Human Eye And Colourful World, Class 10th Notes

THESE NOTES ARE PREPARED ACCORDING TO JKBOSE SYLLABI IN SIMPLIFIED AND CONCISE  PATTERN

Summary of the Unit: 

Myopia (Short Sightedness)	Image forms Infront of retina. Corrected by Concave lens
Hypermetropia (Long Sightedness)	Image formed behind the retina. Corrected by Convex lens
Power of Accommodation	Human eye can see from Infinity up to 25 cm
Prism	Causes Dispersion of White light. First discovered by I. Newton
Dispersion by a Prism	Split of white light into 7 constituent colours based on energy. Violet light bends most where as Red light bends least
Prism Formula	<A + <D = <i + <e
Atmospheric Refraction	Leads to twinkling of stars, Advanced Sunrise, Delayed sunset, Anomaly in actual position of stars, Mirage etc.
Scattering of Light	Leads to Blue colour of Sky, Blue colour of Seas/Oceans, Red colour of sky during sunset

Q. Discuss the Structure of Human Eye?

Ans: The eye helps us in vision. It works like camera. The human eye consists of following parts:

1. Cornea: It is front transparent part and it is bulged outwards.

2. Iris: It is coloured part behind cornea.

3. Pupil: It is small hole in Iris. Light enters into eye through Pupil.

4. Eye Lens: It is convex lens held by Ciliary muscles.

5. Retina: It is screen behind the eye lens where image is formed. It contains Rod cells and Cone cells.

6. Optic Nerve: The optic nerve carries electric signals from Retina to brain.

Aqueous Humour: It is space between cornea and Eye lens. It is filled with fluid.

Vitreous Humour: It is space between Eye lens and Retina. It is also filled with fluid.




Q. What are the various defects in Vision?

Ans:

1. MYOPIA: Myopia means short sightedness. A person sees nearby objects clearly but can't see distant objects.

In myopia, the rays of light meet infront of Retina.
It is treated by Concave lens.

2. HYPERMETROPIA: Hypermetropia means long sightedness. A person sees far off objects clearly but cant see nearby objects.

In hypermetropia, the days of light meet behind Retina.
It is treated by convex lens.

Q. Why do stars twinkle?

Ans: The stars twinkle because of atmospheric refraction. As the light of stars enter earth's atmosphere, It bends due to increase in density of air.

When light is refracted towards us, stars appear brighter. When light refracted away from us, the stars appear dim.

Q. Explain why the Planets do not twinkle?

Ans: The planets are very close to earth as compared to stars. They can be considered as collection of point sized sources of light. 

The total amount of light from planets reaching to us is more than stars which cancels the twinkling effect.

Q. A person needs a lens of power - 5.5 D to correct distant vision. What is the focal length of the lens?

Ans: Given,

P = - 5.5 D

F = ? 

We know,

    P =  1 / F

- 5.5 = 1 / F

  F  =  1/ - 5.5

  F =  - 0.18 m

Q. A person needs a lens of power + 1.5 D to correct near vision. What is the focal length of the lens?

Ans: Given,

P = + 1.5 D

F = ? 

We know,

    P =  1 / F

 + 1.5 = 1 / F

  F  =  1/ + 1.5

  F =  + 0.66 m

Q. The far point of Myopic person is 80 cm infront of the eye. What is the nature and power of the lens required to correct the problem?

Ans: The person would require Concave lens to correct the problem.

Power of lens required: 

Given,

U = - ∞ (It means object shall ideally be placed ∞ cm away)
V = - 80  (we take Far point of the person i.e 80 cm as Image distance)
F = ?

We know,
1/V - 1/U = 1/F
- 1/80 - (- 1/∞) = 1/F
- 1/80 = 1/F (Number divided by Infinity is zero)
F = - 80 cm i.e 0.8 m


Now, Power of Lens required to correct the problem (P) = 1/F
P = 1/ - 0.8
P = - 1.25 D

Q. The near point of Hypermetropic person is 1 m infront of the eye. What is the nature and power of the lens required to correct the problem?

Assume the near point of human eye is 25 cm.

Ans: The person would require convex lens to correct the problem.

Power of lens required:

Given,
U = - 25 (It means  object shall ideally be placed only 25 cm away)
V = - 100 (we take near point of the person i.e 1m  (100 cm) as Image distance)
F = ?


We know,
1/V - 1/U = 1/F
- 1/100 - (- 1/25) = 1/F
- 1/100 + 1/25 = 1/F
- 1 + 4/ 100 = 1/ F
3 / 100 = 1/F
F = 100/3 i.e 33.33 cm = 0. 33 m.


Now, Power of Lens required to correct the problem (P) = 1/F
P = 1/ +0.3
P = + 3.33 D

Q. What happens to the image distance in the eye when we increase the distance of an object from the eye? 

Ans: We know that the image is always formed at Retina. So, when we increase the distance of an object  from the eye, the image distance does not change.

