre of curvature
    3. (iii)Principal axis
    4. (iv)Focal length (b) An object of size 4 ​cm is ​placed at a distance of 15 cm in front of a ​concave mirror of focal length ​10 cm. Find the distance of the image from the mirror and size of the image formed.
    Show answer

    The image is 30 cm in front of the mirror ($v=-30$ cm) and 8 cm tall, real and inverted.

    (a) (i) Pole: the centre of the reflecting surface of a spherical mirror; it lies on the surface of the mirror and is denoted by P. (ii) Centre of curvature: the centre of the hollow sphere of which the reflecting surface of the mirror is a part; it is denoted by C and is not a part of the mirror. (iii) Principal axis: the imaginary straight line passing through the pole and the centre of curvature of the mirror. (iv) Focal length: the distance between the pole and the principal focus of the mirror ($f=PF$). (b) $h=+4$ cm, $u=-15$ cm, $f=-10$ cm. Mirror formula: $\frac{1}{v}=\frac{1}{f}-\frac{1}{u}=-\frac{1}{10}+\frac{1}{15}=-\frac{1}{30}$, so $v=-30$ cm: the image is $30$ cm from the mirror, in front of it. Size of the image: $m=-\frac{v}{u}=-\frac{(-30)}{(-15)}=-2$, so $h^{\prime }=m\times h=-8$ cm: the image is $8$ cm tall, real and inverted (twice the size of the object).

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17. Q317 CBSE 2018
    An object of height 4.0 cm is placed at a distance of 30 ​cm from the optical centre ‘O’ ​of ​a convex lens ​of focal length 20 cm. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ‘O’ ​and ​principal focus ‘F’ ​on the ​diagram. Also find the approximate ratio of size of the image to the size of the object.
    Show answer

    The image is real, inverted and 8.0 cm tall, 60 cm from the lens beyond $2F_2$, so image : object is about 2 : 1.

    Ray diagram: the object AB ($4.0$ cm) stands $30$ cm from O, between $F_1$ and $2F_1$. A ray from B parallel to the axis is refracted through $F_2$, and a ray from B through O goes undeviated; the two meet at $B^{\prime }$. Position of the image: $60$ cm from the lens on the other side, beyond $2F_2$ (check: $\frac{1}{v}=\frac{1}{20}-\frac{1}{30}=\frac{1}{60}$, so $v=+60$ cm). Size of the image: $8.0$ cm; the image is real and inverted ($m=-2$). Ratio of the size of the image to the size of the object $\approx 2:1$.

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18. Q318 CBSE 2018
    A student has to ​determine the focal length of ​a given concave ​mirror by ​focussing a ​distant object. Answer the following questions :

    1. (i)Which two precautions should he ​observe while selecting the object ?
    2. (ii)Which distance should he measure to determine the focal length ​and why ?
    Show answer

    Choose a very distant, well-lit object; measure the mirror-to-screen distance, because rays from a distant object meet at the principal focus.

    (i) Any two: the object should be very far away (a distant tree or building), so that the rays from it reach the mirror nearly parallel; the object should be well illuminated (in sunlight) and have a clear outline, so that its image on the screen is bright and sharp; nothing should obstruct the path between the object and the mirror. (ii) He should measure the distance between the mirror and the screen on which the sharp image of the distant object is formed. Rays from a distant object are nearly parallel to the principal axis, and after reflection from a concave mirror they meet at its principal focus; so the sharp image forms at F, and the mirror-to-screen distance is the focal length.

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19. Q319 CBSE 2018
    While tracing the path ​of a ray of light passing ​through ​a reactangular ​glass slab a student tabulated his observations. If in his experiment for ​two readings he takes two ​values of $\angle i$ as $40^{\circ }$ ​and $50^{\circ }$, what should be the correct ​values of $\angle e$ and $\angle r$ in each case ?
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    The angle of emergence equals the angle of incidence ($40^{\circ }$ and $50^{\circ }$); for glass, $\angle r$ is about $25^{\circ }$ and $31^{\circ }$.

    $\angle e=\angle i$ in each case: for $\angle i=40^{\circ }$, $\angle e=40^{\circ }$; for $\angle i=50^{\circ }$, $\angle e=50^{\circ }$ (the emergent ray is parallel to the incident ray). $\angle r$ must be less than $\angle i$ in each case, because the ray bends towards the normal on entering glass; for ordinary glass ($n=1.5$), $\angle r\approx 25^{\circ }$ ($25.4^{\circ }$) when $\angle i=40^{\circ }$ and $\angle r\approx 31^{\circ }$ ($30.7^{\circ }$) when $\angle i=50^{\circ }$.

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20. Q320 CBSE 2018
    A ​student has to trace the ​path ​of a ray ​of ​light through a glass slab. List four precautions ​he should observe ​for better results.
    Show answer

    Keep the slab fixed, the angle of incidence between $30^{\circ }$ and $55^{\circ }$, and the pins vertical and at least 5 cm apart.

    Any four of the following: (i) Mark the boundary (outline) of the glass slab with a sharp pencil, and do not move the slab until all the pins are fixed. (ii) Keep the angle of incidence between $30^{\circ }$ and $55^{\circ }$. (iii) Fix the two pins on each side at least $5$ cm apart. (iv) Draw the rays and lines with a sharp pencil. (v) Fix the pins vertically, and look at the feet of the pins, not their heads, while lining them up. (Other correct precautions, such as using a slab with clean, smooth and parallel faces, also earn credit.)

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