CBSE Class 10 Science The Human Eye and the Colourful World

Human Eye and Colourful World is Chapter 11 of the CBSE Class 10 Science syllabus. These notes cover the power of accommodation, near point and far point, myopia, hypermetropia, presbyopia, cataract, dispersion of light, atmospheric refraction, the Tyndall effect, the defects of vision table and the question bank with solved answers.

POWER OF ACCOMMODATION

Power of accommodation is the ability of the eye lens to adjust its focal length.
Relaxation of ciliary muscles makes the lens thin, which increases the focal length.
Contraction of ciliary muscles makes the lens thick, which decreases the focal length.

NEAR POINT AND FAR POINT

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

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

RODS

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

MYOPIA OR NEAR-SIGHTEDNESS

The eye cannot see distant objects clearly.
The image of a distant object forms in front of the retina.
Reasons:
(i) Excessive curvature of the eye lens.
(ii) Elongation of the eyeball.
Correction: use a concave lens.

HYPERMETROPIA OR FAR-SIGHTEDNESS

The eye cannot see nearby objects clearly.
The image of a nearby object forms behind the retina.
Reasons:
(i) The focal length of the eye lens is too long.
(ii) The eyeball becomes small.
Correction: use a convex lens.

PRESBYOPIA

The eye suffers from myopia as well as from hypermetropia.
It is caused by the gradual weakening of the ciliary muscles and the diminishing flexibility of the eye lens.
Correction: use a bifocal lens.

CATARACT

Cataract is the milkiness of the eye lens due to ageing. It can be cured by surgery.

DISPERSION OF LIGHT

Dispersion is the 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 the least and violet light bends the most.
A rainbow is formed due to refraction, dispersion and total internal reflection of sunlight by tiny droplets of water.

Formation of rainbow

[Figure: Figure 11.5 Dispersion of white light by a glass prism. A white light beam enters a glass prism and comes out as a spectrum of seven colours, with red at the lower edge and violet at the upper edge.]

ATMOSPHERIC REFRACTION

Refraction of light by the earth's atmosphere causes twinkling of stars, advanced sunrise, delayed sunset and flattening of the disc of the sun at sunrise and sunset.

[Figure: Figure 11.9 Apparent star position due to atmospheric refraction. A star is seen at a position higher than its actual position because light bends as it passes through air of increasing refractive index towards the Earth.]

TYNDALL EFFECT

When a beam of light strikes fine particles in air, the path of the beam becomes visible.
Very fine particles scatter mainly blue light, while particles of larger size scatter light of longer wavelengths.

Scattering of light (apparatus)

DEFECTS OF VISION

SOME NATURAL PHENOMENA AND THEIR CAUSES

MIND MAP

Human eye and the colourful world

Human eye: cornea, iris, pupil, ciliary muscles, lens and retina.
Iris controls the size of the pupil.
Lens adjusts its focal length (power of accommodation).
Retina acts as a screen.
Near point is 25 cm and far point is infinity.
Defects of vision: myopia, hypermetropia and presbyopia.

Refraction

Refraction through a prism leads to dispersion.
Dispersion splits white light into its constituent colours, which form a band of colour called the spectrum.
The colour sequence of the spectrum is VIBGYOR.
Recombination of the spectrum gives white light.
Atmospheric refraction causes twinkling of stars.
Planets are large in size, so the twinkling of planets is nullified.

FIGURES OF THE CHAPTER

[Figure: Human eye diagram with labelled parts: ciliary muscles, crystalline lens, aqueous humour, pupil, iris, cornea, retina, optic nerve and vitreous humour.]

[Figure: Figure 11.2 (a), (b) and (c): the myopic eye, with the image forming in front of the retina, and the correction for myopia with a concave lens.]

[Figure: Figure 11.3 (a), (b) and (c): the hypermetropic eye, with the image forming behind the retina, and the correction for hypermetropia with a convex lens.]

[Figure: Figure 11.6 Recombination of the spectrum of white light. White light passes through a first prism, which disperses it, and a second inverted prism recombines the colours into white light on a screen.]

QUESTION BANK

1 mark questions (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 25 cm and the far point is infinity. Thus, a normal eye can see objects clearly that are between 25 cm and infinity.

Teacher's Note:
a) The near point is the closest distance the eye can focus on.

2. List the three phenomenon of light which is responsible for formation of rainbow in sky?

Answer: Refraction, dispersion and total internal reflection.

Teacher's Note:
a) Each raindrop acts as a small prism and a mirror for the sunlight that enters it.

2 mark questions

1. 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.

