Class X Science Term - 1 Chapter 6 The Human Eye And The Colourful World with MCQ'Sbtype question

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 CHAPTER AT A GLANCE 



1. An optical prism has two triangular bases and three rectangular lateral surfaces, which are inclined to each other. Angle between it's two lateral faces is called the angle of prism.


 Whenever, refraction of light takes place through a prism, the emergent ray is called the angle of deviation D.  

2. When a beam of white light passes through a glass prism, it splits up into its constituent seven colours. These seven colours are remembered by the acronym 'VIBGYOR'. The splitting of white light into its component colours is called 'dispersion of light's. The band of the coloured components formed due to dispersion is called 'spectrum'. 
 
 
Dispersion is on account of different amount of deviations for light of different colours. Red coloured ray bends the least but violet ray bends the most. Thus, spectrum is formed. 
Seven colours of VIBGYOR can be recombined so as to form white light .



3. Rainbow is a natural spectrum formed in the sky after a rain shower. Tiny water droplets hanging in atmosphere act like small prisms. They refract and disperse  the incident sunlight, then reflect it internally and finally refract it again and the light comes out of drop. Due to dispersion and internal refraction of light different colours reach the observer's eye at different angles and hence rainbow is formed. Rainbow is always formed in a direction opposite to that of sun. 


4. Atmosphere air layer just near the earth surface is optically denser and upper layers of atmosphere are successively rarer  and more rarer. Hence, a light ray passing through atmospheric air undergoes refraction. Since the physical conditions of air are not stationary, the apparent position of the distant object, as seen through the air, fluctuates. It is known as an effect of atmospheric refraction. 

5. Light coming from a distant star on entering into the earth's atmosphere gradually bends towards the normal on account of atmospheric refraction. Hence, the star appears slightly higher than its actual position when viewed near the horizon. 

6. Since the physical conditions of the earth's atmosphere are not stationary, the path of light ray's coming from a star, which behaves almost as a point source of light, goes on varying slightly and it causes the twinkling of star. 
  As planets are much closer to the earth, hence average light coming from them in observer's eye remains the same and hence planets do not twinkle. 

7. On account of atmospheric refraction the sun is visible to us 2 minutes before actual sunrise and for about 2 minutes after actual sunset . Thus, apparent duration of day (from sunrise to sunset) is increased by 4 minutes. 



8. Scattering of light is spreading of light in different directions due to interplay of light with tiny particles like air molecules, dust particles, colloidal solutions etc. 
 Tyndall effect proves experimentally the phenomenon of scattering of light.
 The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue violet but particles of larger size may scatter light of longer wavelengths too. 

9. Blue colour of clear sky is due to scattering os possible. As a result when seen from surface of moon, satellites and from planes flying at very high altitudes the sky appears dark.

10.  Reddish colour of sun at sunrise or sunset, blue colour of water in deep sea are also on account of scattering effect.
 

11. Danger signal lights are of red colour because scattering of red light is least by fog or smoke. Hence, danger signal can be seen even from a longer distance.  
 
 




MULTIPLE CHOIC QUESTION
                   (MCQ) 

MOST IMPORTANT MCQ'S  OF CHAPTER 6 term 1 .. 

Select the correct option out of the four possible options given after the statement of the question. 

1. Stars twinkle due to 

(a) atmospheric refraction.   

(b) atmospheric reflection.

(c) scattering of light.

(d) dispersion of light. 


2. At noon the sun appears white as 

(a) light is least scattered. 

(b) all the colours of the white light are scattered away. 

(c) blue colour is scattered the most.

(d) red colour is scattered the most. 


3. On account of atmospheric refraction 

(a) sun appears 4 minutes before actual sunrise.

(b) sun disappears 4 minutes before actual sunset 

(c) sun is visible 2 minutes before actual sunrise 

(d) sun becomes visible 2 minutes after actual sunrise 


4. If the earth had no atmosphere, the sky would have looked. 

(a) brown 

(b) black 

(c) white 

(d) blue 


5. Which of the following scientist first of all proved that white sunlight is a combination of light of seven different colours ? 

(a) Newton 

(b) Galileo 

(c) Huygen 

(d) Fresnel 


6. When a beam of white light passes through a prism, light of which colour undergoes the least deviation ? 

(a) Red 

(b) Yellow 

(c) Violet 

(d) Blue 


7. Twinkling of stars is due to atmospheric 

(a) dispersion of light by water droplets 

(b) scattering of light by dust particles

(c) refraction of light by different layers of varying refractive indices 

(d) internal reflection of light by clouds 


8. When a ray of light passes through a triangular prism, the light ray 

(a) goes straight 

(b) is deviated towards the base of prism 

(c) is deviated away from the base of prism

(d) is reflected back 


9. An astronaut lands his spacecraft on the surface of moon and observes the sky. He will find the colour of sky as 

(a) deep blue 

(b) light blue

(c) white 

(d) dark 


10. Seven colours of spectrum of white sunlight are remembered by the acronym : 

(a) VIBGYOR 

(b) VBGIOYR 

(c) GIBVYOR

(d) GYORVIB 


11. The angle between the two refracting faces in a prism is called: 

(a) angle of prism

(b) incidence angle

(c) angle of emergence

(d) angle of deviation 


12. Inside of glass prism light of which colour travels fastest ?

(a) Blue 

(b) Red

(c) Green

(d) Indigo 


13. Spreading of light in all directions by minute particles and gas molecules is called : 

(a) scattering of light

(b) dispersion of light 

(c) diffraction of light

(d) atmospheric refraction 


14. Refractive index of glass is maximum for light of 

(a) red colour

(b) violet colour

(c) yellow colour

(d) blue colour 


15. The sky appears reddish at the sunset due to the fact, that 

(a) blue-violet light has been scattered least

(b) red light is scattered most

(c) blue light has been mostly scattered but red light has scattered the least 

(d) for red light the angle of refraction is less than for blue light 



 ~ANSWERS_____


1) a   2) a  3) c  4) b  5) a  6) a  7) c  8) b  9) d  10) a
 11) a  12) b 13) a  14) b  15)  c .........

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