Class 10 Science Chapter 10 Light Reflection And Refraction
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NCERT Solutions For Class 10 Science Chapter 10 Light Reflection And Refraction, in this step-by-step answer guide. In some of State Boards and CBSE schools, students are taught thru NCERT books. As the chapter comes to an end, students are requested few questions in an exercising to evaluate their expertise of the chapter.
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NCERT Solutions for Class 10 Science Chapter 10 Light Reflection And Refraction
Class 10 Science Chapter 10 Light Reflection And Refraction
Q. 1. Which one of the following materials cannot be used to make a lens ?
( a ) Water
( b ) Glass
( c ) Plastic
( d ) Clay
Ans . Correct option : ( d )
Explanation : A lens is a transparent material that means light can pass through it . Since , clay is not transparent so , it cannot be used to make a lens .
Q. 2. 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 ?
( 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
Ans . Correct option : ( d )
Explanation : When an object is placed between the pole and principal focus of a concave mirror , the image formed is virtual , erect , and enlarged .
Q. 3. 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
Ans . Correct option : ( b )
Explanation : When an object is placed at the centre of curvature in front of a convex lens , its image is formed at the centre of curvature on the other side of the lens . The image formed is real , inverted , and of the same size as the object .
Q.4 . 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 .
( a ) both concave .
( b ) both convex .
( c ) the mirror is concave and the lens is convex .
( d ) the mirror is convex , but the lens is concave .
Ans . Correct option : ( a )
Explanation : As per the sign convention , the focal length of a concave mirror and a concave lens are taken as negative . Hence , both the spherical mirror and the thin spherical lens are concave in nature .
Q. 5. 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
Ans . Correct option : ( d )
Explanation : Image formed by a convex mirror is always virtual , erect , and diminished in size than the object .
Similarly , a plane mirror always produces a virtual and erect image of same size as that of the object .
Q. 6. 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
Ans . Correct option ( c )
Explanation : A magnified image of an object will be obtained when it is placed between the radius of curvature and focal length of a convex lens . Magnification is also higher for convex lenses having shorter focal length . Therefore , for reading small letters , a convex lens of focal length 5 cm should be used .
Q. 7. We wish to obtain an erect image of an object , using a concave mirror of focal length 15 cm . What should be the range of distance of the object from the mirror ? What is the nature of the image ? Is the image larger or smaller than the object ? Draw a ray diagram to show the image formation in this case .
Ans . A concave mirror gives an erect , virtual and enlarged image if the object is placed between the pole ( P ) and principal focus ( F ) .
The given focal length is 15 cm . So , to obtain an erect the object must be placed anywhere in between P and F , that is , between 0 cm and 15 cm . Since all the distances are measured from pole so at pole distance is zero . The image formed will be virtual and magnified .
So , range of distance of object is 0-15 cm .
Q. 8. Name the type of mirror used in the following situations :
( a ) Headlights of a car
( b ) Side / rear – view mirror of a vehicle
( c ) Solar furnace
Support your answer with reason .
Ans . ( a ) Mirror used : Concave
Reason : Concave mirror produces powerful bright and parallel beam of light if the light source ( bulb ; in this case ) is placed at their principal focus i.e. , near the reflector .
( b ) Mirror used : Convex
Reason : Convex mirror produces a virtual , erect , and diminished image of the objects placed in front of it . So , thereby providing a wide field of view . It enables the driver to see most of the traffic behind him / her .
( c ) Mirror used : Concave
Reason : Concave mirrors are convergent mirrors . That is why they are used to construct solar furnaces . Concave mirrors converge the light incident on them at a single point known as principal focus . Hence , they can be used to produce a large amount of heat at that point .
Q.9 . One – half of a convex lens is covered with a black paper . Will this lens produce a complete image of the object ? Verify your answer experimentally . Explain your observations .
Ans . The convex lens will form complete image of an object , even if it’s one half is covered with black paper . It can be understood by the following two cases .
