Here you get the CBSE Class 10 Science chapter 11, Human Eye and Colourful World: NCERT Exemplar Problems and Solutions (Part-III). This part of the chapter includes solutions for Question No. 25 to 30 from the NCERT Exemplar Problems for Class 10 Science Chapter: Human Eye and Colourful World. These questions include only the Long Answer Type Questions framed from various important topics in the chapter. Each question is provided with a detailed explanation.
NCERT Exemplar problems are a very good resource for preparing the critical questions like Higher Order Thinking Skill (HOTS) questions. All these questions are very important to prepare for CBSE Class 10 Science Board Examination 2017-2018 as well as other competitive exams.
Find below the NCERT Exemplar problems and their solutions for Class 10 Science Chapter, Human Eye and Colourful World:
Long Answer Type Questions
Question. 25 Explain the structure and functioning of the human eye. How are we able to see nearby as well as distant objects?
Structure of human eye: Cornea is the transparent window of eye that serves in refraction of most of the light rays towards the retina.Iris is the coloured part of the eye that controls the amount of light by regulating the pupil size. Pupil is a hole shaped opening in the middle of the eye. Retina is the innermost layer of the eye and contains an outer pigmented layer and an inner nervous layer. It has photo receptors (rods and cones). Optic nerve consists of nerve fibres from innermost layer of retina and serves to transmit impulse to brain. Eye lens is made up of a fibrous, jelly-like material and transparent structure. It serves to form an inverted real image of the object on the retina. Vitreous humour is clear, semi-solid structure that supports the eye ball.
Functioning of human eye: Light enters the eye through pupil and then the eye lens converges these light rays on the retina. An inverted real image of the object is formed on the retina and image-electric signals are to brain via the optic nerves. The brain reconstruct erect image of objects and we can see the objects.
We are able to see nearby as well as distant objects due to the eye’s ability to adjust its focal length which is known as power of accommodation. Relaxation of muscles makes the lens thinner and its focal length increases to make us see the distant objects clearly. Contraction of ciliary muscles increases the curvature of the eye lens and makes the eye lens thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
Question. 26 When do we consider a person to be myopic and hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?
Myopia: This eye defect is caused by elongation of eyeball which results in focusing of the image in front of retina. This causes formation of blurry images of the distant images but the nearby images are clear.
Correction of Myopia: Wearing of concave lens diverge the light rays to help the image to be focused on the retina, hence myopia is corrected using the concave lens.
Hypermetropia: Loss of flexibility or too short eyeball causes focusing of image behind the retina. In both cases, distant images are clear but the nearby images are blurry and the condition is termed as hypermetropia.
Correction of hypermetropia:Wearing a convex lens (plus powered) in front of a hypermetropic eye moves the image forward and focuses it correctly on the retina.
Question. 27 Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence, define the angle of deviation.
The light ray PE enters from air to glass (rarer to denser medium) at surface AB and therefore, bends towards the normal. The refracted ray EF now exits from glass and enters air (from denser to rarer medium) and therefore, bends away from the normal. Extrapolation of incident and emergent ray gives the angle of deviation.
Question. 28 How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?
It is caused by Rayleigh scattering of sunlight. The molecules in the air scatter blue light more than red light as blue light has shortest wavelength as compared to that of red light which has about 1.8 times greater than blue light. Sky appears blue. During sunrise/sunset, the sun looks reddish because at this stage, Sun rays travel longer distance in atmosphere and blue and green light (having shortest wavelength) of visible spectrum is scattered away fully while the red light having largest wavelength is scattered the least.
Sun is directly over head and sunlight travel relatively shorter distance causing only little of the blue and violet colors to be scattered.
Question. 29 Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.
Splitting of light into its constituent colors is referred to as dispersion. When a narrow beam of white light falls on a triangular glass prism, light of different colors have different refractive indices in glass. However, the speed of light is same irrespective of its colors. Different refractive indices of different colors of light lead to their different bending pattern. This causes splitting of white light into light of seven colors called as VIBGYOR. V= violet, I= indigo, B=blue, G=green, Y= yellow, O=orange, R=red.
The light of red color bends the least on passing through the prism and appears at the top while violet colour bends through maximum angle and appears at bottom.
Question. 30 How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
Optical densities of air particles change with changing height. This in turn produce different pattern of refraction of light by the earth's atmosphere. The continuously changing physical conditions air produce different pattern of refraction of light stars. The greater the refraction is, the brighter the star look. The lesser the refraction is, the dimmer the star look. This different refraction pattern through different air layers makes stars to look twinkling.
The planets are present much closer to the earth as compared to stars which makes planets to look like an extended sources of light, not like a point-sized sources of light as stars appear. This results in average of least and maximum refraction as zero making the planets to look stationary not twinkling as stars.
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