"AKC SCIENCE CLASSES"
CLASS 10 TH (CBSE AND MP BOARD)
CHAPTER 10
LIGHT-REFLECTION AND REFRACTION
INTEXT QUESTIONS AND ANSWERS
NCERT INTEXT QUESTIONS (PAGE NO. 168)
Q.01:- Define the principal focus of a concave mirror.
Ans:- It is the point on the principal axis at which a beam of light incident parallel to the principal axis converges after reflection from the concave mirror.
Q.02:- The radius of curvature of a spherical mirror is 20 cm. Which is its focal length?
Ans:- Focal length = ½ ✖ Radius of curvature = ½ ✖ Radius of curvature
Q.04:- Why do we prefer a convex mirror as a rear-view mirror in vehicles?
or
For driving a car what type of mirror would you prefer to see traffic at your back and why?
Ans:- Convex mirrors are used as rear-view mirrors in vehicles to see the the traffic at the rear side. Convex mirrors are preferred because
- They always give an erect, through diminished image.
- They have a wider field of view as they are curved outwards.
NCERT INTEXT QUESTIONS (PAGE NO. 171)
Q.01:- Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Ans:- As the radius of curvature and focal length of a convex mirror are positive, so
R = +32 cm
and f = R/2 = + 16 cm. Ans.
Q.02:- 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?
Ans:- As the image is real, so magnification m must be negative.
∴ m = h'/h = -v/u = -3
or v = 3u
But u = - 10 cm
v = 3 🗙 (-10) = -30 cm. Ans.
Thus the image is located at a distance of 30 cm from the mirror on the object side of the mirror.
NCERT INTEXT QUESTIONS (PAGE NO. 176)
Q.01:- 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?
Ans:- Light travels faster in rarer medium air and slower in denser medium water. As a ray of light travelling in air enters obliquely into water, it slows down and bends towards the normal.
Q.02:- Light enters from air to glass having refractive index 1.50. What is the speed of light in glass? The speed of light in vacuum is 3🗙10⁸ ms⁻¹.
Ans:- Refractive index of glass, ng = 1.50
Speed of light in vacuum, c = 3🗙10⁸ ms⁻¹
Speed of light in vacuum
= c/ng = (3🗙10⁸ ms⁻¹)/1.5 = 2🗙 10⁸ ms⁻¹. Ans.
Q.03:- Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.
Ans:- From Table 10.3 (NCERT book page 175), diameter has highest refractive index ( = 2.42), so it has largest optical density. Air has lowest refractive index ( = 1.0003), so it has lowest optical density.
Q.04:- You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3 (NCERT book page 175).
Ans:- For kerosene, n = 1.44
For turpentine oil, n = 1.47
For water, n = 1.33
As water has lowest refractive index, so light travels fastest in this optically rarer medium than kerosene and turpentine oil.
Q.05:- The refractive index of diamond is 2.42. What is the meaning of this statement?
Ans:- The refractive index of diamond is 2.42. It indicates that the ratio of speed of light in air to that in diamond is 2.42.
NCERT INTEXT QUESTIONS (PAGE NO. 184)
Q.01:- Define 1 dioptre of power of a lens.
Ans:- One dioptre is the power of a lens whose focal length is 1 metre.
Q.02:- 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.
Ans:- Here, v = +50 cm [v is positive for real image]
As the real image is of the same size as the object, so
m = h'/h = v/u = -1
u = -v = -50 cm. Ans.
Now,
2 1
or f = 25 cm = 0.25 m
1 1
P = ----------- = -------------- = +4 D. Ans.
f + 0.25 m
Q.03:- Find the power of a concave lens of focal length 2 m.
Ans:- As the focal length of a concave lens is negative, so f = -2 m
1 1
Power, P = --------------- = ---------- = -- 0.5 D. Ans.
