10. Light - Reflection and Refraction SEBA/ASSEB Class 10 Science– Important Questions and Answers | Assam HSLC

SEBA/ASSEB Class 10 Science Chapter 10: Light - Reflection and Refraction Important Questions and Answers | HSLC Assam

Chapter 10: Light - Reflection and Refraction

Multiple Choose Question answer :

Question: Where should an object be placed in front of a convex lens to get a real image of the size of the object ? [2021]

(a) At infinity     (b) At the principal focus of the lens    (c) At twice of the focal length        (d) Between the optical center of the lens and its principal focus.

Answer: (c) At twice the focal length of the lens.

[ When an object is placed at  (twice the focal length) in front of a convex lens, the image is formed at  on the other side of the lens. The image is real, inverted and equal in size to the object.]

Question: 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

(a) – 30 cm               (b) – 20 cm                  (c) – 40 cm                  (d) – 60 cm

Answer : (b) – 20 cm

[ Here,   ,  ,

We have ,     

Using mirror formula , we have  

    ]

Question: Which of the following statements is true?

(a) A convex lens has 4 dioptre power having a focal length 0.25 m

(b) A convex lens has –4 dioptre power having a focal length 0.25 m

(c) A concave lens has 4 dioptre power having a focal length 0.25 m

(d) A concave lens has –4 dioptre power having a focal length 0.25 m

Answer : (a) A convex lens has 4 dioptre power having a focal length 0.25 m

[Here, 

    ]

Question: Magnification produced by a rear view mirror fitted in vehicles

(a) is less than one

(b) is more than one

(c) is equal to one

(d) can be more than or less than one depending upon the position of the object in front of it

Answer:  (a) less than one.

Question: Under which of the following conditions a concave mirror can form an image larger than the actual object?

(a) When the object is kept at a distance equal to its radius of curvature

(b) When object is kept at a distance less than its focal length

(c) When object is placed between the focus and centre of curvature

(d) When object is kept at a distance greater than its radius of curvature

Answer : (c) When the object is placed between the focus and the centre of curvature.

[ In a concave mirror, when an object is located between the focus (F) and the centre of curvature (C), the mirror forms a magnified and virtual image that is larger than the actual object. ]

Question: Rays from Sun converge at a point 15 cm in front of a concave mirror. Where should an object be placed so that size of its image is equal to the size of the object?

(a) 15 cm in front of the mirror         (b) 30 cm in front of the mirror

(c) between 15 cm and 30 cm in front of the mirror        (d) more than 30 cm in front of the mirror

Answer : (b) 30 cm in front of the mirror .

[ Here,   

We have,  

Therefore, the object should be placed 30 cm in front of the mirror. ]

Question: A full length image of a distant tall building can definitely be seen by using

(a) a concave mirror      (b) a convex mirror

(c) a plane mirror           (d) both concave as well as plane mirror.

Answer : (b) a convex mirror .

[ A convex mirror diverges light rays and allows a wider field of view compared to a concave or plane mirror. ]

Question: In torches, search lights and headlights of vehicles the bulb is placed

(a) between the pole and the focus of the reflector

(b) very near to the focus of the reflector

(c) between the focus and centre of curvature of the reflector

(d) at the centre of curvature of the reflector

Answer :  (b) very near to the focus of the reflector.

[ In torches, searchlights and headlights of vehicles, the bulb is placed at the focus of the reflector. ]

Question:  The focal length of a lens is 1 metre. Its power is : [SEBA 2023]

(a) 0.5 D          (b) 1 D                  (c) 1.5 D              (d) 2 D

Answer:  (b) 1D

[ Here, metre

We have,     ]

Question:  If the refractive index of the glass is 1.5, then speed of light in glass is : [SEBA 2023]

(a) m/s           (b)  m/s              (c)  m/s              (d)  m/s

Answer:  (a)  m/s   

[ Here,    and

We have ,       

Question: A spherical mirror and a thin spherical lens have each a focal length of – 15 cm . The mirror and the lens are likely to be [2024]

(a) both convex     (b) both concave    (c) the mirror is concave and the lens is convex   (d) the mirror is convex and the lens is concave

Answer:  (b) both concave     

[Since both the mirror and the lens have a focal length of – 15 , they are likely to be both concave.]

