Std X Science 1 Chapter 6: Refraction of light

Title : Std X Science 1 Chapter 6: Refraction of light

I. Refraction of light II. Laws of refraction III. Refractive index IV. Dispersion of light

Maharashtra state board solutions for class 10 SSC board science part STD X-Science 1 Space Missions

 Know everything about; Reflection of light .Maharashtra state board class 10 SSC .Get detailed Questions and answers for chapter 6 Science 1 Refraction of light II. Laws of refraction III. Refractive index IV. Dispersion of light

Q.1) What is meant by reflection of light?

Answer: Reflection of light is when light bounces off an object. If the surface is smooth and shiny, like glass, water or polished metal, the light will reflect at the same angle as it hit the surface. This is called specular reflection.

 **Q.2) What are the laws of reflection?

Answer: The laws of reflection are as follows:

1. The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.

 2. The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.

3. The reflected ray and the incident ray are on the opposite sides of the normal.

 Refraction of light

Q.1) Define refraction of light.

 Answer: Light changes its direction when going from one transparent medium to another transparent medium. This is called the refraction of light.

Laws of refraction

 **(Q.1) Explain the laws of refraction with the help of the given ray diagram.

 Answer: Here AN is the incident ray and NB is the refracted ray.

1. Incident ray and refracted ray at the point of incidence N are on the opposite sides of the normal to the surface of the slab at that point i.e. CD, and the three, incident ray, refracted ray and the normal, are in the same plane.

2. For a given pair of media, here air and glass, the ratio of sin i to sin r is a constant. Here, i is the angle of incidence and r is the angle of refraction. Light ray entering a glass slab from air Refractive index

Q.1) Write a note on refractive index and absolute refractive index.

Answer: The change in the direction of a light ray while entering different media is different. It is related to the refractive index of the medium. The value of the refractive index is different for different media and also for light of different colours for the same medium. The refractive indices of some substances with respect to vacuum are given in the table. The refractive index of a medium with respect to vacuum is called its absolute refractive index. Refractive index depends on the velocity of light in the medium.

Q.2) What do you mean by Snell’s Law?

Answer: sin i/sin r = constant = n n is called the refractive index of the second medium with respect to the first medium. This second law is also called Snell’s law. A ray incident along the normal (i = 0) goes forward in the same direction (r = 0).

 Q.3) Give the absolute refractive indices of different media.

 Answer: Substance Refractive index Substance Refractive index Substance Refractive index Air 1.0003 Fused Quartz 1.46 Carbon disulphide 1.63 Ice 1.31 Turpentine oil 1.47 Dense flint glass 1.66 Water 1.33 Benzene 1.50 Ruby 1.76 Alcohol 1.36 Crown glass 1.52 Sapphire 1.76 Kerosene 1.39 Rock salt 1.54 Diamond 2.42

Q.5) Explain refraction of light in different media.

Answer: When a light ray passes from a rarer medium to a denser a medium, it bends towards the normal.  When a light ray passes from a denser medium to a rarer medium, it bends away from the normal. When a light ray is incident normally at the boundary between two media, it does not change its direction and hence does not get refracted

 **Q.6) What is a mirage?

Answer: Local atmospheric conditions affect the refraction of light to some extent. the air near the hot road or desert surface and near the holi flames is hot and hence rarer than the air above it. The refractive index of air keeps increasing as we go to increasing heights. In the first case above, the direction of light rays, coming from a distance, keeps changing according to the laws of refraction. This is called a mirage. In the second example, the direction of light rays coming from objects beyond the holi fire changes due to changing refractive index above the fire. Thus, the objects appear to be moving. Rarer medium Denser medium i r Denser medium Rarer medium r Denser medium Rarer medium

 Q.7) Explain the concept behind twinkling of stars

 Answer: Effect of atmospheric conditions on refraction of light can be seen in the twinkling of the stars. Stars are self luminous and can be seen at night in the absence of sunlight. They appear to be point sources because of their being at a very large distance from us. As the desity of air increases with lowering height above the surface of the earth, the refractive index also increases. Star light coming towards us travels from rarer medium to denser medium and constantly bends towards the normal. This makes the star appear to be higher in the sky as compared to its actual position. The apparent position of the star keeps changing a bit. This is because of the motion of atmospheric air and changing air density and temperature. Because of this, the refractive index of air keeps changing continuously. Because of this change, the position and brightness of the star keep changing continuously and the star appears to be twinkling. Apparent position of a star Star Apparent position of a star increasing refractive index Cold air Hot surface Hot air

 Q.8) Why do we not see twinkling of the planets?

 Answer: We do not see twinkling of planets because, planets are much closer to us as compared to stars. They, therefore, do not appear as point sources but appear as a collection of point sources. Because of changes in atmospheric refractive index the position as well as the brightness of individual point source change but the average position and total average brightness remains unchanged and planets do not twinkle.

 **Q.9) Why do we see the Sun even before it is over the horizon?

 Answer: By Sunrise we mean the appearance of the Sun above the horizon. But when the Sun is somewhat below the horizon, its light rays are able to reach us along a curved path due to their refraction through earth’s atmosphere. Thus, we see the Sun even before it emerges above the horizon. Same thing happens at the time of Sunset and we keep seeing the Sun for a short while even after it goes below the horizon.

Dispersion of light

 Q.3) From incident white light how will you obtain white emergent light by making use of two prisms?

Answer: Take 2 identical glass prisms and place them parallel to each other in an inverted position and allow white light to fall on it. Allow the colours of the spectrum to fall from the second prism. It is seen that the beam of light emerges from the other side of the second prism.

 Q.4) You must have seen chandeliers having glass prisms. The light from a tungsten bulb gets dispersed while passing through these prisms and we see coloured spectrum. If we use an LED light instead of a tungsten bulb, will we be able to see the same effect?

Answer: We can see that the same effect for LED light also. But the spectrum obtained is not continuous as in sunlight.

Q.5) Explain partial and total refraction .

Answer: When light enters a rarer medium from a denser medium, it gets partially reflected i.e. part of the light gets reflected and comes back into the denser medium as per laws of reflection. This is called partial reflection. The rest of the light gets refracted and goes into the rarer medium. Glass Prism 9 As light is going from denser to rarer medium, it bends away from the normal i.e. the angle of incidence i, is smaller than the angle of reflection r. If we increase i, r will also increase according to Snell’s law as the refractive index is a constant. For a particular value of i, the value of r becomes equal to 90o . This value of i is called the critical angle. For angles of incidence larger than the critical angle, the angle of refraction is larger than 90o . Such rays return to the denser medium as shown towards the right in figure 6.9. Thus, all the light gets reflected back into the dense medium. This is called total internal reflection.

 **(Q.7) How is the rainbow formed?

Answer: Rainbow is a beautiful natural phenomenon. It is the combined effect of a number of natural processes. It is the combined effect of dispersion, refraction and total internal reflection of light. It can be seen mainly after a rainfall. Small droplets of water act as small prisms. When light rays from the Sun enter these droplets, it gets refracted and dispersed. Then there is internal reflection as shown in the figure, and after that once again the light gets refracted while coming out of the droplet. All these three processes together produce the rainbow.