Case Based Questions for Ch 10 The Human Eye and the Colourful World Class 10 Science
Important Questions1
The hotter air is lighter (less dense) than the cooler air above it, and has a refractive index slightly less than that of the cooler air. Since the physical condition of the refracting medium (air) are not stationary, therefore, the light goes from rarer medium to denser medium in atmosphere. This phenomenon is called atmospheric refraction. The twinkling of stars and advanced sunrise and delayed sunset are common examples of atmospheric refraction.
(i) Stars appear to twinkle because of
(a) movement of air
(b) atmospheric refraction
(c) both (a) and (b)
(d) none of these
(ii) Which of the following is not caused because of atmospheric refraction?
(a) Apparent image of Sun is formed closer to the Earth.
(b) Dawn or dusk are formed
(c) Sun can be seen 2 minutes before actual sunrise and 2 minutes after actual sunset.
(d) Clouds look white
(iii) When sunlight enters the atmosphere the colours which scatter first are
(a) only red
(b) red, orange and yellow
(c) blue and green
(d) violet, indigo and blue
(iv) The order of wavelength of seven colours in atmosphere is
(a) V < I < B < G < Y < O < R
(b) V > I > B > G > Y > O > R
(c) V < B < I < G < Y < O < R
(d) V > I > B > G > O > R > Y
Answer
(i) – (b); Due to atmospheric refraction and moving air, the refractive index of medium is not stationary. So due to multiple refraction through atmospheric layers, stars appears to twinkle.(ii) – (d); Clouds look white because water droplets present in the atmosphere scatter all wavelengths due to their large size.
(iii) – (d); Violet, indigo and blue due to their short wavelengths gets scattered more.
(iv) – (a); V < I < B < G < Y < O < R because red colour has less deviation angle and more wavelength than other colours. So, the wavelengths vary as above.
2
Issac Newton was the first to use a glass prism to obtain the spectrum of sunlight. He tried to split the colours of the spectrum of white light further by using another similar prism. He then placed a second identical prism in an inverted position with respect to the first prism. This allowed all the colours of the spectrum to pass through second prism. He found a beam of white light emerging from the other side of the second prism. This observation gave Newton the idea that the sunlight is made up of seven colours. A rainbow is a natural spectrum appearing in the sky after rain.
(i) The range of wavelength of visible light spectrum is
(a) 500 nm to 780 nm
(b) 450 nm to 750 nm
(c) 380 nm to 780 nm
(d) 300 nm to 700 nm
(ii) Many different wavelength of energy produced by light source through prism is called
(a) hydrogen spectrum
(b) light spectrum
(c) corona discharge
(d) frequency modulation
(iii) The critical angle between an equilateral prism and air is 45°, if the incident ray is perpendicular to the refracting surface, then
(a) it is reflected totally from the second surface and emerge perpendicular to the third surface.
(b) it gets reflected from second surface and third surface and emerges from the third surface.
(c) it keeps reflecting from all the three sides of the prism and never emerges out.
(d) after deviation, it gets refracted from the second surface.
(iv) The dispersive power of prism depends upon
(a) the shape of the prism
(b) the angle of the prism
(c) height of the prism
(d) the material of the prism
Answer
(i) – (c); For visible light spectrum range is 380 nm to 780 nm.(ii) – (b); Light spectrum is the different wavelength of energy produced by light source.
(iii) – (a); It is refracted by first surface then reflected from second surface, emits out perpendicular from third surface.
(iv) – (d); Depends on nature of material of prism.
3
The Earth's atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air.
When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches us after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particle is known as Tyndall effect.
Tyndall effect can also be observed when sunlight passes through a canopy of dense forest. The colour of the scattered particle's light depend upon size of scattering particles.
(i) The phenomenon of scattering of light by colloidal particles is called
(a) Corona effect
(b) Tyndall effect
(c) dispersion effect
(d) none of these
(ii) The colour of scattering light depends upon
(a) volume of particles
(b) nature of particles
(c) size of particles
(d) none of these
(iii) The colour of scattered light depends upon
(a) frequency of the scattered particles
(b) wavelength of the scattered particles
(c) velocity of the scattered particles
(d) all of the above
(iv) If the size of the scattering particles is large enough then
(a) the scattered light may appear red
(b) the scattered light may appear white
(c) the scattered light may appear blue
(d) none of these
Answer
(i) – (b); The scattering of light by colloidal particle is called Tyndall effect.(ii) – (c); Size of particle because less size means more scattering of light and more size means less scattering of light then scattering nature depends on wavelength of colour.
(iii) – (b); Wavelength of the scattered particles because colour of light depends on wavelength of
chromatic light.
(iv) – (b); The scattered light may appear white as particle of larger size scatter light of longer wavelengths.
