SSLC Notes Class 10 Science – Refraction of Light Multiple Choice Questions:
1.) The speed of light in vacuum is found to be
a) 2.25×10^8 m/s
b) 3×10^8 m/s
c) 2×10^8 m/s
d) 3×10^8 km/s
Ans: b) 3×10^8 m/s
2.) As optical density of the medium increases the speed of light in that medium
a) increases
b) remains same
c) decreases
d) cannot say
Ans: c) decreases
3.) During refraction of light, the light rays get deviated in media’s because of the
a) difference in refractive index
b) difference in optical densities
c) difference in speed of light
d) all of the above
Ans: d) all of the above
4.) When light ray enters from medium of lower optical density to medium of greater optical density then refracted ray deviates
a) away from the normal
b) does not changes direction
c) towards the normal
d) none
Ans: c) towards the normal
5.) When a ray of light enters from a medium of greater optical density to medium of lower optical density then refracted ray deviates
a) towards the normal
b) away from the normal
c) goes in same direction
d) none
Ans: b) away from the direction
6.) The ratio of sine of angle of incidence to the sine of angle of refraction (sini/sinr) is constant and that constant is called as
a) speed of light
b) optical density of medium
c) refractive index
d) none
Ans: c) refractive index
7.) Optical fibres, endoscope works on the principal of
a) reflection
b) refraction
c) scattering
d) total internal reflection
Ans: d) total internal reflection
8.) Concave lens is also called as
a) converging lens
b) diverging lens
c) both a and b
d) none
Ans: b) diverging lens
9.) Convex lens is also called as
a) converging lens
b) diverging lens
c) both a and b
d) none
Ans: a) converging lens
10.) As the convex lens is a converging lens it’s principal focus is
a) virtual
b) real
c) both a and b
c) cannot predict
Ans: b) real
11.) As the concave lens is a diverging lens it’s principal focus is
a) virtual
b) real
c) both a and b
d) none
Ans: a) virtual
12.) The lens equation is given as
a) 1/f = 1/v + 1/u
b) 1/f = 1/u – 1/v
c) 1/f = 1/v – 1/u
d) 1/v = 1/f – 1/u
Ans: c) 1/f = 1/v – 1/u
13.) The reciprocal of the focal length of the lens is called as
a) refractive index
b) optical density
c) power of lens
d) radius of curvature
Ans: c) power of lens
14.) The SI unit of power of lens is
a) meter
b) dioptre
c) m/s
d) watt
Ans: b) dioptre
15.) Power of concave lens is
a) positive
b) neutral
c) zero
d) negative
Ans: d) negative
Very Short Answer Questions:
1.) What is the Snell’s law of refraction?
Ans:
Snell’s law of refraction states that the ratio of sine of angle of incidence to the sine of angle of refraction (sini/sinr) is constant and that constant is called as refractive index.
2.) What is the relationship between speed of light and optical density of the medium?
Ans:
The optical density of the medium and speed of light are inversely proportional to each other. Because as the optical density of the medium increases, the speed of light through those medium decreases.
3.) If a ray of light incident normally on the air water interface, then what is the direction of refracted ray after refraction?
Ans:
When a ray of light incident normally on a air water interface then the ray of light does not changes its direction and goes along the same direction within any deviation.
4.) What is relative refractive index?
Ans:
The refractive index of one medium with respect to another medium is called as relative refractive index.
5.) What is absolute refractive index?
Ans:
The refractive index of the medium with respect to vacuum is called as absolute refractive index.
6.) Give the formula for absolute refractive index.
Ans:
The formula for absolute refractive index is as follows:
Absolute refractive index= speed of light in air/speed of light in medium = c/v
7.) What is the unit of refractive index?
Ans:
The quantity refractive index is unit less.
8.) What is the refractive index of glass and water?
Ans:
The refractive index of glass and water is found to be 4/3 and 3/2 respectively.
9.) What is the value of critical angle in water?
Ans:
The critical angle in water is found to be 48.6°
10.) Define a lens.
Ans:
A lens is the transparent medium which has spherical surfaces.
11.) What is optic centre in case of lenses.
Ans:
In case of lenses optic centre is the midpoint of the lens and denoted by P.
12.) What is the principal focus of the convex lens?
Ans:
When rays of light are incident parallel to the principal axis then after refraction all these rays converges at a single point along principal axis of the convex lens and that point is called as principal focus of the convex lens.
13.) Why the principal focus of concave lens is virtual?
Ans:
In case of concave lens, it is impossible to get real convergence of light rays and hence the principal focus of the concave lens is virtual.
