Lecture-1 : CLASS-X: SCIENCE : Chapter: COLOURFUL WORLD

CLASS X  |    SCIENCE    |   Colourful World

      notes prepared by subhankar Karmakar

                                                                         

GLASS PRISM:

A prism is a wedge shaped portion of a transparent refracting medium bounded by two plane faces inclined to each other at a certain angle.

The two plane faces (ABED and ACFD) inclined to each other are called refracting faces of the prism.

The line (AD) along which the two refracting faces meet is called the refracting edge of the prism.

The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.

The angle A included between the two refracting faces is called angle of the prism.

Any section of the prism cut by a plane perpendicular to the refracting edge is called principal section of the prism.

The third face (BCFE) of the prism opposite to the refracting edge is called the base of the prism.

ANGLE OF DEVIATION FOR A PRISM:

The angle of deviation D is the angle between the incident ray and the emergent ray. 

Difference between when light travels through a glass slab and light travels through a prism:

 

In refraction through a glass slab , the emergent ray is parallel to the incident ray, but in refraction throigh a glass prism, the emergent ray is not parallel to the incident ray.

DISPERSION OF LIGHT:

The phenomenon of splitting of white light into its component colours on passing through a refracting medium is called "dispersion of light." The pattern of the coloured bands obtained on the screen is called "spectrum."

CAUSES OF DISPERSION:

The dispersion of white light occurs because different colours of white light travel at different speeds through the glass prism. 

"More is the speed, less is the deviation"

(i) Red colour travels with maximum speed in glass prism, hence, red colour deviates least or minimum.

(ii) Violet colour travels with minimum speed in glass prism, therefore, violet colour deviates maximum.

RECOMBINATION OF LIGHT:

Opposite of dispersion is called recombination of light. 

Where as splitting of white light into seven colours is called dispersion, but when seven colours combined together to give white light, it is called "recombination" of light.

RAINBOW FORMATION:

The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sun's light by raindrops in the atmosphere. Therefore, we can say, a rainbow is produced by the dispersion of white sunlight by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum of seven colours.

(i) Why planets don't twinkle

In comparison to the stars planets  are very near to us. Therefore size of the planets seem to bigger to us. The continuously changing atmosphere is unable to cause variations in the light coming from a big sized planet because of which the planet does not twinkle at all.

(ii) Atmospheric Refraction:

The refraction of light caused by the earth's atmosphere give two different layers of  varrying optical densities is called atmospheric refraction

The different phenomena occur due to optical refraction of light

1) twinkling of stars

2) the stars seem higher then they actually are

3) advance sunrise and delayed sunset

(iii) Why does twinkling of a star  occur?

Twinkling of a star occurs due to atmospheric refraction of star's light. The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next. Therefore the starlight reaching our eyes increases and decreases continuously due to atmospheric refraction and hence the star appears to twinkle at night.

In the fig the actual position of a star is given, but due to atmospheric refraction the light from the star gradual curved as shown in the figure. When it ultimately reach our eyes, we see the stars at a height which is more than its actual height. The light bends due to the difference of optical densities of different layers of atmosphere. Hence ,"The Stars Seem Higher Than They Actually Are."

(iv) Advance Sunrise and Delayed Sunset :

We can see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction.

So we observe an advance sunrise and a delayed sunset.

When the sun is below the horizon actually the rays of sunlight get bend due to atmospheric refraction and reaches our eyes and we observe the sun rise although the sun is still below the horizon. 

Similarly when the Sun already goes below the horizon after actual sunset, but still rays from the sun get bend due to atmospheric refraction and we observe that sunset still not happen. Therefore, we see a delayed sunset.

SCATTERING OF LIGHT

Q1. What is scattering of light? 

Ans. Scattering of light means to through light in various random directions. When light rays fall on various types of suspended particles in its path, it is scattered in random directions. Depending on the size of particles, different colour of light are scattered. The most prominent examples of scattering of light are (i) blue colour of the sky, (ii) red colour of the sun at sunrise as well as at sunset. 

Q2. What is Tyndall effect?

Ans. The scattering of light by particles in its path is called tyndall effect. When a beam of sunlight enters a dusty room through his window the tiny dust particles present in the air of room scatter the beam of light all around the room, and thus its path becomes visible to us. 

Q3) On what factor the colour of scattered light depends upon. Explain.

Ans. The colour of scattered light depends upon the size of the scattering particles in the atmosphere. Like, the larger particles of dust and water droplets present in the atmosphere scatter the light of all the colours due to which the scattered light appears to be white. As larger particles scatter all the colours present in the white light. But air molecules in the upper atmosphere where the amount of dust particles are low, they are very small compared to air particles and hence they scatter only light with low wavelength like blue. Due to this reason, the atmosphere looks blue to us.

Q4. Why the colour of the sky is blue?