Q. Why does sky appear blue in colour?

Ans:  The sunlight is scattered in all directions by dust particles present in the atmosphere.The blue colour has shorter wavelength and is scattered most all over the sky. hence the sky appears blue.

Q. Why does sky appear dark instead of blue to the astronaut?

Ans: The sky appears blue due to the scattering of white light by dust particles, gases and water vapours present in the atmosphere. 

The sky appears dark to astronaut since there is no atmosphere present in the space to scatter the rays of light.

Q. Why sun appears reddish in sunrise and sunset?

Ans: During sunrise and sunset, the light has to travel greater distance. Most of the blue light is scattered away but red light scatters the least and reaches to us. Thus sky appears Reddish.

Q. Explain how there is advanced sunrise and delayed sunset?

Ans:  The sun is visible 2 minutes before sunrise and 2 minutes after sunset.
This occurs due to atmospheric refraction. The rays of light upon entering atmosphere bend towards the earth and reach us before actual sunrise and after sunset.

Q. Explain the dispersion of light by Prism?

Ans: Dispersion means the split of white light into seven colours.

When the rays of light enter Prism, it splits  into spectrum of seven colours called VIBGYOR i.e, Violet, Indigo, Blue, Green, Yellow, Orange, Red.

The light splits because different colours bend at different angles inside prism.

The red light bends least and Violet bends most.

Q. Explain the formation of Rainbow?

Ans: Rainbow is formed when sun comes out during rain. Rainbow is formed opposite to the direction of sun.

It is formed due to dispersion of light by tiny droplets of water.

The light bends upon entering into water drops and then reflects internally.

This causes light to split into constituent colours.

Q. What is meant by Power of Accomodation of eye?

Ans: Power of accomodation means ability of eye to adjust its focal length to see objects at different distances.

It is possible due to change in thickness of eye lens.

To see distant objects, the lens becomes thin. To see near objects, the lens becomes thick.

Q. What is the far point and near point of human eye with normal vision?

Ans: Far point is maximum distance upto which an eye can see. It is infinite for normal eye.

Near Point is minimum distance upto which an eye can see. It is 25 cm for normal eye.

Q. Why is normal eye not able to see objects clearly closer than 25 cms?

Ans. To see nearer objects, the eye lens becomes thicker due to relaxing of ciliary muscles to adjust the focal length.

The ciliary muscles cannot relax beyond a certain point, so we can't see things clearly closer than 25 cm.

Q. What happens to the image distance in the eye when we increase the distance of an object from the eye?

Ans: There is no change in the image distance when we increase the distance of an object from the eye.

This is because the image is always formed at the retina.


Q. Why cannot we read a printed page by holding it very close to our eyes. Why?

Ans: Our eyes have the power of accommodation in which the focal length of our eye lens adjusts according to the object distance.

Our eye lens cannot adjust if the object is less than 25 cm far from the eye. Hence we cannot read a page by holding it very close to our eyes.

Objective Type Questions

Q. Which natural phenomenon  among the following takes place because of scattering of light?
A. Blue colour of sky
B. Twinkling of stars
C. Advanced sunrise and delayed sunset
D. Both B and C

Ans: A. Blue colour of sky

Q. When we just enter dark room, it takes some time to see the objects. Why

Ans: In sunlight the size of the pupil gets small. When we enter dark room, the light entering through small pupil is very less. It takes some time for pupil to adjust.



Q. What is Dispersion?

Ans: Dispersion is the separation of white light into its constituent colours in form of VIBGYOR.

Q. A student has difficulty reading blackboard while sitting in last row. What could be the defect the child is suffering from? How could it be corrected?

Ans: The child is suffering from myopia.
It can be corrected by concave lens.

Q. The human eye can focus objects at different distances by adjusting focal length of the eye lens. This is due to


1. Presbyopia          2. Accomodation
3. Near sightedness    4. Far sightedness


Ans:
Accomodation

Q. The human eye forms image at its:

1. Cornea   2. Iris      3. Pupil    4. Retina

Ans: Retina


Q. The least distance of distinct vision for adult is about:

1. 25 m   2. 2.5 cm   3. 25 cm    4. 2.5 m

Ans: 25 cm

Q. The change in focal length of an eye lens is caused by action of

1. Pupil   2. Retina   3.  Ciliary muscles  4. Iris

Ans: Ciliary muscles

Q. Most sensitive part of retina is

Ans: Yellow spot

Q. Most insensitive part of retina is

Ans: Black spot

Q. When light enters eye, most of refraction occurs at:
1. Lens   2. Iris    3. Cornea  4. Pupil

Ans : Lens


Q. Human eye forms image of object at

1. Cornea   2. Retina   3. Pupil   4. Iris

Ans: Retina

Q. Which defect of eye cannot be corrected by spectacles.

1. Myopia   2. Presbyopia   3. Cataract   4. Hypermetropia

Ans: Cataract


Q. A person is wearing spectacles of concave lens of suitable focal length. What kind of defect of vision is he having?
A. Myopia
B. Hypermetropia
C. Presbiopia
D. Both B & C