14.) What is the focal length in case of lenses?
Ans:
In case of lenses, focal length is the distance between optic centre P and principal focus F.
And it is denoted by f.
15.) What is the SI unit of focal length.
Ans:
The SI unit of focal length is meter.
Short Answer Questions:
1.) State the laws of refraction.
Ans:
Following are the two laws of refraction.
First law: The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.
Or the angle of incidence i, the angle refraction r and the normal at the point of incidence all lie in the same plane.
Second law: The ratio of sine of angle of incidence to the sine of angle of refraction that is sini/sinr is constant and is called as refractive index of that medium. This law is also called as Snell’s law.
2.) What is the refractive index of glass if the speed of light in glass medium is 2×10^8 m/s.
Ans:
Given that,
Speed of light in glass = 2×10^8 m/s
Speed of light in air = 3×10^8 m/s
As we know that,
Refractive index of glass = speed of light in air/ speed of light in glass
= 3×10^8/2×10^8 = 3/2 = 1.5
Thus, refractive index of glass is found to be 1.5
3.) Define critical angle in case of total internal reflection and what is the value of critical angle for water?
Ans:
When ray of light enters from a medium of greater optical density to the medium of lower optical density then the value of angle of incidence for which the angle of refraction is 90° that value of angle of incidence is called as critical angle.
The value of critical angle for water is 48.6
4.) What is total internal reflection. Give its applications.
Ans:
When a ray of light enters from a medium of greater optical density to the medium of lower optical density at an angle of incidence which is greater than critical angle for that medium, then at that time the ray of light get reflected in the same medium without undergoing refraction. That means the ray of light get totally internally reflected and this phenomenon is called as total internal reflection.
Following are the applications of total internal reflection:
In medical field, in endoscope the phenomenon of total internal reflection is used.
In telecommunication field, in optical fibre cables total internal reflection phenomenon is observed due to which thousands of signals of different frequencies can be sent to distant places without loosing intensity of light.
5.) How we can say a convex lens as converging lens and it’s focus is real. Explain with diagram.
Ans:
- As we know that, in case of convex lens when rays of light incident parallel to the principal axis converges at a single point along the principal axis after refraction as shown in figure. This point of convergence is called as principal focus of convex lens.
- And hence convex lens is called as converging lens.
- As the light rays converges at a point in front of convex lens, the principal focus of the convex lens is said to be real.
6.) How we can say a concave lens as diverging lens and it’s principal focus is virtual. Explain with diagram.
Ans:
- When rays of light incident parallel to the principal axis of the concave lens, the light rays diverge from one another after refraction and all these rays appears to be originated from the single point on the same side of the lens. And the point is called as principal focus of concave lens.
- And hence we can call concave lens as diverging lens.
- Also, it is impossible to produce convergence of light rays by using concave lens and hence principal focus of concave lens is said to be virtual.
7.) If the ray of light is entering from X medium to Y medium and angle of incidence is 60° and angle of refraction is 45° then find refractive index of the medium B with respect to medium A.
Ans:
Given that,
Angle of incidence i = 60°
Angle of refraction r = 45°
As we know that, refractive index of medium B with respect to medium A is given as,
u = sini/sinr = sin60/sin45 = (√3/2)/(1/√2) = √3/√2
Thus, refractive index of medium B with respect to medium A is found to be √3/√2.
8.) If the focal length of the convex lens is 0.25m then find its power.
Ans:
Given that,
For convex lens:
Focal length f= 0.25m
As we know that, the power of lens is given by
P = 1/f = 1/0.25 = 4D
Thus, the power of lens is found to be 4 dioptre.
9.) How extent of bending depends on refractive index of the medium. Give example.
Ans:
- As we know that, when a ray of light enters from rarer medium to denser medium after refraction the ray of light bends towards the normal.
- For example: If a pencil is dipped in water, we can see the pencil appears like bent at the air water interface. Because of change in path of the ray of light at air water interface. According to law of refraction when light enters from air to water because of change in refractive index (air and water) the speed of light changes and hence the path of the light also changes. And finally it bents or changes direction while entering from air to water medium, here it bends towards the normal.
- Thus, we observe the pencil is bent at the air water interface.
- Similarly, if we are using kerosene or turpentine instead of water we can see the bending of pencil is different in case of water and kerosene. Because, the refractive index of water and kerosene is different that means speed of light will be also different through that medium.
- In this way, we can explain the extent of bending how varies with refractive index of the medium and speed of light also.
