CLASS 8 COMBUSTION AND FLAME.

 COMBUSTION AND FLAME

NOTES

Combustion

• A chemical process in which a substance reacts with oxygen to give off heat and light is called combustion.
• The burning of wood is an example of combustion.
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• Combustible and Non-Combustible Substances

  • Substances which easily catch fire are combustible substances, such as paper, coal, wood.
  • Substances which do not catch fire readily are non-combustible substances, such as sand, water, glass.

  • Types of Combustion

    Let us now study the types of such reactions.

  1. Complete combustion: When the reaction takes place in the presence of abundant Oxygen, the substances combine with Oxygen to their maximum extent. Such reactions have heat and light as a visible by-product.
  2. Incomplete combustion: These are defined as the reactions that occur in the absence of sufficient oxygen because of which substances are unable to burn completely. Such reactions leave Soot in the container due to this process along with the formation of Carbon monoxide which is an air pollutant
  3. .Why is combustion so important?

  4. Combustion reactions are a very important class of chemical reactions. Such responses are vital to our daily lives. The combustion reaction happens as the fuel and oxygen react, creating fire or heat and light. Combustion happens as the gasoline, most commonly a fossil fuel, reacts with the oxygen in the air to create heat. The heat produced by the combustion of fossil fuels is used in the operation of machinery such as boilers, furnaces, ovens and engines.

  • History of Wood and Candle Flame

    Fuel

  • Any substance, which upon combustion produces a usable amount of energy is known as fuel. For example, fossil fuels, biogas, nuclear energy, etc.
  • Fuels can be solid, liquid or gas depending on their state.
  • On the basis of their occurrence, it can be either natural or artificial.

  • Ignition Temp

    The lowest temperature at which a combustible substance catches fire when heated in air is called its ignition temperature.

    Inflammable Substances

    The substances, which have very low ignition temperature and can easily catch fire with a flame are known as inflammable substances. Examples: diesel, LPG, acetone.

    Fire

    • Fire is the result of a chemical combustion reaction between oxygen and some sort of fuel.​​​​​​
    • How long a fire lasts depends on how much fuel and oxygen are available.

    Candle Flame

    Fire Triangle

    For the generation of fire, we need three things to be present simultaneously:-

  • Some sort of fuel or combustible material.
  • A heat source to raise the temperature of the fuel to its ignition temperature.
  • Enough oxygen to sustain combustion. So, if we remove any one of these resources, the fire can be controlled.
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    Flame

  • Flame is the visible and gaseous part of the fire.
  • What we see as the flame is the light energy released due to the combustion of fuel.
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  • Structure of Flame

    • The outermost zone is the hottest among all zones and is blue in colour and this is due to complete combustion. It is the non-luminous part of the flame.
    • The middle zone of the candle flame is moderately hot and is yellow in colour, and partial combustion of fuel takes place. It is the bright part of the flame.
    • The innermost zone of the flame is the least hot and is black in colour. This is due to the presence of unburnt wax vapours.

    • Smoke

      • Smoke is an example of solid (unburnt particles) dispersed in a gas (air).
      • The black colour of smoke is due to the presence of unburnt carbon particles in the smoke.

      Matchstick

      Types of Combustion

      • The type of combustion in which heat and light are released in a very short span of time is called rapid combustion. For e.g. combustion of L.P.G.
      • The type of combustion in which substances catch fire on their own, without the application of heat is termed as spontaneous combustion. For e.g. forest fires.

      To know more about Types of Combustion, visit here.

      Working of a Matchstick

    • The main component of the bulb of a matchstick is red phosphorus which turns into white phosphorus on heating.
    • White phosphorus spontaneously ignites, thereby increasing the temperature of the wooden stem to the ignition point and the matchstick starts burning.

      Calorific Value

      Ideal Fuel

      • The ideal fuel is cheap, easily available and readily combustible.
      • It has high calorific value.
      • It does not produce harmful gases or residues that pollute the environment.

      Calorific Value and Efficiency of a Fuel

      • The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
      • Efficiency is that proportion of energy released by a fuel combustion process which is converted into useful work.
      • Calorific value is directly proportional to its efficiency. If the value is high, it’s efficiency will also be high. If the value is low, it’s efficiency would also be low.

      • Pollution

        Harmful Products from Burning of Fuel

        • The burning of fuels like wood, coal and petroleum products releases unburnt carbon particles in the air which causes respiratory problems.
        • Incomplete combustion of fuels produces a very poisonous gas called carbon monoxide.
        • The burning of fuels releases carbon dioxide in air in the environment which causes global warming.