Case I : When the upper half of the lens is covered
In this case , a ray of light coming from the object will be refracted by the lower half of the lens . These rays meet at the other side of the lens to form the image of the given object , as shown in the following figure .
Case II : When the lower half of the lens is covered
In this case , a ray of light coming from the object is refracted by the upper half of the lens . These rays meet at the other side of the lens to form the image of the given object , as shown in the following figure .
Q. 10. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm . Draw the ray diagram and find the position , size and the nature of the image formed .
Ans . Given , height of object = 5 cm
Position of object , u = – 25 cm
Focal length of the lens , f = 10 cm
Hence , position of image , v = ?
We know that ,
Thus , distance of image is 16.66 cm on the opposite side of lens .
Now , magnification = v / u
That is , m = 16.66 / -25 = -0.66
Also , m = height of image / height of object
Or , -0.66 = height of image / 5 cm
Therefore , height of image = -3.3 cm
The negative sign of height of image shows that an inverted image is formed .
Thus , position of image = At 16.66 cm on opposite side of lens
Size of image = -3.3 cm at the opposite side of lens
Nature of image : Real and inverted .
Q. 11. A concave lens of focal length 15 cm forms an image 10 cm from the lens . How far is the object placed from the lens ? Draw the ray diagram .
Ans . Focal length of concave lens ( OF₁ ) , f = -15 cm
Image distance , v = -10 cm
According to the lens formula ,
The negative value of u indicates that the object is placed 30 cm in front of the lens . This is shown in the following ray diagram .
Q. 12. 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 .
Ans . Focal length of convex mirror , f = +15 cm
Object distance , u = -10 cm
According to the mirror formula ,
The positive value of magnification indicates that the image formed is virtual and erect . [ 1 ½ ]
Q. 13. The magnification produced by a plane mirror is +1 . What does this mean ?
Ans . Magnification produced by a mirror is given by the relation :
The magnification produced by a plane mirror is +1 , which means that the image formed by the plane mirror is of the same size as that of the object . The positive sign shows that the image formed is virtual and erect .
Q. 14. 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 .
Ans . Object distance , u = -20 cm
Object height , h = 5 cm
Radius of curvature , R = 30 cm
The height of image is positive which shows that image formed is erect .
So , a virtual , erect and smaller than object image is formed .
Q. 15. An object of size 7.0 cm 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 focused image can be obtained ? Find the size and the nature of the image .
Ans . Object distance , u = -27 cm
Object height , h = 7 cm
Focal length f = -18 cm
Using the mirror formula
The height of image is negative which shows that image is inverted .
Q. 16. Find the focal length of a lens of power – 2.0 D. What type of lens is this ?
A concave lens has a negative focal length . Hence , it is a concave lens .
Q. 17. 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 ?
Q. 1. Define the principal focus of a concave mirror . [ NCERT Q. 1 , Page 168 ]
Ans . Principal focus also called focal point is a point where incident rays which are parallel to principal axis get converged after reflection from the mirror .
The principal focus always lies on the principal axis , and all the rays which is parallel to principal axis will pass through principal focus after reflection .
Q. 2. The radius of curvature of a spherical mirror is 20 cm . What is its focal length ? [ NCERT Q. 2 , Page 168 ]
Ans . Radius of curvature , R = 20 cm
Radius of curvature of a spherical mirror ,
R = 2 x focal length ( f )
R = 2f
20 = 2f
⇒ f = 20/2 = 10 cm
Therefore , the focal length of given spherical mirror is 10 cm .
Q. 3. Name the mirror that can give an erect and enlarged image of an object . [ NCERT Q. 3 , Page 168 ]
Ans . A concave mirror will give an erect , enlarged and virtual image of an object .
To obtain an erect , virtual and enlarged image from a concave mirror the object must be placed in between the principal focus and pole of the mirror .
Q. 4. Why do we prefer a convex mirror as a rear – view mirror in vehicles ? [ NCERT Q. 4 , Page 168 ]
Ans . Rear view mirrors should have a large field of view so that drivers can easily see most of the traffic behind them . Convex mirrors always form , diminished , erect , and virtual images of the objects .