4
Light of all the colour travel at the same speed in vacuum for all wavelengths. But in any transparent medium (glass or water), the light of different colours travel with different speeds for different wavelength that means that the refractive index of a particular medium is different for different wavelength. As there is a difference in their speeds, the light of different colour bend through different angles. The speed of violet colour is maximum and the speed of red colour is minimum in glass so, the red light deviates least and violet colour deviates most. Hence, higher the wavelength of a colour of light, smaller the refractive index and less is the bending of light.
λr > λv and rn < vn Also frequency, υ = c/λ .
(i) Which of the following statements is correct regarding the propagation of light of different colours of white light in air?
(a) Red light moves fastest.
(b) Blue light moves faster than green light.
(c) All the colours 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.
(ii) Which of the following is the correct order of wavelength?
(a) Red Green > Yellow
(b) Red > Violet > Green
(c) Yellow > Green > Violet
(d) Red Yellow > Orange
(iii) Which of the following is the correct order of speed of light in glass?
(a) Red Green > Blue
(b) Blue Green > Red
(c) Violet > Red > Green
(d) Green > Red > Blue
(iv) Which colour which has maximum frequency
(a) Red
(b) Violet
(c) Blue
(d) Green
(v) Which of the following is the correct order of angle of deviation?
(a) Red > Green > Blue
(b) Blue > Yellow > Orange
(c) Orange > Red > Green
(d) Blue > Green > Violet
Answer
(i) – (c); All the colours of the white light move with the same speed in air.
(ii) – (c); The increasing order of wavelength of visible spectrum is
Violet < Indigo < Blue < Green < Yellow < Orange < Red
So, the correct order is
Yellow > Green > Violet
(iii) – (b); The more be the wavelength, more be the speed.
(iv) – (b); Frequency is inversely proportional to the wavelength. Violet has minimum wavelength among all these colours, so violet has maximum frequency.
(v) – (b); The angle of deviation is more for more refractive index.
5
The spreading of light by the air molecules is called scattering of light. The light having least wavelength scatters more. The sun appears red at sunrise and sunset, appearance of blue sky it is due to the scattering of light. The colour of the scattered light depends on the size of particles. The smaller the molecules in the atmosphere scatter smaller wavelengths of light. The amount of scattering of light depends on the wavelength of light. When light from sun enters the earth's atmosphere, it gets scattered by the dust particles and air molecules present in the atmosphere. The path of sunlight entering in the dark room through a fine hole is seen because of scattering of the sun light by the dust particles present in its path inside the room.
(i) To an astronaut in a spaceship, the colour of earth appears
(a) red
(b) blue
(c) white
(d) black
(ii) At the time of sunrise and sunset, the light from sun has to travel.
(a) longest distance of atmosphere
(b) shortest distance of atmosphere
(c) both (a) and (b)
(d) can't say
(iii) The colour of sky appears blue, it is due to the
(a) refraction of light through the atmosphere
(b) dispersion of light by air molecules
(c) scattering of light by air molecules
(d) all of these
(iv) At the time of sunrise and sunset
(a) Blue colour scattered and red colour reaches our eye
(b) Red colour scattered and blue colour reaches our eye
(c) Green and blue scattered and orange reaches our eye
(d) None of these
(v) The danger signs made red in colour, because
(a) thee red light can be seen from farthest distance
(b) the scattering of red light is least
(c) both (a) and (b)
(d) none of these
Answer
(i) – (b); Light is scattered by the air molecules present in atmosphere.(ii) – (a); As the distance between us and sun is more at the time of sunrise and sunset.
(iii) – (c); Due to the more scattering of blue colour by molecules of air.
(iv) – (a); Red light being of largest wavelength blue scatter more, red scattered least.
(v) – (c); Scattering is least but velocity of red light is more.
6
Atmospheric refraction is the phenomenon of bending of light on passing through earth's atmosphere. As we move above the surface of earth, density of air goes on decreasing. Local conditions like temperature etc. also affect the optical density of earth's atmosphere. On account of atmospheric refraction, stars seen appear higher than they actual are; advanced sunrise; delayed sunset, oval appearance of the sun at sunrise and sunset; stars twinkle, planets do not.
(i) Due to atmospheric refraction, apparent length of the day
(a) increases
(b) decreases
(c) remains the same
(d) all of these
(ii) Apparent position of the star appears raised due to
(a) atmospheric refraction
(b) scattering of light
(c) both (a) and (b)
(d) none of these
(iii) The sun appears oval shaped or flattened due to
(a) dispersion
(b) scattering
(c) atmospheric refraction
(d) cannot say
(iv) Twinkling of stars and non – twinkling of planets is accounted for by
(a) scattering of light
(b) dispersion of light
(c) atmospheric refraction
(d) none of these
(v) In absence of atmosphere, the colour of sky appears
(a) blue
(b) black
(c) red
(d) vellow
Answer
(i) – (a); Due to atmospheric refraction, apparent length of the day increases by 4 minutes.(ii) – (a); Apparent position of the star appears raised due to atmospheric refraction.
(iii) – (c);
(iv) – (c); Twinkling of stars and non – twinkling of planets is on account of atmospheric refraction.
(v) – (b); Due to no scattering of light.