CBSE Class 10 Science 85 Most Important Question with Solution 2023

It contains a comprehensive list of CBSE Class 10 Science 85 most important questions that are likely to appear in the board exam. The book comes with detailed solutions to all the questions, making it an ideal tool for last-minute revision.

The solutions are designed to help candidates easy understand the concepts in a clear and concise manner.

They can focus on the most important topics and practice solving questions similar to the ones that appear in the exam. The book also provides tips and tricks to help students improve their scores.

MOST VVI SCIENCE QUESTIONS FOR CLASS 10TH

1. Why is it necessary to balance a chemical equations ? (CBSE 2019)

Answer: We know that, according to the law of conservation of mass, the total mass of all the atoms present in the reactants should be equal to that of all the atoms present in the products.

This law will hold good only when the number of atoms of all types of elements on both sides is equal. Thus  balancing of chemical equation is necessary so that the law of conservation of mass may be obeyed.

2. Write the use of decomposition reactions. (CBSE 2018)

Answer: The decomposition reactions carried out by electricity are used to extract several metals from their naturally occurring compounds like chlorides and oxides.

When passing electricity decomposes the fused metal chloride or metal oxide, then metal is produced at the cathode.

3. What are the characteristics of chemical reactions? (CBSE 2017)

Answer: Evolution of gas , formation of a precipitate , change in colour, change in temperature, change in state.

4. What are bases and alkalies ? (CBSE 2016)

Answer: Oxides and hydroxides of metals and metal like radicals (e.g.,NH4+ ions) are called bases. Bases ionise to give OH– ions in aqueous solution.

Bases may be soluble or insoluble in water.

The soluble bases are called alkalies. Thus all alkalies are bases but all bases are not alkalies.

Examples

NaOH and Cu (OH)2 both are bases, but, since NaOH is soluble in water, it is an alkali. On the other hand, since Cu (OH)2 is insoluble in water, it is not an alkali. Other examples of alkalies

are KOH, Ca (OH)2 and NH4OH.

5. Name the two stages of photosynthesis. (CBSE 2018)

Answer: the two stages are : Light reaction and dark reaction.

6. Name the factors affecting photosynthesis.

Answer: The factors affecting photosynthesis are light , water, temperature, humidity, age  of the leaf and carbon dioxide.

7. What are stomata and lenticels ?(CBSE 2019)

Answer: Stomata are tiny apertures found on the surface of the leaf, which regulate the exchange of respiratory gases and transpiration.

Lenticels are the raised pores in the woody plants that allow the exchange of gases between the atmosphere and the internal tissues.

8. Give two points of difference between respiration in plants and respiration in animals. (CBSE 2019)

Answer: The respiration in plants differ from respiration in animals in two respects, they are

(i) There is minimal transport of gases from one part of the plant to another, unlike the animals.

(ii) Plant respiration occurs at a much slower rate than animal respiration.

9. Name the respiratory organs of :-

fish , mosquito, earthworm, dog

Answer: The respiratory organs of

(i) fish – gills

(ii) mosquito – tracheoles

(iii) earthworm – skin

(iv) dog – lungs.

10. What are chemotrophs? (CBSE 2014)

Answer: Chemotrophs are organisms, which do not require light. They manufacture their food from inorganic substances in the presence of energy derived from the oxidation of simple inorganic compounds of iron, sulphur, etc

e.g. bacterium Nitrosomonas.

11. Which is the largest prominent part of the brain?

Answer: cerebrum is the largest and most prominent part of the brain

12. Define reflex arc ? (CBSE 2013)

Answer: the pathway taken by the netve impulse in a reflex action , from receptor organ to spinal cord and back to effector organ of reflex action is called reflex arc. Recepter organ could be sense organs such as eyes, skin, etc.

13. State one example of chemotropism?

Answer: Growth of pollen tube towards the ovule due to chemical stimulus during the process of fertilisation in a flower is an example of chemotropism.

14. Define phototropism. Name the plant hormone which is responsible for phototropism. (Cbse 2014)

Answer: Phototropism is the movement of a part of the plant in response to light.

Shoots generally grow towards light and are said to be positively phototropic, while roots grow away from light and are said to be negatively phototropic.

The growth movement of the plant part (stem) is caused by the action of auxin hormone. Auxin causes cell elongation. Thus, causing growth of stem towards the light stimulus.

