10. Light - Reflection and Refraction

CHAPTER  10.
LIGHT - REFLECTION AND REFRACTION
PAGE No: 168

1. Define the principal focus of a concave mirror.

Rays of light falling parallel to the principal  axis of a concave mirror  after reflection converges at a common fixed point on the principal axis .
This common fixed point on the principal axis is known as the principal focus of a concave mirror

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

Radius of curvature = 2  x  focal length
20  = 2 x f
f   =   20/2
f  =10.cm
Focal length is 10 cm

3. Name a mirror that can give an erect and enlarged image of an object?

A concave mirror can give an erect and enlarged image when the object is placed between the principal focus and the pole.

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

We prefer a convex mirror as a rear view mirror because of
1. It gives an erect and diminished image of a large area in front of the mirror.
2. It gives a wider field of view as it curved outwards than a plane mirror

PAGE No: 171

1.Find the focal length of a convex mirror whose radius of curvature is 32 cm ?

For a convex mirror
Radius of curvature and focal length are positive
Radius of curvature = 2  x  focal length
32  = 2 x f
f   =   32/2
f  =16.cm
Focal length is  + 16 cm

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

m =  - 3 ( since image is three times magnified and negative sign since  the image is real)
u =  -10 ( Negative sign indicates the object is in front , that is to the left of the mirror)
v = ?
Magnification  m  = -v/u
                        -3  = -v / -10

        -v   =  -10 x -3  =  + 30cm

          v =  -30 cm
The image is located at a distance of 30 cm in-frond of the mirror.

PAGE No: 176

1.A  ray of light travelling in air enters obliquely into water. Does the light ray bent towards the normal or away from the normal why ?

The light ray bents towards the normal .
Air is a rarer medium and the speed of light is greater  in a rarer medium than a denser medium.
Water is a denser medium compared to air, hence the speed of light is less in water
Hence the light ray slows down when entering obliquely from air to water and bend towards the normal.

But if the ray incident along the normal the ray undergo no deviation

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 x 10⁸ m /s

Refrative index of glass  n  = 1.50
Speed of light in vaccum  c = 3 x 10⁸ m /s
 Speed of light in medium   v = ?


n = c / v
  1.50 = 3 x 10⁸  / v

  v = 3 x 10⁸  / 1.50
       =  2 x 10⁸  m/s

3. Find out ( from table 10.3) the medium having highest optical density Also find the medium        with lowest density  

Air has the lowest refractive index of 1.0003
This implies that air has the lowest optical density

Diamond has the highest refractive index of 2.4
This implies that diamond has the highest optical density

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

Kerosene has a refractive index of 1.44 

Turpentine has a refractive index of  1.47

Water  has a refractive index of  1.33

From these information  turpentine has the highest optical density and water has the lowest

As water has the lowest optical density, light travels the fastest in water than the other two.

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

The refractive index of diamond  is  2.42 indicates that the ratio of the speed of light in vacuum / air to that of   diamond is 2.42

PAGE No: 184

1.Define 1 dioptre of power of a lens

One dioptre is the power of a lens having a focal length of 1 meter

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 

Convex lens.
Real and inverted image
v = + 50 cm  ( positive for the image is real and formed along the direction of incident ray )

m = -1 ( size of the image and size of the object are equal, negative sign since the image is real)

m = v/u


-1 = 50 /u

u = -50 cm

Negative sign indicates that the object is placed in the opposite direction of the incident ray
The needle is placed at a distance of 50 cm in-front  of the convex lens


To find the power of lens

1/f  = 1/v - 1/u


     = 1/(+50 )   -    1/( -50)

  1/f     =  2/50  =  1/25

f =25 cm  = 0.25 m 

Power  P = 1/f  ( in meter )

              = 1/0.25
              = + 4 D     ( Dioptre)

The power of the lens is 4 Diopter

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

EXERCISES

PAGE No: 185

1.Which one of the following materials cannot be used to make a lens?

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?

3.Where should an object be placed in front of a convex lens to get a real image of the size of the object?

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-

5. No matter how far you stand from a mirror , your image appears erect . The mirror is likely to be -

6.Which of the following  lenses would you prefer to use while reading small letters found in a dictionary?

7.We wish to obtain an erect image of an object, using a concave mirror of focallength 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the immage? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case

8.Name the type of mirror used in the following situations

(a)Headlight of a car

(b) Side / Rear view mirror of a vehicle.

(c)  Solar furnace

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

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 nature of the image formed

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

12.An object is placed at a distance of 10cm from a covex mirror of focal length 15 cm. Find the position and nature of the immage

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