        Unburnt Carbon Particles

        • Carbon fuels like wood, coal, candle,  petroleum release unburnt carbon particles.
        • These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.

        • CO Emission

          • Carbon monoxide is a poisonous gas, which is produced by incomplete combustion of fuels.
          • It is dangerous to burn coal in a closed room as the carbon monoxide produced can kill people sleeping in that room.

          Global Warming

          • The rise in the average temperature of the earth’s atmosphere due to the release of carbon dioxide on combustion of fuels is termed as global warming.
          • Melting of polar ice-caps or change in the rainfall pattern are the consequences of global warming.
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      Acid Rain

    • Acid rains are caused by emissions of sulphur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acid.
    • It has a very harmful effect on plants, land and aquatic animals and infrastructure.

    • Fire Control

      Fire can be controlled by removing any or all of the factors of combustion, i.e. fuel, oxygen (air) and ignition temperature (by lowering the temperature).

      Fire Extinguisher

    • The fire extinguisher is a device used by the fire brigade to control fire.
    • The role of the fire extinguishers is to cut off the supply of oxygen or bring down the temperature of the fuel or both.

    
    

  • NCERT EXCERCISES

  • 1. List conditions under which combustion can take place.

    ANSWER-

    The burning of a substance in the presence of oxygen is defined as combustion.

    The conditions under which combustion can take place are

    • The presence of air or oxygen.
    • The presence of fuel plays a significant role.
    • Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest temperature.)
    • 2. Fill in the blanks.

    • (a) Burning of wood and coal causes Pollution of air.

      (b) A liquid fuel, used in homes is Kerosene.

      (c) Fuel must be heated to its Ignition Temperature before it starts burning.

      (d) The fire produced by oil cannot be controlled by Water

    • 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

      Soln:

      CNG played an important role in reducing pollution among automobiles for the following reasons

      • CNG is comparatively a cleaner fuel.
      • The CNG can be an alternative for diesel, petrol and propane/LPG.
      • It usually contains a few undesirable gases than the other fuels mentioned above.
      • The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide, which leads to respiratory diseases.

      4. Compare LPG and wood as fuels

      Soln:

      Wood

      • It is considered as a traditional fuel used for both domestic and industrial purposes.
      • Wood produces a lot of smoke which pollutes the atmosphere and cause respiratory diseases.
      • The usage of wood to a large extent causes deforestation.
      • The calorific value of wood ranges between 17000 to 22000 kJ/kg
      • However, wood may be used as a furnace, stove or fireplace indoors while it is used for a campfire, furnace outdoors.

            LPG

      • The usage LPG (Liquefied petroleum gas) has replaced wood.
      • It doesn’t release smoke and other pollutants.
      • It is a cleaner fuel.
      • The fuel efficiency of LPG is more than that of wood.
      • The calorific value of LPG is 55000 kJ/kg.
      • Hence, LPG is the most preferred choice.

      5. Give reasons.

      (a) Water is not used to control fires involving electrical equipment.

      (b) LPG is a better domestic fuel than wood.

      (c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

      Soln:

      a)

      • Water is a good conductor of electricity.
      • If added to an electrical fire, the water would just spread the electricity further.
      • The person dousing the fire might get an electric shock

      b)

      • LPG being a cleaner fuel than wood doesn’t release smoke and other pollutants.
      • Wood, on the other hand, releases a lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
      • Hence, LPG is a better domestic fuel than wood.

      c)

      • The paper by itself catches fire easily because of its low ignition temperature.
      • The piece of paper wrapped around an aluminium pipe doesn’t catch fire because aluminium is a good conductor of electricity.
      • While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So, there is a transfer of heat from paper to the aluminium pipe. Hence it doesn’t catch fire.
      • 6. Make a labelled diagram of a candle flame.
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        7. Name the unit in which the calorific value of a fuel is expressed.

        Soln:

        Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg

        kJ=kilo joules and kg=kilogram

        8. Explain how CO2 is able to control fires.

        Soln:

        CO2 is a non-combustible gas and extinguishes fire in two ways:

        (i) It is heavier than oxygen and it covers the fire like a blanket and cuts off the contact between oxygen and fuel.

        (ii) In cylinders, CO2 is kept in the liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.

        9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

        Soln:

        A heap of green leaves contains a lot of moisture in it, hence its ignition temperature is high. Therefore, it does not catch fire easily.

        But dry leaves have no moisture content in it, hence its ignition temperature is low. Therefore, it catches fire easily.

