Class 10 Science Chapter 10 Case Based Questions - Human Eye and Colourful World

Case study - 1

The Tyndall effect is related to scattering of light. The blue colour of the sky, colour of water in the deep sea, the reddish colour of sun at sunrise and sunset are the colourful phenomenon of light related to scattering. The spreading of light when it strikes to small dust particles, smoke particles which are present in the air is called as scattering of light. When we will send the beam of light through the true solution we will observe the path of beam is not visible. While in case of colloidal solution the path of the beam is visible because of colloidal particles which are somewhat larger in size. We know that atmosphere contains large number of very small particles it means it is a heterogeneous mixture which contains smoke particles, tiny water droplets, suspended particles of dust and air. When light passes through such a heterogeneous mixture the path of the light becomes visible because it strikes with such particles. This effect is called as Tyndall effect. Sometimes we can see the visible beam of light entering into the smoke filled room through a small hole. In canopy of dense forest also, the light coming shows the visible path. Here the scattering of light is due to the water droplets present in the mist. In case of scattering of light, if the particles are having very small size then the light scattered more is of shorter wavelength that is blue colour and scattered red light is less of longer wavelength.

Q1: The reddening of the sun at sunset and sunrise is because of
(a) dispersion of light
(b) scattering of light
(c) refraction of light
(d) total internal reflection of light
Ans: (b)
Explanation: When the sun is close to the horizon during sunrise or sunset, its light passes through a larger portion of Earth's atmosphere than when it's directly overhead. This increased path length through the atmosphere leads to a phenomenon called scattering of light.

Q2: Which colour is having longest wavelength?
Ans: The red colour is having longest wavelength and wavelength increases from violet to red in the sequence VIBGYOR.

Q3: The advance sunrise and delayed sunset are because of
(a) Scattering of light
(b) dispersion of light
(c) reflection of light
(d) atmospheric Refraction of light
Ans: (d)
Explanation: The advance sunrise and delayed sunset are primarily due to the atmospheric refraction of light. Atmospheric refraction is the bending of light as it passes through the Earth's atmosphere, which causes the apparent position of the Sun to be slightly different from its actual position. This bending of light is a result of the varying density of the Earth's atmosphere.

Q4: Which coloured light scatter more and why?
Ans: The blue coloured light scatter more because the wavelength of blue colour light is shortest. And scattering of light increases as the wavelength of light decrease.

Case study - 2

There are various natural  phenomenon associated with light. Refraction of light is the phenomenon in which when light travels from one transparent medium to another transparent medium it changes its direction. The change in direction of light is due to the change in velocity of light in different media. And hence the path of the light also changes in different media. In case of refraction of light through rectangular glass slab we must observe that the incident ray and the emergent ray are parallel to each other. We can see the pencil immersed in water as bent at the water air interface only because of the refraction of light. Again second phenomenon is dispersion of light in which white light when passed through the prism it splits into seven coloured spectrum. And these seven colours are VIBGYOR. We can see here the angle of deviation is different for different colour because different wavelengths of different colour. Also, Newton observed that when second prism is placed inverted to first prism the white light incident on the first prism will come out as white light only when emerges out from second prism. The phenomenon of formation of rainbow is also because of the dispersion of light. In that case the tiny water droplets acts as prism.
But most importantly, the refraction of light occurs only because of the change in refractive index of medium. The twinkling of stars is due to the atmospheric refraction of light.

Q1: Rainbow formation takes place because of which phenomenon related to light?
Ans: Rainbow is formed because of total internal reflection of light, dispersion of light and refraction of light.

Q2: What is mean by total internal reflection?
Ans: When a ray of light travels from one medium to other, if the angel of incidence is greater than the critical angle then the incident light get reflected totally in the same medium is called as total internal reflection.

Q3: Why atmospheric refraction occurs?
Ans: The atmosphere contains different layers which contains hot and cold air. The hotter air is lighter than the cooler air. And hence the refractive index of hot air is less than the cooler air. As there is continuous change in refractive index of different layers of atmosphere, the atmospheric refraction takes place.

Q4: For which colour the angle of deviation is more in case of dispersion?
Ans: The angel of deviation is more for violet colour. And it decreases from violet to red in VIBGYOR.

Case study - 3

As we know that, the ciliary muscles are responsible for change in focal length of the eye lens. And this ability of eye lens to change the focal length is referred as accommodation. The least distance of distinct vision of normal eye is a about 25 cm and the far point of normal eye is found to be at infinity. Because of refractive defects of vision in human being 3 defects are possible they are myopia, hypermetropia and presbyopia. In case of myopia, person is able to see nearby objects clearly but cannot able to see distant objects distinctly. In this defect, the image is formed in front of retina and hence to correct this defect concave lens which is a diverging lens of suitable power is used to form the image on the retina. In case of hypermetropia, the person is not able to see nearby objects clearly but he is able to see distant objects distinctly. And hence in this case, the image is formed behind the retina because of that to correct this defect convex lens which is converging lens of suitable power is used to form the image on the retina.
Presbyopia is the defect of vision which occurs with ageing. Such person is not able to see nearby objects clearly and distinct objects distinctly without proper eye glasses. The persons which are suffering from both myopia and hypermetropia uses bi-focal lenses.

Q1: Which reasons are responsible for myopia?
Ans: The defect myopia occurs because of excessive curvature of eye lens and elongation of the eye balls.

Q2: Hypermetropia is also called as?
Ans: Hypermetropia is also called as far-sightedness because person suffering from hypermetropia is able to see distant objects clearly but cannot able to see nearby objects clearly.

Q3: A student who is sitting on the first bench in the class is not able to see what is written on the board. The student is suffering from which defect and which type of lens is suitable for him?
Ans: The student sitting on the first bench in the class is not able to see what is written in the board because that student is suffering from hypermetropia in which person is able to see distant objects distinctly but cannot able to see nearby objects clearly. And hence to correct his defect he should have to use convex lens of suitable power.

Q4: What is cataract?
Ans: Sometimes, at old age the crystalline lens of people becomes cloudy and milky and that condition is called as cataract because of which the person may lose partial or complete vision. And it is cured by cataract surgery only.