Teacher's Note:
a) The printed answer says red light has a lower wavelength; the correct statement is that red light has the longer wavelength.
b) Scattering is least for long wavelengths, which is why fog does not block red light easily.

2. Give reasons why the planets do not twinkle

Answer:
1. Planets are much closer to the earth than the stars.
2. Planets are bigger when we observe them from the earth. Planets are made up of a large number of point sources.
3. Each point source twinkles due to atmospheric refraction, but the total effect of all the point sources is nullified.

Teacher's Note:
a) A star is a single point source, so its twinkling is noticeable.
b) The average of many small twinkles is almost zero for a planet.

Short Answer Type Questions (3 marks)

1. A person needs a lens of power 4.5 D for correction of her vision. [3 Marks]
(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:
1. A positive power means a convex lens, which is used for hypermetropia. So, (a) she has hypermetropia.
2. Focal length f = \( \frac{1}{P} \) = \( \frac{1}{4.5} \) m = 0.22 m, so (b) the focal length is 0.22 m.
3. The lens has positive power, so (c) it is a convex lens.

Teacher's Note:
a) A positive power lens converges light, so it is convex.
b) Use f = 1/P with P in dioptres and f in metres.
c) Hypermetropia is corrected by a 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?

Answer: Ritu shows care and concern for her friend. She is helpful, and she calms Dolly by explaining that the problem can be corrected.

Teacher's Note:
a) Ritu does not panic and advises a doctor's visit.
b) Helping a friend in time is a valuable habit.

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

Answer: Dolly is suffering from myopia, also called near-sightedness.

Teacher's Note:
a) She can see nearby objects but not the distant blackboard clearly.

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

Answer:
1. The eyeball is elongated, so the image of a distant object forms in front of the retina.
2. The eye lens has excessive curvature, which means its focal length is shorter than normal.

Teacher's Note:
a) Either cause brings the image in front of the retina.
b) The eyeball can become longer with age or with eye strain.

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

Answer:
1. For a myopic eye, the corrective lens must form the image of an object at infinity at the far point, 50 cm from the eye.
2. Here, x = 50 cm, f = -50 cm.
3. Power P = \( \frac{100}{f} \) = \( \frac{100}{-50} \) = -2 D.
4. Hence, the power is -2 D, which means a concave lens.

Teacher's Note:
a) The minus sign shows that the lens is concave.
b) Use f in centimetres in P = 100/f to get the power in dioptres.
c) Myopia is always corrected with a concave lens of the right power.

Long answer type questions (5 marks)

1. Explain myopia and hypermetropia with the help of ray diagrams and show how these defects can be corrected?

Answer:
1. Myopia: the eye cannot see distant objects clearly. The image of a distant object forms in front of the retina because the eyeball is elongated or the lens is too curved.
2. Correction for myopia: a concave lens of suitable power is placed in front of the eye, so that the image of a distant object is formed on the retina.
3. Hypermetropia: the eye cannot see nearby objects clearly. The image of a nearby object forms behind the retina because the focal length of the lens is too long or the eyeball is short.
4. Correction for hypermetropia: a convex lens of suitable power is placed in front of the eye, so that the image of a nearby object is formed on the retina.
[Figure: Figure 11.2 (a), (b) and (c) for myopia and Figure 11.3 (a), (b) and (c) for hypermetropia, from the NCERT textbook: the defective eye, and the eye with the corrective lens.]
5. The type of lens used: concave lens for myopia and convex lens for hypermetropia.

Teacher's Note:
a) Myopia is caused by an image in front of the retina, and a concave lens moves it back.
b) Hypermetropia is caused by an image behind the retina, and a convex lens brings it forward.
c) The ray diagrams must show the image position clearly on the retina after correction.
d) Always name the lens type with the defect, because students often swap the two.

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:
1. Dispersion: the splitting of white light into its component colours on passing through a prism is called dispersion.
2. Recombination: when an inverted prism is kept in the path of these seven colours, they combine to form white light. This is called recombination.
3. Spectrum: the band of seven colours formed due to dispersion of white light is called the spectrum.
4. Sequence of the spectrum: violet, indigo, blue, green, yellow, orange and red, also known as VIBGYOR.
[Figure: Figure 11.5 Dispersion of white light by a glass prism and Figure 11.6 Recombination of the spectrum of white light, from the NCERT textbook.]

Teacher's Note:
a) Red light bends the least and violet light bends the most in a prism.
b) A second, inverted prism is what brings the colours back together.
c) The spectrum is always written in the order VIBGYOR.
d) Students often write ROYGBIV in the reverse order, which is the same colours but from the other end.

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

(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

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