15. State the fuction of Thyroxine in human body.

Answer: thyroxine regulates carbohydrates, proteins and fat metabolism in the body . It promotes growth of body tissues also.

16. How can we control hyperthyroidism?

Answer: hyperthyroidism can be controlled by using iodised salt.

17. What is DNA ? (CBSE 2011)

Answer: DNA (deoxyribonucleic acid) is a polymer made up of large number of nucleotide units.

It carries genetic information from generation to generation.

18. What is DNA copying ? And why is it important? (CBSE 2021)

Answer: DNA copying is the production of similar copies of DNA present in a cell using various chemical reactions. DNA copying is essential for reproduction through which the organisms pass on their body features to their offspring.

Moreover, minor alternations during the process of DNA copying result in the production of variations. Such variations are useful for the survival of species over time. 

19. Name the method by which Spirogyra reproduces under favourable conditions. Is this method sexual or asexual?

Answer :-The method by which Spirogyra reproduces under favorable conditions is fragmentation. This is an asexual mode of reproduction.

20. List four modes of asexual reproduction other than fission in the living organisms. (CBSE 2020)

Answer : The four modes of asexual reproduction other than fission in living organisms are :

• Budding
• Spore formation
• Regeneration and
• Fragmentation

21. Define multiple fission. Give its one example.

Answer: Multiple fission is an asexual mode of reproduction in which the parent organism splits to form many new organisms at the same time. Multiple fission occurs in Plasmodium.

22. Which contraceptive method changes the hormonal balance of the body?

Answer: chemical contraceptive methods changes the hormonal balance of the body

23. List two functions of ovary of human female reproductive system ?

Answer: Two functions of ovary of human female are:

*Production of female gametes, i.e., ova

*Secretion of female hormones, i.e., estrogen and progesterone.

24. What is the main difference between sperms and eggs of humans?

Write the importance of this difference.(CBSE 2016)

Answer: The main difference between sperms and eggs of humans is that a sperm has X or Y chromosome whereas egg has X chromosome.

This helps in determination of the sex of a person and maintaining the genetic continuity in the organisms.

25. Mendelian experiment consisted of breeding pea plants bearing  violet flowers with pea plants bearing white flowers. What will be the result in F1 progeny?

Answer: According to the Mendelian experiment, violet coloqr (VV) is a dominant trait while white colour (ww) is a recessive trait. Hence, the colour of the flower in F1 progeny will be violet (Vw).

26. What is a gene? (CBSE 2011)

Answer: gene is a unit of DNA on a chromosome which governs the synthesis of particular protein that controls specific characteristics (or traits) of an organism.

27. What is heredity?

Answer: The inheritance of characters (or trails) from the parents to their off springs is called heredity.

28. State the two laws of reflection of light ?(CBSE 2018)

Answer: Laws of reflection of light states that

(i) The angle of incidence is equal to the angle of reflection.

(ii) The incident ray, the reflected ray and the normal to the mirror at the point of incidence all lie in the same plane.

29. What is the magnification of the images formed by plane mirrors  and why?

Answer: Magnification of images formed by plane mirrors is unity because for plane mirrors, the size of the image formed is equal to that of the object.

30. Draw a labelled ray diagram to show the path of the reflected ray corresponding to an incident ray of light parallel to the principal axis of a convex mirror. Mark the angle of incidence and angle of reflection on it .

Answer: 

31. Name the type of mirrors used in the design of solar fufurnaces. Explain how high temperature is achieved by this device . (CBSE 2017)

Answer: Concave mirrors are used in the designing of solar furnaces.

When a solar furnace is placed at the focus of a large concave mirror, it focuses a parallel beam of light on the furnace. Therefore, a high temperature is attained at the point after some time.

32. List two properties of the images formed by convex mirrors. Draw ray diagram in support of your answer. (CBSE 2016)

Answer: Concave mirrors are always form diminished , virtual and erect images.

33. Define the term power of accommodation. Write the modification in the curvature of the eye lens which enables us to see the nearby objects clearly? (CBSE 2018)

Answer: The ability of the eye lens to adjust its focal length is called power of accommodation.

The ciliary muscles modifies the curvature to some extent.

The change in the curvature of the eye lens can thus change its focal length.

