NCERT Science Class 10 Chapter 11 the Human Eye and the Colourful World CBSE Board Sample Problems Long Answer

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Question

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that near point of the normal eye Is 25 cm.

Solution

Diagram representing the correction of hypermetropia is a follows:

Diagram representing the correction of hypermetropia

Diagram Representing the Correction of Hypermetropia

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Near point of defective eye is 1 m and that of normal eye is 25 cm.

Here,

Using lens formula

Question

A 14 year old student is not able to see clearly the questions written of the black board placed at a distance of 5 m from him.

(a) Name the defect of vision he is suffering from?

(b) Draw the diagram to show this defect?

(C) Name the type of lens used to correct this defect?

(d) Name two possible cause of this defect.

(e) Draw the diagram to show how this defect can be corrected.

Solution

(a) The student is suffering from myopia.

(b)

The student is suffering from myopia

The Student is Suffering from Myopia

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(C) Concave lens

(d) (I) Decrease in the focal length of the eye lens

(ii) Eye ball gets elongated.

(e)

Correction for myopia

Correction for Myopia

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Question

What is Tyndall effect? What ¡s its cause? Explain two phenomena observed in daily life which are based on Tyndall effect.

Solution

  • Tyndall effect: when a beam of light is passed through a colloidal solution, placed in a dark room, the path of beam becomes illuminated when observed through a microscope placed perpendicular to the path of light. This effect is called Tynciall effect.

  • Cause of Tyndall effect the size of the colloidal particle ¡s relatively larger than the solute particle of a true solution. The colloidal particles first absorb energy from the Incident light and then scatter a part of this energy from their surfaces.

  • Thus. Tyndall effect is due to scattering of light by the colloidal particles and the colloidal particles are seen to be moving as points of light moving against a dark background.

Daily life Examples:

(i) When sunlight passes through a canopy of a dense forest, the tiny water droplets in the mist scatter light and become visible.

(ii) When a fine beam of sunlight enters a smoke filled room through a small hole, the smoke particles become visible due to the scattering of light.

Question

i. What is meant by dispersion of light?

ii. Describe the formation of rainbow in the sky.

iii. What is meant by accommodation of eye? Name the part of eye which helps in this phenomenon and state how does it help.

Solution

  • Dispersion of light: The phenomenon due to which a white light splits into its component colours, when passed through a prism. White light is combination of seven different colours of light having different wavelengths.

    • More is the wavelength of light, less is the angle of deviation for the same material and same angle of incidence. This is caused of dispersion of white light.

Dispersion of light

Dispersion of Light

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  • The rainbow is produced due to the dispersion of sunlight by tiny droplets of water suspended in air, just after rain. From the figure when the sunlight is incident on the side A of the droplet of water, it gets refracted as well dispersed.

  • The dispersed rays on striking the surface B of tiny droplets, suffer total internal reflection and moves towards surface A, the rays further suffer refraction and emerge out as the band of colours in the form of a circular arc (rainbow) along the horizon. The red colour appears upper arc and violet colour on the innermost arc of the rainbow.

  • Accommodation of eye: The phenomenon by which the ciliary muscle alter the focal length of the crystalline lens, so as to focus nearer or far-off objects clearly on the retina is called accommodation of the eye.

  • In order to focus at far-off objects, the ciliary muscles relax to make crystalline lens thin and its focal length increases. In order to focus nearer objects, ciliary muscles contract to make crystalline lens thick and its focal length decreases.

Question

A student finds the writing on the blackboard as blurred and unclear when sitting on the last desk of the class room. He however sees clearly when sitting on the front desk of an approximate distance 2 m from the blackboard. Name the defect of vision the student is suffering from. Also, list two causes of this defect. iii. Name the kind of lens that would enable him to see clearly when he is seated at the last desk. Draw the ray diagram to illustrate the formation of image of the blackboard writing by his eye lens when he sits at the last desk and how this lens helps him to see clearly

Solution

Eye ball gets elongated

Eye Ball Gets Elongated

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Defect of vision — myopia (Short-sightedness)

Two causes of this defect:

(a) Ciliary muscles get weak.

(b) Eye ball gets elongated.

Concave lens is used for the correction of myopic eye (seated at last desk)

The correction of myopic eye

The Correction of Myopic Eye

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Question

(a) Draw a ray diagram to explain the term angle of deviation.

(b) Why do components of incident white light splits into a spectrum while passing through a glass prism?

(c) Draw a well labelled diagram to show the formation of a rainbow.

Solution

(a)

Draw a ray diagram to explain the term angle of deviation

Draw a Ray Diagram to Explain the Term Angle of Deviation

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(b) Different colours of white light bend through different angles as they have different Wavelengths, so, they split into seven colours.

(C)

Diagram to show the formation of a rainbow

Diagram to Show the Formation of a Rainbow

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Solution

(b) Different colours of white light bend through different angles as they have different wavelengths. 50, they split into seven colours.

Question

What is Hypermetropia? State its two causes and with the help of diagram show:

1. This defect and

2. Correction using a lens

Solution

Person with Hypermetropia cannot see nearby objects clearly.

1. Focal length of the eye lens is too long.

2. Eyeball has become too small.

Diagrams:

Correction using a lens

Correction Using a Lens

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Question

(a) Rahul’s doctor told him that length of his eyeball is increased i.e. distance of retina from the eye lens has increased. Identify the defect of vision Rahul has, and suggest a correction of his defect with the help of diagram. (1+2)

(b)If ‘x’ is the distance of far point of defective eye, ‘f’ is the focal length of lens used for correcting this defect. Derive an expression for ‘f’. (1)

Solution

(a) Rahul has myopia or short-sightedness (1)

Correction use a concave lens. (1)

Concave lens

Concave Lens

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(2)

(b) x= distance of far point of myopic eye f=focal length of concave lens to be used,

(1)

Focal length of concave lens used for correcting the myopic eye is equal distance of far point of the myopic eye.