        10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

        Soln:

        The goldsmith mainly uses non-luminous flame which is termed to be the outermost part of the flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered as the hottest part of the flame.

        11. In an experiment, 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

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        12. Can the process of rusting be called combustion? Discuss.

        Soln:

        No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.

        13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

        Soln:

        The water placed in the outermost part of the flame will be heated in a short time since it is a non-luminous flame and is regarded as the hottest part of the flame. So Ramesh’s beaker will be heated first. However, Abida who placed the beaker in the luminous flame (yellow flame) is comparatively less hot.

        
        

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Human eye And the colourful world.

 Human eye and the colourful world.

As you all know, light is important for us as it enables us to see. Likewise, eye is very important as it produces a sensation of vision in us .

Eyes are called as jewels of our body.its a famous quote.

Human eye

It is a naturally occurring optical instrument. As you all know that we have a pair of eyes and its function is to enable us to see. Without it the whole world would have been a dark place for us.

Structure of eye

It is a spherical ball with a small bulge in the front part.

It is located in the eye socket.

It has two layers that cover it: scleroid and choroid.

The function of scleroid: It is the outermost covering that consists of white fibres and its function is to protect all parts of the eye.

The function of choroid: It is a grey membrane attached to choroid from inner side. Its function is to darken the eye from inside so that no internal reflection takes place.

Apart from these layers, it consists of :

Cornea

Iris

Pupil

Ciliary muscle

Lens

Retina

Blind spot

Optic nerve

Cornea:

It is the white part of the eye that allows light to enter.

It acts as a window to the world.

Iris

It is colored part of the eye.

It holds the pupil and also adjust the size of pupil according to the intensity of light.

Pupil:

It is black in color and absorbs all the light rays falling on it.

It gets constricted when the intensity of light is high.

It gets expanded when the intensity of light is low.

Ciliary muscles

They hold the lens.

They adjust the focal length of the lens.

Convex lens

A cellular structure resembling convex lens (diverging lens).

Retina :

It is the screen of an eye where image is formed.

It consists of two types of cells:

Cone cells: those cells which respond to colours.

Rod cells: those cells which respond to the intensity of light.

Yellow spot:

A point on the retina where the most clear image is formed.

Blind spot: It is that point on the retina where no image is formed.

Optic nerve: A nerve that connects the eye to the brain.

The fluid which is present between cornea and lens is called aqueous humour.

Function : It is a watery fluid present in the interior part of the eye and its function is to protect the exterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. Also, it is the fluid that flows out from the eye when we wink our eyes. So, it washes the eye and also keeps it moist.

The fluid is present between lens and retina and is called vitreous humor.

Function: It is a dense jelly – like fluid present in the posterior part of the eye and its function is to protect the posterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. It also helps in focusing the image clearly on the retina.

Adjustment of the size of the pupil according to the intensity of light: When we are exposed to bright light the iris constricts the pupil partially or we can say that the pupil shortens so that the right amount of light enters and a clear image is formed. Whereas in a dark room, Pupil expands itself to gather more light in order to obtain a clear image.

Accommodation of eye: It is the ability of the eye lens to adjust its focal length so that a clear image is formed on the retina that can be easily recognized by our brain.

In case of far off objects.

In order to see a far off object, our ciliary muscles, lens and focal length undergo a change i.e. the ciliary muscles relax, lens become thin and elongated and focal length increases.

In case of nearby objects.

In order to see nearby objects, focal length of the lens and ciliary muscles undergo a change.

ciliary muscles contract, lens become thick and short and the focal length decreases.

How we can see by our eyes

SPECIAL NOTE POINTS-

1.To see any object comfortably and distinctly you must hold it at about 25 cm from your eyes.from this distance you can see any object without strain .it is also called the near point of your eyes 

2 The farthest point up to which the eye can see objects clearly is called the far point of eye .it is infinity for a normal eye.

Defects of vision and their corrections-

Sometime eyes may lose the power of accomodation due to the refractive defects or errors.Refractive error means that the shape of your eye does not bend light correctly, resulting in a blurred image

- THERE  are major 3 types of refractive defects

1.MYOPIA(NEAR SIGHTEDNESS)

2 HYPERMETROPIA (FAR SIGHTEDNESS)

3.PRESBYOPIA

A)MYOPIA OR NEAR SIGHTEDNESS--->

A person with myopia can see nearby objects clearly.but he is unable to see the far object clearly.their far point is nearer than infinity. Image of the object is made infront of retina not on retina.

Causes of myopia-

A)Due to excessive curvature of eye lens

B)elongation of eye ball.