When the ciliary muscles contract, the lens becomes thick and its focal length decreases, thus enables us to see nearby objects clearly.

34. List two causes of hypermetropia?

Answer: Hypermetropia is caused due to following reasons:

(i) Shortening of the eyeball

(ii) Focal length of crystalline lens is too long.

35. Define electric current . (2019)

Answer: electric current is the amount of charge flowing through a particular unit of area.

36. Define one ampere.

Answer: one ampere is constituted by the flow of one coulomb of charge per second 1A= 1Cs/1 

38. State Ohm’s law . (CBSE 2020)

Answer: It states that the potential difference V, across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it, provided its temperature remains the same.

Mathematically, V ∝ I V= RI where R is the resistance of the conductor.

39. What is meant by magnetic field.?

Answer: Magnetic field : It is defined as the space surrounding the magnet in which magnetic force can be experienced.

40. What us solenoid? (2016)

Answer: Solenoid : A coil of many circular turns of insulated copper wire wrapped in the shape of cylinder is called solenoid.

41. What is magnetic field lines?(2020)

Answer: Magnetic field lines : These are the imaginary close curves which are used to represent the magnetic field around the magnet.

42. List two biotic components of a biosphere? ( 2020)

Answer: Two biotic components of a biosphere are:

(i) Producers – Include organisms which can produce their food using simple inorganic compounds, e.g., all green plants, blue green algae (cyanobacteria).

(ii) Consumers – Include organisms which are unable to synthesise their food, therefore, utilise materials and energy stored by the producers or eat other organisms, e.g., all the animals

43. Why are green plants called producers?

Answer: Green plants are called producers because they manufacture their own food with the help of CO2 and H2O in the presence of sunlight and chlorophyll.

44. List two examples of natural ecosystem?

Answer: Forest ecosystem And River ecosystem.

45. What is meant by the term Biomass?

Answer: biomass is the total amount of living organic matter in an ecosystem at any time.

46. What is meant by biological magnifications? (CBSE 2019)

Answer: Biological magnification or biomagnification refers to the process of accumulation of non- biodegradable chemicals (pesticides, etc.) into the body of organisms through the food chain which go on increasing in its concentration at each trophic level.

47. What is an ecosystem? (CBSE 2016)

Answer: An ecosystem is defined as a structural and functional unit of the biosphere. It comprises of living organisms and their non-living environment that interact by means of food chains and biogeochemical cycles resulting in energy-flow, biotic diversity and material cycling to form stable self-supporting system.

48. What are covalent compounds? Why are they different from ionic compounds? List their three characteristic properties. (Delhi 2016)

Answer: Covalent compounds are those compounds which are formed by sharing of valence electrons between the atoms e.g., hydrogen molecule is formed by mutual sharing of electrons between two hydrogen atoms.

They are different from ionic compounds as ionic compounds are formed by the complete transfer of electrons from one atom to another e.g., NaCl is formed when one valence electron of sodium gets completely transferred to outer shell of chlorine atom. The characteristic properties of covalent compounds are:

(i) They are generally insoluble or less soluble in water but soluble in organic solvents.

(ii) They have low melting and boiling points.

(iii) They do not conduct electricity as they do not contain ions.

49. Give reasons for the following:

(i) Element carbon forms compounds mainly by covalent bonding.

(ii) Diamond has high melting point.

(iii) Graphite is a good conductor of electricity. (3/5, Foreign 2011)

Answer:  

(i) As carbon has four valence electrons and it can neither loose nor gain lour electrons thus, it attains noble gas configuration only by sharing of electrons. I bus, it forms covalent compounds.

(ii) In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure. This makes diamond the hardest known substance. Thus, it has high melting point.

(iii) In graphite, each carbon atom is bonded to three other carbon atoms by covalent bonds in the same plane giving a hexagonal array. Thus, only three valence electrons are used for bond formation and hence, the fourth valence electron is free to move. As a result, graphite is a good conductor of electricity.