Question: The image formed by a concave mirror is observed to be virtual , erect and larger than the object. Where should be the position of the object ? [2024]

(a) Between the principal focus and the centre of curvature    (b) At the centre of curvature  

(c) Beyond the centre of curvature         (d) Between the pole of the mirror and its principal focus .

Answer: (d) Between the pole of the mirror and its principal focus.

[ When an object is placed between the pole (P) and the principal focus (F) of a concave mirror, the image formed is virtual, erect and enlarged. It is formed behind the mirror.]

Question: The refractive indices of four mediums P, Q, R , S are 1.23 , 1.5 , 1.25 and 2 respectively. Through which medium, the speed of light is maximum ? [2025]

(a) P       (b) Q           (c) R           (d) S

Answer:  (a)  P

[Since medium P has the lowest refractive index (1.23), the speed of light is maximum in medium P.]

Question: A concave lens has a focal length of 20 cm . What is the power of the lens ? [2025]

(a) – 5 dioptre       (b) – 0.05 dioptre      (c) + 0.05 dioptre  (d) + 5 dioptre

Answer: (a) – 5 dioptre.        

[ Here,  

We have,    ]

Question: A student conducts an activity using a concave mirror with focal length of 10 cm. He places an object 15 cm from the mirror. Where is the image likely to form ? [2025]

(a) At 6 cm behind the mirror      (b) At 30 cm behind the mirror       (c) At 6 cm in front of the mirror          (d) At 30 cm in front of the mirror

Answer: (d) At 30 cm in front of the mirror .

[ Here,  

We have,  

   ]

Question:  The   mirror is used as rear view in vehicles . [SEBA 2023]

Answer:  Convex .

[Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles.]

Question:  Which one of the following materials cannot be used to make a lens?
(a) Water                (b) Glass                    (c) Plastic               (d) Clay

Answer: (d) Clay

[ Lenses need transparent materials to refract light, but clay is opaque, so it cannot focus or bend light properly.]

Answer: (d) Between the pole of the mirror and its principal focus.
Question:  Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.

Answer: (b) At twice the focal length .

[When the object is placed at 2F, the convex lens forms a real, inverted image of the same size at 2F on the other side.]

Question:  No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
(a) plane.          (b) concave.             (c) convex.         (d) either plane or convex.

Answer: (d) either plane or convex.

[ Both plane mirrors and convex mirrors always form erect images, regardless of the object’s distance.]
Question:  Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm.         (b) A concave lens of focal length 50 cm.
(c) A convex lens of focal length 5 cm.           (d) A concave lens of focal length 5 cm.

Answer: (c) A convex lens of focal length 5 cm. 

[ A convex lens with short focal length acts as a magnifier, producing a larger, clearer image of small letters when held close to the eye.]

Question:  Which mirror is used as a rear-view mirror in vehicles?

(a) Plane mirror       (b) Concave mirror     (c) Convex mirror    (d) Cylindrical mirror

Answer:  (c) Convex mirror

Question:  The SI unit of power of a lens is:

(a) Metre                   (b) Dioptre        (c) Centimetre          (d) Watt

Answer:   (b) Dioptre .      

Question:  The focal length of a spherical mirror is 20 cm. Its radius of curvature is:

(a) 10 cm     (b) 20 cm        (c) 40 cm     (d) 60 cm

Answer:  (c) 40 cm

[Here,  

We have, ]

Question:  Which lens is thicker at the centre than at the edges?

(a) Concave lens      (b) Convex lens     (c) Both lenses         (d) Neither lens

Answer:  (a) Concave lens .     

Question:  The image formed by a concave lens is always:

(a) Real and inverted      (b) Virtual and erect     (c) Real and erect            (d) Virtual and inverted

Answer:  (b) Virtual and erect .

Question:  The refractive index of a medium is given by:

(a) v/c       (b) c/v      (c) c × v       (d) c + v

Question:  When an object is placed at the centre of curvature of a concave mirror, the image is formed:

(a) At the focus     (b) At the centre of curvature    (c) Behind the mirror     (d) At infinity

Answer:  (b) At the centre of curvature .

[When an object is placed at the centre of curvature of a concave mirror, the image is formed at the centre of curvature itself.]