Remedy-corrected by using concave lens of suitable power.concave lens will bring the image back on retina.

2.HYPERMETROPIA(FAR SIGHTEDNESS)

Person can see far object clearly but can't see the distant objects clearly.the near point of such person is larger than normal .image is formed beyond the retina

Causes- 

1.focal length of the eye  is too long.

2 The eyeball has become too small.

Remedy- person may use convex lens of appropriate power .

PRESBYOPIA-it is due to age.power of accomodation decreases due to age.they can't see nearby objects comfortably.

Causes-due to weakening of ciliary muscles and decreasing flexibility of eye lens.

Sometime old person may suffer from both myopia and HYPERMETROPIA both.so both the lenses used together in a single frame.

Power of the correcting convex lens:
The Lens formula, 1v−1u=1f can be used to calculate the focal length and hence, the power of the myopia correcting lens.
In this case,
Object distance, u = ∞
Image distance, v =
person’s far point Focal length, f =?
Hence, lens formula becomes

• In case of a concave lens, the image is formed in front of the lens i.e., on the same side of the
  object.
  Focal length = -Far point
  Now, power of the required lens (P) = 1/f
• Power of the correcting convex lens: Lens formula, 1v−1u=1f can be used to calculate focal length f and hence, power P of the correcting convex lens, where,
  Object distance, u = -25 cm, normal near point
  Image distance, v = defective near point
  Hence, the lens formula is reduced to
• i/v -1/25=1/f

REFRACTION OF LIGHT THROUGH PRISM

What is Refraction?

The change in direction or bending of a light wave passing from one transparent medium to another caused by the change in wave’s speed is the refraction.

The extent of bending of light rays entering from one medium to another is the refractive index and is denoted by the ‘n’.

It is represented as n = c/v, where c = velocity/speed of light of a certain wavelength in the air and v = velocity of light in any medium.

What is a Prism?

It is a solid figure having two triangular bases and three rectangular surfaces and is the closed surface. The angle between each surface is the angle of the prism. Here the opposite surfaces are equal surfaces and are parallel. We notice that here there are two refracting surfaces which means the surface where refraction of light takes place.

• When a ray of light moves from one surface to another it bends. This is called Refraction
• The ray of light from air is incident on the glass surface AB. On hitting the surface AB, it bends towards the normal.
• At the surface AC, the light ray moves from glass to air. It again bends.
• T he angle of incidence (i), the angle of refraction (r) and the angle of emergence (e) is as shown in the figure.
• PQ is the incident ray
• QR is the refracted ray
• RS is the emergent ray.
• angle A is called the angle of prism.
• angle d is called angle of deviation.
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• Dispersion of white light by a glass prism: The phenomenon of splitting of white light into its seven constituent colours when it passes through a glass prism is called dispersion of white light. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red. The sequence of colours remembers as VIBGYOR. The band of seven colours is called the spectrum. The different component colour of light bends at a different angle with respect to the incident angle. The violet light bends the least while the red bends most.this coloured bend is called the spectrum of lights.
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  v
  

• Composition of white light: White light consists of seven colours i.e., violet, indigo, blue, green, yellow, orange and red.

  Monochromatic light: Light consisting of single colour or wavelength is called monochromatic light, example; sodium light.

  Polychromatic light: Light consisting of more than two colours or wavelengths is called polychromatic light, example; white light.

  Recombination of white light: Newton found that when an inverted prism is placed in the path of dispersed light then after passing through the prism, they recombine to form white light.

  Issac Newton: He was the first, who obtained spectrum of sunlight by using glass prism. He tried to split the spectrum of white light more by using another similar prism, but he could not get any more colours.
  He repeated the experiment using second prism in inverted position with respect to the first prism. It allowed all the colours of spectrum to pass through second prism. He found white light emerges on the other side of second prism.

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  He concluded that Sun is made up of seven visible colour VIBGYOR.

  Rainbow: It is the spectrum of sunlight in nature. It is formed due to the dispersion of sunlight by the tiny water droplet, present in the atmosphere.

  Formation of the rainbow: The water droplets act like small prism. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.
  Red colour appears on top and violet at the bottom of rainbow.
  A rainbow is always formed in a direction opposite to that of Sun.
  At ‘A’ – Refraction and dispersion take place.
  At ‘B’ – Internal reflection takes place.
  At ‘C’ – Refraction and dispersion take place.

  
  NCERT EXCERCISE AND INTEXT QUESTIONS.