50. Elements forming ionic compounds attain noble gas electronic configuration by either gaining or losing electrons from their valence shells. Explain giving reason why carbon cannot attain such a configuration in this manner to form its compounds. Name the type of bonds formed in ionic compounds and in the compounds formed by carbon. Also explain with reason why carbon compounds are generally poor conductors of electricity. (Foreign 2015, AI 2014)

Answer: Ionic compounds are formed either by gaining or losing electrons from the outermost shells, but carbon which has four electrons in its outermost shell cannot form ionic bonds because

1. If carbon forms ionic bonds by gaining four electrons to attain a noble gas configuration then it would be difficult for six protons in the nucleus to hold ten electrons.

2. If carbon forms ionic bonds by loss of four electrons then it would require a lot of energy to remove these electrons from outermost shell.

Due to these reasons carbon forms covalent bonds by sharing the valence electrons.

Type of bonds formed in ionic compounds are called electrovalent bonds and the type of bonds formed in carbon compounds are called covalent bonds.

51. Reproduction is one of the most important characteristic ‘of living beings. Give three reasons in support of the statement. (AI 2017)

Answer: Reproduction is one of the most important characteristics of living beings because :

it is essential for existence and continuity of a species.

it helps to pass genetic information to next generation.

it brings variations in next generation which is the basis for evolution.

52. Define reproduction. How does it helps in providing stability to the population of species? (AI 2016)

Answer: The production of new organisms by the existing organisms of the same species is known as reproduction. It is linked to the stability of population of a species. DNA replication during reproduction ensures transfer of specific characters or body design features that is essential for an individual of a population to live and use that particular niche. Some variations present in a few individuals of population caused due to reproduction which also helps in their survival at changing niches.

53. What is DNA copying? State its importance. (Delhi 2015)

Answer: DNA copying is the production of similar copies of DNA present in a cell using various chemical reactions. DNA copying is essential for reproduction through which the organisms pass on their body features to their offspring. Moreover, minor alternations during the process of DNA copying result in the production of variations. Such variations are useful for the survival of species over time.

54. What is the effect of DNA copying, which is not perfectly accurate, on the reproduction process? How does the amount of DNA remain constant through each new generation is a combination of DNA copies of two individuals? (AI 2014)

Answer: In the process of reproduction, if DNA copying is not perfectly accurate, variation occurs. These in turn may allow few individuals of a population to survive in an altered niche and becomes the basis of evolution and over time. Such variations are useful for the survival of species.

The combination of DNA copies of two individuals, (male and female) occurs during sexual reproduction. Reduction division (meiosis) during gamete formation halves the chromosome number in both male and female gametes. Since these two gametes fuse during fertilisation, the original number of chromosomes (as in the parent) is restored in the offspring. By this way the amount of DNA remains constant in each new generation.

55. What happens when a Planaria gets cut into two pieces? (Delhi 2016)

Answer: When Planaria is cut into two pieces then each piece grows into a complete organism. This is known as regeneration.

56. What happens when a mature Spirogyra filament attains considerable length? (AI 2016)

Answer: When a mature Spirogyra filament attains considerable length it simply breaks into two or more fragments and each fragment then grows into a new Spirogyra.

57. List four advantages of vegetative propagation. (Delhi 2014)

Answer: The following are the advantages of vegetative propagation:

The characters of the parent plants are preserved hence a good variety produced can be propagated by vegetative means.

The plants, which do not produce viable seeds or produce very few seeds, can be reproduced by this method. For example, banana, potato, grapes, sugarcane, rose, orange, etc.

It is an easier, quicker and cheaper method of propagation.

It is easier to get rid of pathogen from any part of plant by vegetative propagation.

58. (a) What is peristaltic movement?

(b) ‘Stomata remain closed in desert plants during daytime’. How do they do photosynthesis? (Board Term I, 2013)

Answer: (a) The relaxation of gut muscles to move the partially digested food downwards throughout the alimentary canal is called peristaltic movement.

(b) In desert plants, stomata open at night and take in carbon dioxide (CO2). Stomata remain closed during daytime to prevent the loss of water by transpiration. They store the CO2 in their cells until the sun comes out so that they can carry on with photosynthesis during the daytime.

59. (a) Why is nutrition necessary for the human body?

(b) What causes movement of food inside the alimentary canal?

(c) Why is small intestine in herbivores longer than in carnivores?

(d) What will happen if mucus is not secreted by the gastric glands? (2020)

Answer: (a) Human body continuously require energy for their life activities like respiration, circulation, excretion, etc. Energy is required even we are sleeping because a number of biological processes keep on occurring. All these processes require energy and this energy is obtained from nutrition. Nutrition is also needed for growth and repair of human body.