Question:  An object placed at 2F₁ of a convex lens forms an image:

(a) At F₂      (b) Between F₂ and 2F₂      (c) At 2F₂    (d) At infinity

Answer:  (c) At 2F₂

Question:  The power of a concave lens of focal length −0.5 m is:

(a) +2 D          (b) −2 D         (c) +0.5 D       (d) −0.5 D

Answer:  (b) −2 D          

[Here,  

We have,  

  ]

Question:  The mirror formula is:

(a)        (b)         (c)          (d)   

Answer: (a)   

Question:  A ray of light travelling from air into glass generally bends:

(a) Away from the normal      (b) Towards the normal     (c) Along the surface    (d) Back into air

Answer:  (b) Towards the normal

[When light travels from air (a rarer medium) into glass (a denser medium), it slows down and bends towards the normal.]

Question:  The magnification produced by a spherical mirror is:

(a)        (b)        (c)          (d)  

Answer: (b)

Question:  The power of a concave lens is generally:

(a) Positive             (b) Negative            (c) Zero           (d) Always equal to one

Answer: (b) Negative.

Assertion–Reason Questions

(A) Both Assertion and Reason are true, and Reason correctly explains Assertion.

(B) Both Assertion and Reason are true, but Reason does not correctly explain Assertion.

(C) Assertion is true, but Reason is false.

(D) Assertion is false, but Reason is true.

Question: Assertion: A convex mirror is used as a rear-view mirror in vehicles.

Reason: A convex mirror forms erect, virtual and diminished images and provides a wider field of view.

Answer: (A). The Reason correctly explains the Assertion.

Question: Assertion: A concave lens always forms a virtual, erect and diminished image of a real object.

Reason: A concave lens diverges the incident rays of light.

Answer: (A). The Reason correctly explains the Assertion.

Question: Assertion: The power of a concave lens is negative.

Reason: The focal length of a concave lens is negative according to the New Cartesian Sign Convention.

Answer: (A). The Reason correctly explains the Assertion.

Question: Assertion: A ray of light passing through the optical centre of a thin lens emerges without deviation.

Reason: The optical centre of a thin lens is the point through which a ray passes approximately undeviated.

Answer: (A). The Reason correctly explains the Assertion.

Question: Assertion: A convex lens always forms a real image.

Reason: A convex lens converges parallel rays of light.

Answer: (D). The Assertion is false because a convex lens forms a virtual, erect and magnified image when the object is placed between the optical centre and the principal focus. The Reason is true.

Question: Assertion: Light bends towards the normal when it travels obliquely from air into glass.

Reason: The speed of light decreases when it enters glass from air.

Answer: (A). The Reason correctly explains the Assertion.

Question: Assertion: A concave mirror can form both real and virtual images.

Reason: The nature of the image formed by a concave mirror depends on the position of the object.

Answer: (A). The Reason correctly explains the Assertion.

Question: Assertion: The image formed by a plane mirror is laterally inverted.

Reason: A plane mirror forms a virtual and erect image of the same size as the object.

Answer: (B). Both statements are true, but the Reason does not explain lateral inversion.

Very Short Question answer :

Question: What is the relation between the radius of curvature and focal length of a spherical mirror?

Answer: The relation between the radius of curvature and focal length of a spherical mirror is 

Question:  Define the refractive index of a medium.

Answer: The absolute refractive index of a medium is the ratio of the speed of light in vacuum to its speed in that medium.

Question:  Write the SI unit of the power of a lens.

Answer: Dioptre (D).

Question:  Which lens is called a converging lens?

Answer: A convex lens is called a converging lens.

Question:  Which lens is called a diverging lens?

Answer: A concave lens is called a diverging lens.

Question:  What is the nature of the image formed by a plane mirror?

Answer: The image is virtual, erect and of the same size as the object.

Question: The magnification produced by a plane mirror is +1. What does this mean?

Answer: The magnification produced by a plane mirror is +1 . It is indicate that the object distance is equal to the image distance in a plane mirror .

Question: Find the power of a concave lens of focal length 2 m.

Solution:  Here,  (concave lens)

 We have, 

Therefore, the power of a concave lens is  – 0.5 D .

Question: Define 1 dioptre of power of a lens.         

Answer: 1 dioptre is the power of a lens whose focal length is 1 metre.

Question: Define the principal focus of a concave mirror. [1M]

Answer:  A number of rays parallel to the principal axis are falling on a concave mirror and the rays are intersecting at a point on the principal axis of the mirror. This point is called the principal focus of the concave mirror.

Question: The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Solution:  Here ,  

We have,

Therefore, the focal length is 10 cm .