Due to this property of forming diminished , erect , and virtual images , convex mirrors are preferred as rear – view mirrors .
Q. 5. Find the focal length of a convex mirror whose radius of curvature is 32 cm . [ NCERT Q. 1 , Page 171 ]
Ans . The relation between radius of curvature ( R ) and focal length ( f ) is :
R = 2f
R = 2f
⇒ 32 = 2f
f = 32/2 = 16 cm
Q. 6. 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 ? [ NCERT Q. 2 , Page 171 ]
Ans . Magnification produced by a spherical mirror is given by the relation ,
If height of object is h , ⇒ height of image is -3h ( image formed is real )
Object distance is u = -10 cm
Image distance v = ?
Put the values in the formula .
The negative sign denotes that an inverted image is formed in front of the concave mirror at a distance of 30 cm from the pole .
Q. 7. 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 ? [ NCERT Q. 1 , Page 176 ]
Ans . The light ray bends towards the normal . When a ray of light travels from an optically rarer medium to an optically denser medium , it gets bent towards the normal . Water ( 1.33 ) is optically denser than air ( 1.0003 ) , a ray of light travelling from air into the water will bend towards the normal .
Q. 8. 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 x 108 m s-1 . [ NCERT Q. 2 , Page 176 ]
Ans . Refractive index of a medium nm is given by ,
Q. 9. Find out , from the given Table , the medium having highest optical density . Also find the medium with lowest optical density .
Table : Absolute refractive index of some material media
[ NCERT Q. 3 , Page 176 ]
Ans . We know that,
Optical density ∝ Refractive index
It can be observed from the Table that diamond and air , respectively have the highest and lowest refractive index . Therefore , diamond has the highest optical density and air has the lowest optical density .
Q. 10. You are given kerosene , turpentine and water . In which of these does the light travel fastest ? Use the information given in Q. 74 . [ NCERT Q. 4 , Page 176 ]
Ans . Speed of light in a medium is given by the relation for refractive index ( nm ) . The relation is given as :
It can be inferred from the relation that light will travel the slowest in the material which has the highest refractive index and travel the fastest in the material which has the lowest refractive index . It can be observed from the Table that the refractive indices of kerosene , turpentine , and water are 1.44 , 1.47 , and 1.33 , respectively . Therefore , light travels the fastest in water .
Q. 11. The refractive index of diamond is 2.42 . What is the meaning of this statement ? [ NCERT Q.5 , Page 176 ]
Ans . Refractive index of a medium nm is related to the speed of light in that medium v by the relation :
Where , c is the speed of light in vacuum / air
The refractive index of diamond is 2.42 . This suggests that the speed of light in diamond will reduce by a factor 2.42 compared to its speed in air .
Q. 12.Define 1 dioptre of power of a lens . [ NCERT Q. 1 , Page 184 ]
Ans . Power of lens is defined as the reciprocal of its focal length . If P is the power of a lens of focal length Fin meters , then
The S.I. unit of power of a lens is Dioptre . It is denoted by D.
1 dioptre is defined as the power of a lens of focal length 1 metre .
∴ 1D = 1m –¹
Q. 13. 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 . [ NCERT Q. 2 , Page 184 ]
Ans . When an object is placed at the centre of curvature , 2F₁ , of a convex lens , its image is formed at the centre of curvature , 2F₂ , on the other side of the lens . The image formed is inverted and of the same size as the object , as shown in the given figure .
It is given that the image of the needle is formed at a distance of 50 cm from the convex lens . Hence , the needle is placed in front of the lens at a distance of 50 cm .
Object distance , u = -50 cm
Image distance , v = 50 cm
Focal length = f
According to the lens formula ,
Hence , the power of the given lens is + 4 D.
Q. 14. Find the power of a concave lens of focal length 2 m . [ NCERT Q.3 , Page 184 ]
Ans . Focal length of concave lens , f = 2 m
Here , negative sign arises due to the divergent nature of concave lens . Hence , the power of the given concave lens is D.
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