CBSE Class 10 Science The Human Eye and the Colourful World

\ HUMAN EYE AND COLOURFUL WORLD

Power of accommodation:

* Ability of the eye lens to adjust its focal length.

* Relaxation of ciliary muscles → lens becomes thin → increase in focal length.

* Contraction of ciliary muscles → lens becomes thick → focal length decreases.

Near Point (N): The point at closest distance, at which an object can be seen clearly by the eye is called Near Point (N) of the eye. The distance of the near point of a normal eye is called the least distance of distinct vision. It is represented by d. For a normal eye, value of least distance of distinct vision is d=25cm.

Far Point (F): The most distant point at which an object can be seen clearly is called Far Point (F) of the eye. For a normal eye, far point lies at infinity.

Rods: Respond to the intensity of light and enable vision in dim light.

Myopia or Near-sightedness:

* Eye cannot see distant objects clearly.

* Image of distant object forms in front of retina.

* Reasons: (i) Excessive curvature of eye lens.

(ii) Elongation of eyeball

Correction: using concave lens

Hypermetropia or Far-sightedness:

* Eye cannot see nearby objects clearly.

* Image of object nearby forms behind retina.

* Reasons: (i) Focal length of eye lens is too long.

(ii) Eyeball becomes small Correction using convex lens

Presbyopia:

* Eye suffers from myopia as well as from hypermetropia.

* Due to gradual weakening of ciliary muscles and diminishing flexibility of eye lens. Correction using bifocal lens

Cataract: Milkiness of eye lens due to aging can be cured by surgery

Dispersion of Light:

Splitting of light into its component colours.

White light disperses into its seven-colour components in the order VIBGYOR (violet, Indigo, Blue, Green, Yellow, Orange, And Red). Red light bends least, Violet the most.

Rainbowisformed due to refraction, dispersion and total internal reflectionof sunlight by tiny droplets of water

Atmospheric Refraction: Refraction of light by the earth‟s atmosphere Twinkling of stars, Advanced sunrise, Delayed sunset, Flattening of disc of sun at sunrise and sunset are due to atmospheric refraction

Tyndall effect:

* When a beam of light strikes fine particles in air, path of the beam becomes visible.

* Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths

DEFECTS OF VISION

QUESTION BANK

Question. The term “accommodation” as applied to the eye, refers to its ability to:
(a) Control the light intensity falling on the retina
(b) Erect the inverted image formed on the retina
(c) Adjust the focal length of the lens
(d) Vary the distance between the lens and retina
Answer: C

Question. How eyes adjust in order to focus the image of near or distant objects on retina?
(a) The lens moves in or out according to the position of the object
(b) The retina moves in or out according to the position of the object
(c) The lens becomes thicker or thinner according to the position of the object
(d) The pupil gets larger or smaller according to the position of the object
Answer: C

Question. A person cannot see the distant objects clearly (though he can see the nearby objects clearly). He is suffering from the defect of vision called:
(a) Cataract
(b) Hypermetropia
(c) Myopia
(d) Presbyopia
Answer: C

Question. A got his eye tested. The optician’s prescription for the spectacles was:
Left eye: −3 D
Right eye: −3.50 D
The person is having a defect of vision called:
(a) Presbyopia
(b) Myopia
(c) Astigmatism
(d) Hypermetropia
Answer: B

Question. A man finds it difficult to read the odometer on the dashboard of the car but is able to clearly read a distant road sign. Which of the following statement is correct about this man?
(a) The near point of his eyes has receded away.
(b) The near point of his eyes has come closer to him.
(c) The far point of his eyes has receded away.
(d) The far point of his eyes has come closer to him.
Answer: A

Question. With both eyes open, a person’s field of view is about:
(a) 90o
(b) 150o
(c) 180o
(d) 360o
Answer: C

Question. When a beam of white light falls on a glass prism, the colour of light which will deviate least is:
(a) Violet
(b) Red
(c) Green
(d) Blue
Answer: B

Question. The star appear shifted from their actual position due to the phenomenon of:
(a) Diffraction of light
(b) Scattering of light
(c) Refraction of light
(d) Reflection of light
Answer: C