(b) The wall of alimentary tract contains muscles which can contract and expand alternately. The contraction and expansion movement of the walls of foodpipe is called peristaltic movement. The peristaltic movement moves the partially digested food in all the digestive organs throughout the alimentary canal.

(c) Herbivores eat plants which is rich in cellulose. Cellulose takes longer time for complete digestion by the enzymes present in symbiotic bacteria. Therefore, they have longer small intestine. Carnivores, feed on flesh which is easier to digest and do not contain cellulose also. Therefore, they have shorter intestine for digestion of food eaten by them.

(d) Gastric glands secrete HCl, mucus, rennin and pepsin enzymes. Mucus protects the inner lining of stomach from the action of HCl and enzymes. In the absence of mucus, there would be erosion of inner lining of stomach leading to acidity and ulcers.

60. Draw a neat and clean diagram of  open stomata and closes stomata.

Answer: 

61. Draw a neat and clean diagram of digestive system

Answer: 

 

62. Draw a neat and well lebelled diagram of structure of neuron

Answer: 

63. Draw a neat and clean diagram of budding in hydra

Answer:

64. Draw a neat and clean diagram of human respiratory system

Answer: 

65. Draw tye longitudinal section of a flower.

Answer: 

66. (a) State the form in which the following are stored:

(i) Unused carbohydrates in plants.

(ii) The energy derived from food in humans,

(b) Describe the process of nutrition in Amoeba with the help of diagram. (Board Term I, 2016)

Answer: (a) (i) Unused carbohydrates in plants are stored in the form of complex sugar known as starch. They are later broken down into simple sugars (glucose) when energy is needed.

(ii) The assimilated food molecules hold energy in their chemical bonds. Their bond energy is released by oxidation in the cell. This energy is trapped by forming bonds between ADP (adenosine diphosphate) and inorganic phosphate (Pi) to synthesise ATP (Adenosine triphosphate) molecules. These bonds are later broken by enzymatic hydrolysis and the energy released is utilised for cellular processes.

(b) Amoeba is a unicellular animal. Amoeba eats tiny (microscopic) plants and animals which float in water. The mode of nutrition in Amoeba is holozoic. The process of obtaining food by Amoeba is called phagocytosis. When a food particle comes close to Amoeba, it ingests the food particle by forming temporary finger-like projections called pseudopodia around it. The food is engulfed with a little surrounding water to form a food vacuole inside the Amoeba. The food is digested inside food vacuole by digestive enzymes and absorbed directly into the cytoplasm of Amoeba cell by diffusion. A part of absorbed food is used to obtain energy and the remaining part is utilised for growth of Amoeba. When considerable amount of undigested food collects inside Amoeba then its cell membrane ruptures at any place to throw out this undigested food. This process is called egestion.

67. Diffusion is insufficient to meet the oxygen requirement of multicellular organisms like human. State reason. (Board Term 1,2017)

Answer: Due to higher metabolic rate and the volume of human body is so large that oxygen cannot diffuse into all cells of the body quickly as oxygen will have to travel large distances to reach each and every cell. So diffusion is insufficient to meet the oxygen demand of multicellular organisms.

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75. Write the molecular formula of first two members of homologous series having functional group -Cl. (Delhi 2017)

Answer: The molecular formula of first two members of homologous series having -Cl functional group are CH3Cl and CH3CH2Cl.

76. Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is ethene. (AI 2017)

Answer: Homologous series of alkenes have general formula, CnH2n whose first member is ethene.

2nd member of homologous series of alkenes is C3H6 i.e., propene.

3rd member of homologous series of alkenes is C4H8 i.e., butene.

77. Write the next homologue of each of the following:

(i) C2H4

(ii) C4H6 (Delhi 2016)

Answer: 

(i) C2H4 belongs to alkene series having general formula, CnH2n.

Thus, next homologue will be C3H2×3 = C3H6

(ii) C4H6 belongs to alkyne series having general formula, CnH2n-2.

Thus, next homologue will be C5H2×5-2 = C5H8

78. State two properties of carbon which lead to a very large number of carbon compounds. (2/5, AI 2011)

Answer: Carbon forms a large number of carbon compounds like long chains which may be straight or branched chains or ring of different sizes due to its tetravalency ahd unique property of catenation. Carbon due to its small size forms exceptionally stable compounds by forming strong bonds.