10.) Explain optically rarer and optically denser medium.
Ans:
- Initially we will define optical density.
- Optical density is the ability of the medium to pass the light rays through it.
- The medium with greater optical density is the denser medium and the medium with lower optical density is the rarer medium.
- But, speed of light in rarer medium is greater than speed of light in denser medium.
- That means, for optically rarer medium optical density is less and speed of light through it is more and hence refractive index is also less.
- But on the other hand, for optically denser medium like glass the optical density is greater than means speed of light is less and hence refractive index is more.
- You can compare rarer and denser medium with their refractive indices to study in detail.
For example:
Air: R.I. = 1.003
Ice: R.I. = 1.31
Water: R.I. = 1.33
Crown glass: R. I. = 1.52
Diamond: R.I. = 2.42
- Thus, diamond is having greatest refractive index and hence it’s optical density is greater and hence the speed of light through it is lower or less.
11.) Following table shows the medium and their refractive indices. State through which medium the speed of light is less and more with correct reason.
Glass = 1.52
Glycerine= 1.47
Sun Flower Oil = 1.47
Water = 1.33
Flint glass = 1.62
Ans:
As we know that,
- More is the refractive index more is the optical density of the medium and less is the speed of light through that medium.
- Also, we have relationship between refractive index and speed of light in that medium as given below.
- Refractive index of medium= speed of light/speed of light in that medium
- Hence, u = c/v
- That means, speed of light in that medium and refractive index are inversely proportional to each other.
- More the refractive index, less is the speed of light through that medium.
- And less is the refractive index, more is the speed of light through that medium.
- Thus, here from given medium and their refractive indices we can conclude that flint glass is having greatest refractive index and hence speed of light through it is lowest as compared to other medium given here.
- And on the other side, water has lowest refractive index and hence speed of light through it is greatest as compared to other medium given here.
12.) Which lens gives us the image size as same as the object size and when?
Ans:
- As we know that, convex lens gives the image size as same as the object size.
- When the position of the object is at C1 that time image is formed at C2 which is having the same size as the object size.
- Here the figure given shows it. Also, in this case the image formed is having same size, real and inverted as shown.
13.) Find the refractive index of the medium in following cases.
1) angle of incidence i= 20°, angle of refraction r= 13°
2) angle of incidence i = 10°, angel of refraction r= 15°
Ans:
We know that, according to Snell’s law of refraction,
The ratio of sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media and that constant is called as refractive index of the medium.
Thus,
Sini/Sinr = constant = u
1)
sini/sinr = sin(20°)/sin(13°) = 0.34/0.22= 1.5
Here, refractive index is found to be 1.5
2)
Sini/sinr = sin(10°)/sin(15°) = 0.17/0.26 = 0.7
Thus, here refractive index is found to be 0.7
14.) What will be the position and nature of the images in following cases for convex lens.
a)object placed at infinity
b) object placed beyond C1
c) object between F1 and C1
Ans:
For convex lens:
a)
When object is placed at infinity, the image will be formed at focus F2 which is highly diminished, point sized, real and inverted.
b) when the object is placed beyond C1, the image will be formed between F2 and C2 which is diminished, real and inverted.
c) when object is placed between F1 and C1 the image will be formed beyond C2 which is enlarged, real and inverted.
15.) What will be the position and nature of the image in the following cases for concave lens.
a) at infinity
b) between infinity and optical centre O
Ans:
For concave lens:
a) if the object is placed at infinity then image will be formed at F1 which is highly diminished, point sized, virtual and erect.
b) when object is placed between infinity and optical centre O then image will be formed between F1 and O which is diminished, virtual and erect.
Long Answer Questions:
1.) How refraction is responsible for twinkling of stars?
Ans:
- As we know that, refractive index changes if the medium changes.
- When we see the stars there position we observe is the apparent position and which is changing continuously and hence we are able to see twinkling of stars.
- Now we can discuss in detail how refraction is responsible for twinkling of stars.
- The earth’s atmosphere contains different layers which has different refractive indices and when star light travels through this layers of changing refractive index it undergoes continuous refraction.
- Because of that, the star light get bended towards the normal and we see the position of the star which is different than actual position of the star.
- But, this apparent position of the star also changes continuously as the refractive index of atmospheric layers changes.
- Also, stars are at larger distance from us and hence we can observe them as a point source of light and hence light coming from stars and reaching our eyes sometimes appear brighter and sometimes fainter. And this flickering of intensity of star light is called as twinkling of stars.
- Following diagram explains the twinkling of stars.
