CBSE Class 10 Science The Human Eye and the Colourful World Assignment Set C

Question : Which of the following colors is least scattered by fog, dust of smoke?
(a) Violet
(b) Blue
(c) Red
(d) Yellow
Answer :  C

Question : Which of the following statements is correct regarding the propagation of light of different colors of white light in air?
(a) Red light moves fastest.
(b) Blue light moves faster than green light.
(c) All the colors of the white light move with the same speed
(d) Yellow light moves with the mean speed as that of the red and the violet light.
Answer :  C

Question : In the following diagram, the path of a ray of light passing through a glass prism is shown:

In this diagram the angle of incidence, the angle of emergence and the angle of deviation respectively are (select the correct option):
(a) X, R and T
(b) Y, Q and T
(c) X, Q and P
(d) Y, Q and P
Answer :  D

Question : Twinkling of stars is due to atmospheric
(a) Ultraviolet radiations are absorbed in the atmosphere.
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds.
Answer :  B

Question : A student traces the path of a ray through a glass prism for four different values of angle of incidence. On analyzing the diagrams, he is likely to conclude that the emergent ray
(a) is always parallel to the incident ray.
(b) Is always perpendicular to the incident ray
(c) Is always parallel to the refracted ray
(d) Always bends at an angle to the direction of incident ray.
Answer :  D

Question : Why is the sun visible to us 2 minutes before actual sunrise and 2 minutes after actual sunset?
Answer :  We are able to see the sun 2 minutes before actual sunrise and 2 minutes after actual sunset because sunrays undergo atmospheric refraction as they travel from an optically rarer medium to an optically denser medium when they enter the earth’s atmosphere due to which they are refracted downwards and hence appear to be above the horizon.

Question : What is colour blindness? 
Answer : Colour-blindness is the inability to distinguish the differences between certain primary colours. This condition results from an absence of color-sensitive pigment in the cone cells of the retina, the nerve layer at the back of the eye. 

Question : Why do stars appear higher than their actual position?
Answer :  Stars appear higher than their actual position as starlight undergoes refraction continuously on entering earth’s atmosphere and bends towards the normal.

Question : On the basis of which observation did Newton conclude that sunlight is made up of seven colours?
Answer :  The observation on the basis of which Newton concluded that sunlight is made up of seven colours was the emergence of white light from the other side of a second identical and inverted prism placed alongside first prism and through which all colours of the spectrum were passed.

Question : Why is the colour of the clear sky blue?
Answer :  The blue colour of sky is due to the scattering of sunlight by the large number of molecules such as fine dust particles, gases, water vapour etc present in the earth’s atmosphere.
As the size of the scatterer is much smaller than the wavelength of light, light of smaller wavelength such as blue is scattered the most as compared to light of longer wavelength.

Question : Use scattering of light? phenomenon to explain why (A) the Sun appears reddish at sun-rise, and (B) the clear sky appears blue.
Answer :  Scattering of light is the phenomenon of change in the direction of light on striking an obstacle like an atom, a molecule, dust particle, water droplet etc.
(A) At the time of sunrise, the sun is near the horizon due to which sunrays have to travel much larger part of the atmosphere. Most of the blue light is thus scattered away. The red colour having the longest wavelength is scattered the least and hence enters our eyes.
(B) The blue colour of sky is due to the scattering of light by the large number of molecules and fine dust particles present in the earth’s atmosphere which mostly scatter light of shorter wavelength such as blue as the size of the scatterer is much smaller than the wavelength of light. 

Question : Study the diagram given below and answer the questions that follow: 

i. Which defect of vision is represented in this case? Give reason for your answer.

ii. What could be the two causes of this defect?

iii. With the help of a diagram show how this defect can be corrected by the use of a suitable lens?

Answer : i. This defect is hypermetropia (Far- sightedness) as the image of a near point is formed beyond the retina.

Question : A beam of white light falling on a glass prism gets split up into seven colours marked 1 to 7 as shown in the diagram. 

i. The colour at position marked 3 and 5 are similar to the colour of the sky and the colour of gold metal, respectively. Is the above statement made by the student correct or incorrect justify?

ii. Which of the above shown positions corresponds approximately to the colour of

a. a brinjal,

b. danger signal,

c. neel which is applied to clothes,

d. orange.

Answer : i. No the satement made by the student is incorrect as because 3 refers to yellow which is the colour of gold metal and 5 refers to blue colours of the spectrum which is the colour of sky.

ii. a. 7 which is violet

b. 1 which it red

c. 6 which is indigo

d. 2 which is orange

 

Long Answers

Question :  What is atmospheric refraction? Use this phenomenon to explain the following natural events. 

i. Twinkling of stars.

ii. Advanced sunrise and delayed sunset.

Draw diagrams to illustrate your answers.

Answer :  The density of the atmosphere, as we know goes on decreasing as the distance above the sea level increases. Hence, the refractive index of a layer of air level goes on changing with distance. Due to this refraction takes place when light passes through the earth's atmosphere. This phenomenon is called as atmospheric refraction.

i. Twinkling of stars: It is due to atmospheric refraction of star light. Due to variations in the refractive indices of the various layers of air, the light from a star passing through the atmosphere changes its path from time to time and therefore the amount of light reaching the eye is not always the same. This increase or decrease in the intensity of light reaching the eye results in the change in apparent position of the star. Hence, the stars appear to be twinkling.

ii. Advanced sunrise and delayed sunset: The figure below shows the actual position of the sun S at the time of sunrise and S' the apparent position of sun. The advanced sunrise and delayed sunset is because of atmospheric refraction.

The light rays starting from the Sun travel from rarer to denser layers. They bend more and more towards the normal as it moves in denser medium.

However, an observer on earth sees an object in the direction of the rays reaching his eyes. The Sun which is actually in a position S below the horizon appears in the position S' above the horizon for him. Thus, the Sun appears to rise early by about 2 min. Similarly, during sunset due to atmospheric refraction, the observer on earth sees the sunset than it would be without atmosphere. Thus, sunset late by about 2 min than its actual timings. These phenomena together increases the length of the day by about 4 min.