VSAQ for Light Reflection and Refraction Class 10 Science NCERT
Important Questions1
Define the term principal axis of a spherical mirror.
Answer
The line which joins the pole and the centre of curvature of a spherical mirror, is the principal axis of a spherical mirror.
VSAQ
2
What is virtual image?
Answer
If the rays of light do not actually meet after reflection or refraction, but appear to meet when produced backwards, then that point constitutes virtual image.
VSAQ
3
Define the principal focus of a concave mirror.
Answer
Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflection from the mirror. This point is known as the principal focus of the concave mirror.
VSAQ
4
Write two different uses of concave mirrors.
Answer
It is used for torches, searchlights, headlights of vehicle etc.
VSAQ
5
What do you mean by lateral inversion of the image in mirrors?
Answer
If we look into the mirror to see the image, we see that the image is inverted sideways, i.e., if we move our left hand, the image appears to move our right hand.
VSAQ
6
What are the two factors on which the lateral displacement of an emergent ray from a glass slab depends?
Answer
Angle of incidence and thickness of slab.
VSAQ
7
What is the magnification of the images formed by plane mirror and why?
Answer
Its magnification is +1 because plane mirror always forms image equal to object’s size.
VSAQ
8
What is the minimum distance between an object and its real image in case of a concave mirror when the object is at C?
Answer
Zero (0), when the object is at C.
VSAQ
9
We prefer a convex as a rear view mirror in vehicles. Why?
Answer
Due to its wide field of view.
VSAQ
10
When a light rays passes obliquely through the atmosphere in an upward direction, how does its path generally change?
Answer
Light will bend away from its normal direction.
VSAQ
11
Explain why a ray of light passing through the centre of curvature of a concave mirror, gets reflected along the same path.
Answer
The ray passing through the centre of curvature is incident to the mirror along its normal so ∠i = ∠r = 0°. Therefore, the ray retraces its path.
VSAQ
12
Why does the bottom of a tank or a pond containing water appear to be raised?
Answer
Light from the bottom of the pond undergoes change in path at the interface of air and water. As a result, the rays appear to come from a raised surface rather than the actual bottom surface of the pond.
VSAQ
13
What are the values of angle of incidence, i and angle of reflection, r for a normal incidence?
Answer
Normal incidence implies, i = 0°. Hence, from the second law of reflection, r = 0°.
VSAQ
14
The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer
Radius of curvature,
R = 20 cm
Radius of curvature of a spherical mirror,
= 2 × focal length (f)
R = 2f
⇒ f = R/2 = 20/2 = 10 cm
Hence, the focal length of the given spherical mirror is 10 cm.
VSAQ
15
What is meant by power of a lens?
Answer
The ability of a lens to coverage or diverge the rays of light, is called power of lens. It is equal to the reciprocal of the focal length (i.e. P = 1/f).
VSAQ
16
Define angle of incidence and angle of refraction.
Answer
Angle of incidence is the angle I made by the incident ray with the normal. Angle of refraction is the angle r made by the refracted ray in the second medium with the normal.
VSAQ
17
What is the unit of refractive index?
Answer
Refractive index being a ratio of two similar quantities and has no unit.
VSAQ
18
Define 1 dioptre of power of a lens.
Answer
1 dioptre is defined as the power of a lens of focal length 1 metre.
1 D = 1 m-1
⇒ P = 1/f(m)
VSAQ
19
State a condition for no refraction of light entering of light entering from one medium to another.
Answer
(i) Light incident normally.
(ii) Equal refractive indices of two media.
VSAQ
20
What is the change in image observed as the object is moved from initially towards the concave lens?
Answer
The size of the image increases slightly, through it remains diminished in comparison to the size of the object.
VSAQ
21
Why is refractive index of atmosphere different at different altitudes?
Answer
Refractive index of atmosphere is different at different altitudes because the air density changes with altitudes.
VSAQ
22
How does the size of the image change as the object is brought closer from infinity towards the convex lens?
Answer
The size of the image is formed keeps on increasing as the object is brought closer towards the convex lens.
VSAQ
23
The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer
The refractive index of diamond 2.42 suggests that the speed of light in diamond will reduce by a factor 2.42 as compared with its speed in air.
VSAQ
24
Why does light change its path as the medium changes during the transit?
Answer
Speed of light is different in different media. As the medium changes, the light has to choose a path of minimum time. Hence, the direction of the light changes. This phenomenon is known as refraction of light.
VSAQ
25
Arrange the following common substances in the increasing order of refractive indices, Ice, Kerosene, Glass, Diamond, Alcohol and Water.
Answer
Order of increasing refractive indices:
Ice, Water, Alcohol. Kerosene, Glass, Diamond.
VSAQ