Ans: A. Myopia

Q. Which natural phenomena among the following takes place because of atmospheric refraction.

A. Advanced sunrise and delayed sunset
B. Twinkling of stars
C. Blue colour of sky
D. Both A and B


Ans: D:  Both A and B

Q. Formation of rainbow is an example of

A. Atmospheric refraction
B. Scattering of light
C. Dispersion of light
D. Both A and B

Ans: D. Both A and B

Q. The colored light having maximum speed in the glass prism is:

A. Blue   B. Green    C. Violet     D. Yellow

 

Ans: Yellow

 

Q Which of the following color of white light has the least wave length?

a. Red    b. Orange    c. Violet    d. Blue

 

ans: c. Violet

 

Q. There are two types of light sensitive cells in the human eye:

i. Where are they found?       ii. What is each type called?

Ans: The two types of light sensitive cells in human eye are found inside the Retina.

ii. They are called as Rod cells and Cone cells.

 

Q. Make two diagrams to explain refraction and dispersion

Ans:

 

 

 

Q. Why do stars seem higher than they actually are?

Ans: The stars seem higher than they actually are is due to atmospheric refraction.

 

Q. Why do stars twinkle in a clear night?

Ans: The stars twinkle in a clear night due to atmospheric refraction.

 

Q. What is atmospheric refraction? Give its causes.

Ans: Atmospheric refraction is defined as the refraction of light caused by the earth's atmosphere. Atmospheric Refraction occurs because the different layers of earth's atmosphere vary in terms of optical densities.

 

Q. A beam of white light falling on the glass prism cannot be:

a. Deviated   b. Dispersed   c. Focused   d. Refracted

Ans: c. Focused

Q. At the time of sunset and sunrise, the sun and the surrounding sky appear red. This red colour of the sun and the sky is because:
A. Red colour is least scattered B. Blue colour is least scattered
C. Red Colour is most scattered D. All colours are equally Scattered
Ans: A. Red colour is least scattered

Q. A student has difficulty in reading blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Ans: Since the child cannot see the blackboard clearly, she/he is suffering from 'myopia' or 'short - sightedness'.

Q. Assertion: When a compass needle is moved away from a current carrying straight conductor, the deflection of the needle decreases

Reason: The strength of magnetic field decreases on moving away from the conductor

A. Both A and R are true and R is the correct explanation of A

B. Both A and R are true and R is not the correct explanation of A

C. A is True, R is False

D. A is False, R is True

Ans: A. Both A and R are true and R is the correct explanation of A



Q. The danger signals installed at the top of building are red in colour. These can be easily seen from a distance because among all the colours, the red light:

a. Is scattered the most by the smoke or fog b. Is scattered the least by the fog

c. Is absorbed the most by the fog d. Moves fastest in the air

Ans: b. Is scattered the least by the fog


Q. Which of the following statements is correct?

a. A person with myopia can see distant objects clearly

b. A person with hypermetropia can see nearby objects clearly

c. A person with myopia can see nearby objects clearly

d. A person with hypermetropia cannot see distant objects clearly


Ans: c. A person with myopia can see nearby objects clearly



Q. If you are not able to read small letters in a dictionary, which of the following lenses would you prefer to use?

a. Concave lens of focal length 50 cm b. Concave lens of focal length 50 cm

c. Convex lens of focal length 5 cm d. A concave lens of focal length 5 cm

Ans: c. Convex lens of focal length 5 cm



Q. 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. Yellow light moves with the mean speed as that of the red and the violet light

d. All the colours of the white light move with the same speed

Ans: d. All the colours of the white light move with the same speed

Hint: All colours move at the same speed in the vacuum and air but in much denser medium, different colours move at different speeds.

Q. When a fine beam of sunlight enters a smoke filled room through a small hole, the phenomenon is known as:

(A) Reflection of light.  (B) Refraction of light  (C) Tyndall effect.  (D) None of these

Ans: (C) Tyndall effect

Q. The common refractive defects of vision include Myopia, Hypermetropia and Presbyopia. Choose the correct match:

(A) Myopia-Farsightedness, Hypermetropia-Age related farsightedness, Presbyopia-Shortsightedness

(B) Myopia-Shortsightedness, Hypermetropia-Age related forsightedness, Presbyopia-Farsightedness

(C) Myopia-Shortsightedness, Hypermetropia-Farsightedness, Presbyopia-Age related farsightedness

(D) None of this match is correct

Ans: (C) Myopia-Shortsightedness, Hypermetropia-Farsightedness, Presbyopia-Age related farsightedness

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