NCERT SCIENCE 10 CH10 LIGHT - IN TEXT QUESTION ANSWER

THE PHYSICS CAFE                                                      

CHAPTER TEN

LIGHT: REFLECTION AND REFRACTION

Ncert in-text Question Answer
Page 168 Ncert Textbook
 

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

Answer: Light rays coming parallel to the principal are converged at a point on its principal axis. Then this point is known as the principal focus of the concave mirror.

Question 2: The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Answer: Given Data: Radius of curvature R = 20 cm

We know that, R = f / 2

              20 = f / 2

Hence, the focal length of the given spherical mirror is 10 cm.

Question 3: Name the mirror that can give an erect and enlarged image of an object.

Answer: Concave mirror

Question 4: Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Answer: Convex mirror gives an erect image and wider field of view. Wider field of view helps to see in more area behind and erect image helps in interpretation about the things behind the vehicle, easily and fast. That why convex mirrors are preferred as a rear-view mirror in vehicles.

 

Page 171 Ncert Textbook

Question 1: Find the focal length of a convex mirror whose radius of curvature is 32 cm.   Answer: Given Data: Radius of curvature, R = 32 cm

We know that,         R = f / 2

                             32 = f / 2

 Hence, the focal length of the given convex mirror is 16 cm.

Question 2: A concave mirror produces three times magnified (enlarged) real image of object placed at 10 cm in front of it. Where is the image located?

Answer: Given Data: u = 10 cm

Enlarged real image means image is formed same side of object

                                m = -3

After using sign convention, u = -10 cm

Magnification produced by a spherical mirror is given by the relation,  

                             m = -v / u

                                 -3 = -v / -10

                             v = -30 cm

Here, the image is located at a distance of 30 cm in front of the given concave mirror.  

 

Page 176 Ncert Textbook

Question 1: 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: Yes, the light ray will bend towards the normal. Because when a ray of light travels from a rarer medium to a denser medium, it gets bent towards the normal. We know air is rarer medium as compared to water and light is travelling form rare medium to denser medium here.

Question 2: 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 × 10⁸ m sˉ ˡ

Solution: Given Data: light is coming from air to glass so refractive index of glass with respect to air is given µg = 1.50

                                      C = 3 × 10⁸ m sˉ ˡ

Now,         µg = Speed of light in air / Speed of light in glass

                 1.50 = 3 × 10⁸ / Speed of light in glass

So speed of light in glass is 2 × 10⁸⁸ m sˉ ˡ

Question 3: Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.

Answer: Diamond medium highest optical density and air medium has lowest optical density

Hint:  Refractive index of a medium = Speed of light in air / Speed of light in medium

Speed of light in medium decrease means medium is denser. It means also refractive index increases. It means refractive index and optical density both changes in same aspect.

Conclusion: Optical density directly proportion to refractive index.

Speed of light in medium is inversely proportion to refractive index 

Question 4: You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3.

Answer: Light travel fast means refractive index is low. In all three water has the lowest refractive index. So light will travel fastest in water

Question 5: The refractive index of diamond is 2.42. What is the meaning of this statement?   

Answer: Refractive index of a Diamond µd = 2.42

                  µd = Speed of light in air / Speed of light in diamond

 

Speed of light in diamond = Speed of light in air / µd

Speed of light in diamond = Speed of light in air / 2.42

It means speed of light in diamond is 1 / 2.42 times of Speed of light in air.

 

Page 184 Ncert Textbook

Question 1: Define 1 dioptre of power of a lens.

Answer: Power of lens is reciprocal of focal length.

                   P = 1 / f

                   If f = 1 m

Then           P = 1 mˉ ˡ or 1 D

So, Power of a lens is 1 dioptre if its focal length is 1 meter.

Question 2: 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 Data: v = 50 cm

                                 hi = ho = x

    Real and inverted image 

After using sign convention, v = 50 cm

                                            hi = x

                                            ho = -x

                                           m = hi / ho = v / u                           

       m = -1 = 50 / u

                                                u = - 50 cm

So, needle must placed 50 cm in front of convex lens.

Now by using lens formula, 1 / f = 1 / -u + 1 / v

By solving                                f = -25 cm or .25 m

Now, power of lens is given by P = 1 / f (value of f must be in meter)

                                              P = 1 / .25 = 4 D

Question 3: Find the power of a concave lens of focal length 2 m.

Answer: Given Data: Focal length of concave lens f = 2 m

After using sign convention, f = -2 m

Power of lens is reciprocal of focal length.

                   P = 1 / f

                   If f = -2 m

Then           P = -.5 D

The power of this concave lens is -.5 Dioptre.

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