Question. Blue colour of the sky is due to the phenomenon of:
(a) Reflection of light
(b) Refraction of light
(c) Dispersion of light
(d) Scattering of light
Answer: D

Question. Figure a, b, c respectively; indicate the point in case of:
(a) The Hypermetropia eye, the myopic eye and normal eye
(b) The normal eye, the myopic eye and Hypermetropia eye
(c) The normal eye, the Hypermetropia eye and myopic eye
(d) The myopic eye, the normal eye and Hypermetropia eye
Answer: C

Question. A: Astigmatism usually is caused by an irregularly shaped cornea.
B: Rainbow is the natural phenomenon in which dispersion takes place.
(a) Both 'A' and 'B' are true statements.
(b) Both 'A' and 'B' are false statements.
(c) ‘A' is true while 'B' is false.
(d) ‘A' is false while 'B' is true.
Answer: A

Question. Having two eyes facilitates in:
A: Increasing the field of view
B: Bringing three dimensional view
C: Developing the concept of distance/size.
Then the correct option is/are:
(a) A only
(b) A and B only
(c) B only
(d) A, B and C
Answer: D

very short answer questions

1. What is the far point and near point of the human eye with normal vision?
Answer: For normal vision, the near point is about 25cm and far point is infinity. Thus, a normal eye can see objects clearly that are between 25cm and infinity.

2. List the three phenomenon of light which is responsible for formation of rainbow in sky?
Answer: Refraction, dispersion and total internal reflection.

3. Why are „danger‟ signal lights red in colour?
Answer: Danger signals are red in colour because the red coloured light having lower wavelength is scattered the least by fog or smoke. Therefore, it can be seen clearly from a distance.

4. Give reasons why the planets do not twinkle Planets are much closer to the earth as compared to the stars they are bigger when we observe them from earth. Planets are made up of large number of point sources. Due to atmospheric refraction each point source will appear to twinkle, the total effect will be nullified.

1.A person needs a lens of power 4.5 D for correction of her vision.

(a) What kind of defect in vision is she suffering from?

(b) What is the focal length of the corrective lens?

(c) What is the nature of the corrective lens?

Answer: (a)Hypermetropia. (b) f=1/4.5 = 0.22m (c) Convex lens

VALUE BASED QUESTION

Dolly and Ritu are two friends studying together in V grade. They prefer to sit together. Dolly is uncomfortable reading the blackboard when they are sitting on the last bench. Dolly is depressed fearing that she may turn blind one day. Ritu explains her that the problem may be due to some minor eye defect and there is no cause for worry. Dolly accompanied by Ritu visits the doctor who prescribes spectacles of suitable power for her. Dolly is now all smiles and thanks Ritu.

Read the above passage and answer the following questions:

(i) What values are displayed by Ritu ?

(ii) Name the defect of eye Dolly is suffering from.

(iii) What could be the cause of this defect?

(iv) The far point of Dolly is 50cm. What is the power of the lens she should use to read from the blackboard?

Answer: (i) Ritu has displayed concern for her friend. She is helpful

ii) Dolly is suffering from myopia or short sightedness.

(iii) Two possible causes of this effect are: Increase in size of eye ball or decrease in focal length of eye lens.

(iv) Here, x=50cm f=-50cm p=100/f=100/-50=-2D

Long answer type questions

1. Explain myopia and hypermetropia with the help of ray diagrams and show how these defects can be corrected?
Answer: (a) Explanationof Myopia and Hypermetropia

(b) Fig.11.2&11.3NCERT TEXTBOOK
(c) The type of lens: Concave lens, convex lens

2. What is meant by dispersion and recombination? Explain with the help of a diagram? What is a spectrum? Name the various colours of spectrum of white light in proper sequence.

Answer: The splitting of white light into its component colours on passing through a prism is called dispersion. When an inverted prism is kept in the path of these seven colours, they combine to form white light. This is called recombination.The band of seven colours formed due to dispersion of white light is called „spectrum‟. Seven colours of spectrum are violet, indigo, blue, green, yellow, orange and red also known as „VIBGYOR‟

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