Question: Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer:  Here, 

We have,  

Therefore, the focal length of a convex mirror is 16 cm .

Question: Name a mirror that can give an erect and enlarged image of an object. [1M]
Answer:  Concave mirror . 

Question: Why do we prefer a convex mirror as a rear-view mirror in vehicles ?

Answer:  Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. [1M]

Question: Can a convex lens form a virtual image?

Answer: Yes. A convex lens forms a virtual, erect and magnified image when the object is placed between its optical centre and principal focus.

Short Question answer :

Question: What is the reflection of light ? State the two laws of reflection.

Answer: When a ray of light falls on a smooth/polished surface, it is sent back into the same medium. This bouncing back of light is called reflection.

The two laws of reflection are :

(i) The angle of incidence is equal to the angle of reflection.

(ii) The incident ray, reflected ray and normal at the point of incidence lie in the same plane.

Question: What is refraction of light ? State the two laws of refraction of light.

Answer: The change in direction of light when it passes obliquely from one transparent medium to another due to a change in its speed is called refraction.

 The two laws of refraction of light are :

(i) The incident ray, refracted ray and normal at the point of incidence lie in the same plane.

(ii) For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant

Question: Find the focal length of a lens of power – 2.0 D. What type of lens is this?

Answer:  Here,

We have,  

 
The lens is concave lens .

Question: 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?

Answer:  Here,  

We have,  

 

The focal length of the lens is 0.67 m

The prescribed lens is converging (convex lens) .

Question: An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw its ray diagram . [2M 2019]

Answer: The ray diagram is : 

                       

Question: 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 ?

Answer:   A ray of light travelling from the air ( a rarer  medium ) to the water (a denser  medium) slows down and bends towards the normal. When it travels from  water (a denser medium) to the air (a rarer  medium), it speeds up and bends away from the normal . So, the ray of light travelling in air enters obliquely into water .

Question: Define power of a lens. What is its S.I unit ? [2020 2M]

Answer: Power of a lens is the reciprocal of its focal length .

The S.I. unit of power of a lens is dioptre (D).

Question: 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 ? [2M]

Solution:  For concave mirror :  Here, and

We have,  

Therefore, the image is located within 30 cm .

Question: 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.

Answer:  Given, the image distance is equal to the size of the object distance ( i.e.,) .

Here, u  ,

Using lens formula,    

      

    

  

          

 

Power of lens  

Therefore, the power of the lens is – 4 D .

Question: An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.  [2M 2019]

Answer: Here,  , ,

Using mirror formula , we have  

        

   

     

The position of the image distance is 6 cm  and nature of the image is virtual and erect .

The size of the image is diminished .

Question: 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  .

Solution: Here, The refractive index ,  and  
We know that ,

 

 

Therefore, the speed of light in the glass is  .

Question: A convex mirror used for rear-view on an automobile has a radius of curvature of 3.00 m. If a bus is located at 5.00 m from this mirror, find the position, nature and size of the image. [3M]

Solution: Here,  , ,   and

Using mirror formula, 

     

 

 

 

Therefore, the position of the object is 1.15 m .

Magnification

The nature of the of the image is virtual and erect .  The size of the image is diminished .
Question: An object, 4.0 cm in size, is placed at 25.0 cm in front of a concave mirror of focal length 15.0 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the nature and the size of the image.  [3M]

Answer:  Here, ,  , ,  and

Using mirror formula, 

  

 

 

 

 

 

Magnification

The screen should be placed at 37.5 cm from the mirror.

and

The height of the image is 6 cm . The nature of the image is real and inverted . The size of the image is enlarged.

Question: How is the refractive index of a medium related to the speed of light? Obtain an expression for refractive index of a medium with respect to another in terms of speed of light in these two media?

Answer:  The refractive index  of a medium is inversely related to the speed of light  in that medium. It measures how much the light slows down as it travels through the medium compared to its speed in a vacuum.

The refractive index of a medium is  

Where ,  is the speed of light in a vacuum (  m/s), is the speed of light in the medium.

To find the refractive index of one medium relative to another, consider two media: Medium 1 with speed of light ​ and Medium 2 with speed of light  ​. The refractive indices of these media are:    and 

The relative refractive index  of Medium 1 with respect to Medium 2 is: 

Thus, the refractive index of one medium relative to another can be expressed as the ratio of the speeds of light in those two media.