• Page No: 190

  1. What is meant by power of accommodation of the eye?

  Answer-

  The ability of the lens of the eye to adjust its focal length to clearly focus rays coming from distant as well from a near objects on the retina, is known as the power of accommodation of the eye.

  2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of corrective lens used to restore proper vision?

  Answer-

  An individual with a myopic eye should use a concave lens of focal length 1.2 m so that he or she can restore proper vision.

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

  Answer-

  The minimum distance of the object from the eye, which can be seen distinctly without strain is called the near point of the eye. For a normal person’s eye, this distance is 25 cm.

  The far point of the eye is the maximum distance to which the eye can see objects clearly. The far point of a normal person’s eye is infinity.

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

  Answer-

  The student is suffering from short-sightedness or myopia. Myopia can be corrected by the use of concave or diverging lens of an appropriate power.

  Page No: 197

  Exercise

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

  (a) presbyopia

  (b) accommodation

  (c) near-sightedness

  (d) far-sightedness

  Answer-

  (b) accommodation

  Due to accommodation the human eye can focus objects at different distances by adjusting the focal length of the eye lens.

  Page No: 198

  2. The human eye forms an image of an object at its

  (a) cornea

  (b) iris

  (c) pupil

  (d) retina

  Answer –

  (d) retina

  The retina is the layer of nerve cells lining the back wall inside the eye. This layer senses light and sends signals to the brain so you can see.

  3. The least distance of distinct vision for a young adult with normal vision is about

  (a) 25 m

  (b) 2.5 cm

  (c) 25 cm

  (d) 2.5 m

  Answer –

  (c) 25 cm

  25 cm is the least distance of distinct vision for a young adult with normal vision.

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

  (a) pupil

  (b) retina

  (c) ciliary muscles

  (d) iris

  Answer-

  (c) ciliary muscles

  The action of the ciliary muscles changes the focal length of an eye lens

  5. A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

  Answer-

  The power (P) of a lens of focal length f is given by the relation

  Power (P) = 1/f

  (i) Power of the lens (used for correcting distant vision) = – 5.5 D

  Focal length of the lens (f) = 1/P

  f = 1/-5.5

  f = -0.181 m

  The focal length of the lens (for correcting distant vision) is – 0.181 m.

  (ii) Power of the lens (used for correcting near vision) = +1.5 D

  Focal length of the required lens (f) = 1/P

  f = 1/1.5 = +0.667 m

  The focal length of the lens (for correcting near vision) is 0.667 m.

  6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

  Answer-

  The individual is suffering from myopia. In this defect, the image is formed in front of the retina. Therefore, a concave lens is used to correct this defect of vision.

  Object distance (u) = infinity = ∞

  Image distance (v) = – 80 cm

  Focal length = f

  According to the lens formula,

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  A concave lens of power – 1.25 D is required by the individual to correct his defect.

  7. 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 the near point of the normal eye is 25 cm.

  Answer-

  An individual suffering from hypermetropia can see distinct objects clearly but he or she will face difficulty in clearly seeing objects nearby. This happens because the eye lens focuses the incoming divergent rays beyond the retina. This is corrected by using a convex lens. A convex lens of a suitable power converges the incoming light in such a way that the image is formed on the retina, as shown in the following figure.

• 

• The convex lens creates a virtual image of a nearby object (N’ in the above figure) at the near point of vision (N) of the individual suffering from hypermetropia.

  The given individual will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.

  Object distance, u= – 25 cm

  Image distance, v= – 1 m = – 100 m

  Focal length, f

  Using the lens formula,

  

  
  

• A convex lens of power +3.0 D is required to correct the defect.

  8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

  Answer-

  A normal eye is not able to see the objects placed closer than 25 cm clearly because the ciliary muscles of the eyes are unable to contract beyond a certain limit.

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

  Answer-

  The image is formed on the retina even on increasing the distance of an object from the eye. The eye lens becomes thinner and its focal length increases as the object is moved away from the eye.

  10. Why do stars twinkle?

  Answer-

  The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index.

  11. Explain why the planets do not twinkle?

  Answer-

  Unlike stars, planets don’t twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when viewed through a telescope. Since all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).

  12. Why does the Sun appear reddish early in the morning?

  Answer-

  White light coming from the sun has to travel more distance in the atmosphere before reaching the observer. During this, the scattering of all colored lights except the light corresponding to red color takes place and so, only the red colored light reaches the observer. Therefore, the sun appears reddish at sunrise and sunset.

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

  Answer-

  The sky appears dark instead of blue to an astronaut, as scattering of light does not take place outside the earth’s atmosphere.

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