79. Why are most carbon compounds poor conductors of electricity?

Answer: Due lo catenation, carbon forms covalent bonds with the constituent elements in the carbon compounds, hence it does not have mobile electrons and carbon compounds do not dissociate themselves into ions and hence, they are poor conductor of electricity.

80. To. a solution of sodium hydroxide in a test tube, two drops of phenolphthalein are added.

(i) State the colour change observed.

(ii) If dil HCl is added dropwise to the solution, what will be the colour change?

(iii) On adding few drops of NaOH solution to the above mixture the colour of the solution reappears. Why? (Board Term I, 2013)

Answer:

(i) On adding phenolphthalein to NaOH solution, the colour becomes pink.

(ii) On adding dilute HCl solution dropwise to the same test tube, the pink colour disappears and the solution again becomes colourless.

(iii) On again adding NaOH to the above mixture, pink colour reappears because the medium becomes basic again.

81. (a) Write the chemical name and formula of marble.

(b) It has been found that marbles of Taj are getting corroded due to development of industrial areas around it. Explain this fact giving a chemical equation.

Answer: (a) The chemical formula of marble (lime stone) is CaCO3. Its chemical name is calcium carbonate.

(b) Taj Mahal, one of the seven wonders of the world situated at Agra, is continuously losing its luster day by day due to rapid industrialisation which causes acid rain.

The sulphuric acid present in the acid rain causes the marble (CaCO3) to be washed off as calcium sulphate (CaSO4), leading to the deterioration of such a splendid piece of architecture.

82. Complete and balance the following chemical equations :(cbse 2015)

(i) NaOH(aq) + Zn(s) →

(ii) CaCO3(s) + H2O(l) + CO2(g) →

(iii) HCl(aq) + H2O(l) →

Answer: 

(i) 2NaOH(aq) + Zn(s) → Na2ZnO2(aq) + H2(g)

(ii) CaCO3(s) + CO2(g) + H2Ol → Ca(HCO3)2(aq)

(iii) HCl(aq) + H2Ol > H3O+ Cl–(aq)

83. How the following substances will dissociate to produce ions in their solutions?

(i) Hydrochloric acid

(ii) Nitric acid

(iii) Sulphuric acid

(iv) Sodium hydroxide

(v) Potassium hydroxide

(vi) Magnesium hydroxide (Board Term 1, 2017)

Answer: Dissociation of various substances to produce ions in their solutions are :

(i) Hydrochloric acid (HCl):

HCl(aq) ⇌ H+(aq) + Cl–(aq)

(ii) Nitric acid (HNO3

HNO3(aq) ⇌ + H+aq + NO–3(aq)

(iii) Sulphuric acid (H2SO4):

H2SO4(aq) ⇌ 2H+(aq) + SO2-4(aq)

(iv) Sodium hydroxide (NaOH):

NaOH(aq) ⇌ Na+(aq) + OH–(aq)

(v) Potassium hydroxide (KOH) :

KOH(aq)⇌ K+(aq) + OH–(aq)

(vi) Magnesium hydroxide [Mg(OH)2] :

Mg(OH)2(aq) ⇌ Mg2++(aq) + 2OH– (aq)

84. Trace the sequence of events which occur when a bright light is focused on your eyes. (Delhi 2019)

Answer: When a bright light enters the eye then most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. Then, the crystalline lens merely provides the finer adjustment of focal length required to focus object at different distances on the retina. The pupil regulates and controls the amount of light entering the eye. At retina, the light-sensitive cells get activated upon illumination and generate electric signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals and finally, processes the information so that we perceive objects as they are.

85. Write about power of accommodation of human eye. Explain why the image distance in the eye does not change when we change the distance of an object from the eye? (Delhi 2017)

Answer: The ability of the eye lens to adjust its focal length is called power of accommodation.

The ciliary musqles modifies the curvature to some extent. The change in the curvature of the eye lens can thus change its focal length. Thus, the focal length of the human lens increases or decreases depending on the distance of the object value to this distance of the image does not change. For example, when the ciliary muscles are relaxed, the lens becomes thin and its focal length increases, thus enables us to see distant object clearly.