 Question: Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.

Answer: Here ,   and

The refractive index of diamond with respect to glass is  

 

 

Thus, the absolute refractive index of diamond is 2.4 .

Question: An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Answer:  Here, , , 

Using mirror formula , we have   

            

       

    

   

 

Therefore, the image distance is 8.6 cm (the position of the image).

   

 

The image is virtual, erect and smaller in size by a factor of 0.43 .

    

The height of the image is 2.14 cm .

Question: Size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to 1/3rd of its size. At what distance the object has been placed from the mirror? What is the nature of the image and the mirror?

Answer : Here ,   and  

   

Using mirror formula , we have  

   

 

 

Therefore, the object distance of the convex mirror is 40 cm . The nature of the image is virtual and erect .  

Question: Differentiate between a concave mirror and a convex mirror.

Answer: The difference between a concave mirror and a convex mirror :

Concave Mirror	Convex Mirror
(a) Reflecting surface is curved inward	(a) Reflecting surface is curved outward
(b) Principal focus lies in front of the mirror	(b) Principal focus lies behind the mirror
(c) Can form real and inverted images	(c) Always forms virtual, erect and diminished images
(d) Image can be enlarged, same size or diminished	(d) Image is always diminished
(e) Focal length is positive .	(e) Focal length is negative.

Question: Write two differences between a real image and a virtual image.

Answer: The differences between a real image and a virtual image are :

Real image	Virtual image
(i) It can be obtained on a screen.	(i) It cannot be obtained on a screen.
(ii) It is generally inverted for a single mirror or lens forming an image of a real object.	(ii) It is erect for a single mirror or lens forming an image of a real object.

Question: Define the refractive index and calculate it if light travels through a medium at  m/s.

Answer: The refractive index is the ratio of the speed of light in vacuum to its speed in a medium.

Here,  m/s , m/s ,

We have,  

Question: Write three differences between reflection and refraction.

Answer: The differences between reflection and refraction are :

Reflection	Refraction
(i) Light returns to the same medium after striking a surface.	(i) Light enters another transparent medium and changes direction when incident obliquely.
(ii) It occurs at a reflecting surface.	(ii) It occurs at the boundary between transparent media.
(iii) The angle of incidence equals the angle of reflection.	(iii) The angles of incidence and refraction are related by Snell's law.

 Long Question answer :

Question: A 20 mm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm, then –

(a) find the nature, position and size of the image.      (b) find the magnification of the image . [5M 2019]

Answer:  Here,  ,  , ,  and 

Using lens formula , we have 

      

 

  

  

Therefore, the image distance is 30 cm .

   

The magnification of the image is 2 . The image is two times enlarged.

and   

 

The height of the image is 4 cm .  The nature of the image is real and inverted.

Question: Find the power of a concave lens of focal length 2 m. A concave lens has focal length of 15 cm. At what distance should the object from the lens be placed so that it forms an image at 10 cm from the lens? Also, find the magnification and nature of the image produced by the lens. [5M 2019 , 2020]

Answer: Here,  (concave lens)

 

Therefore, the power of a concave lens is  – 0.5 D .

Here,   ,  and

Using lens formula ,     

       

 

          

  

The image distance is 6 cm .

 

Therefore, the magnification produced by the lens is 0.6  .

Question: An object of size 70 mm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.

Answer:  Here,  ,

Using mirror formula , we have  

     

   

   

 

  

  

The screen should be placed at 54 cm from the mirror. The image is real.

           

The size is inverted and enlarge .

and     

The height of the image is 14 cm .

Question: Draw the ray diagram of image formation by a convex mirror when the object is placed at its centre of curvature (C) . An object 5 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.  [2018 5M]

Answer: The ray diagram of a convex mirror :

            

  Here, ,

 

Using mirror formula , we have  

  

 

  

 

Therefore, the image distance is 8.6 cm (the position of the image).

 

 

The image is virtual, erect and smaller in size by a factor of 0.43 .

   

The height of the image is 2.14 cm .

Question: Draw the ray diagram necessary to observe virtual ,erect, diminished (not point size) image of an object using convex mirror of focal length 15 cm .  Magnification produced by a convex mirror is . If the focal length is , find the object distance, image distance in terms of curvature .   [5M 2017]

Answer: The ray diagram convex mirror :

      

 Given,

and 

We have, 

and 

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