The scattering of blue component of the white sunlight by air molecules of small size are responsible for blue colour of the sky. As we know scattering of light depends upon the size of the scattering particles. Where as dust particles are larger and scattered all the colours of visible light but air molecules like nitrogen and oxygen are much smaller in size hence they only scatter colours with low wavelengths like blue. It is the real reason behind the fact that our sky looks blue in colour.

Q5. Why the sun appears red at sunrise and sunset.

Ans. The sun and the surrounding sky appear red at sunrise and at sunset because at that time most of the blue colour present in sunlight has been scattered out and away from our line of sight leaving behind mainly red colour in the direct sunlight beam that reaches our eyes.

Q6. Perform an experiment to study the scattering of light.

Ans.

 To show the scattering of light we can perform an experiment. For this we shall take a source of light S placed at the focus of a convex lens L1 so as to produce a parallel beam of light rays. A transparent glass tank T is filled with about 2 litres of clear water. A cardboard disc d having a circular hole C at its centre is kept on the other side of the water tank. Another convex lens L2 is kept behind the cardboard disc to focus the light rays to form an image on the screen R. If we dissolve about 200 grams of sodium thiosulphate in water of the glass tank. Then add 1 or 2 ml of concentrated sulphuric acid to the water, we will see the fine microscopic particles of sulphur begin to form in water and colloidal solution is obtained. As the sulphur particles begin to form in water we will see the blue light coming from the sides of the glass tank. This is due to the scattering of short wavelength blue light by the minute colloidal sulphur particles. This is how the sky looks blue.

©Subhankar Karmakar

LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 9 : REPRODUCTION IN ANIMALS

CLASS VIII   |    SCIENCE    |    CHAPTER 9

      notes prepared by subhankar Karmakar

                                                                         

Reproduction in Animals:

REPRODUCTION: 

The production of new organisms from the existing organisms of the same species is known as reproduction. It is essential for the survival of species on this earth. The process of reproduction ensures continuity of life on earth. 

METHODS OF REPRODUCTION: 

There are two main methods of reproduction in living organisms.

a. Asexual reproduction

b. Sexual reproduction

Special reproductive cells are called sex cells or gametes.  

ASEXUAL REPRODUCTION: 

The production of a new organism from a single parent without the involvement of sex cells or gametes is called asexual reproduction. 

In asexual reproduction, a part of the parent organism separates of and grows into a new organism. Thus, in asexual reproduction, only one parent is needed to produce a new organism. 

Some of the examples of asexual reproduction are binary fission in amoeba and budding in hydra.

SEXUAL REPRODUCTION:

The production of a new organism from two parents by making use of their sex cells or gemetes is called sexual reproduction. 

In sexual reproduction, the sex cell of one parent fuses with the sex cell of the other parent to form a new cell called zygote.

This zygote then grows and develops to form a new organism. 

Thus, in sexual reproduction, two parents are needed to produce a new organism. The two parents which are involved in sexual reproduction are called male and female. 

Male and female parents have special organs in which produce male sex cells and female sex cells respectively.

The humans, fish, frog, cats and dogs all produced by the method of sexual reproduction.

In asexual reproduction, the young one produced is an exact copy of the parent whereas in sexual reproduction, the young one produced is not an exact copy of the parents.

MALE AND FEMALE:

An animal having male sex cells called sperms in its body is called male and having female sex cells called eggs or ova in its body is called female. 

GAMETES:

The cells involved in sexual reproduction are called gametes. There are two types of gametes, (a) male gametes and (b) female gametes. The male gametes in animals is called sperm and female gamete in animals is called egg or ovum.

SPERMS: 

The sperms are extremely small cells and can be seen only through microscope. It is is about 0.05 millimetre long. It has a head, middle piece and a tail. Sperm is a single cell with all the  cell components like nucleus, cytoplasm and cell membrane. 

The purpose of tail of sperm is to move it by waving its tail from side to side. 

EGGS or OVA:

The eggs or ova are also very small and we need a microscope to see the eggs are ova. The egg or ovum is also a single cell having a nucleus, cytoplasm and a cell membrane. Outside the cell membrane, an egg or ovum has a thin layer of jelly called jelly coat which allows only one sperm to enter into it during fertilisation. 

The nuclei of sperm and egg contain chromosomes which carry genes and transmit the characteristics of parents to the offspring. The fusion of a male gamete called sperm with the female gamete called egg gives rise to a new cell called zygote. Thus, the new cell which is formed by the fusion of a male gamete and female gamete is called zygote.

Zygote is a single cell which contains one nucleus. The nucleus of zygote is formed by the combination of nuclei of sperm and egg.

The process of fusion of gametes is called fertilisation. 

FERTILISATION:

Sexual reproduction will occur when a male gamete combines with a female gamete. the fusion of male gamete called sperm with female gamete called ovum to form a zygote  during sexual reproduction is called fertilisation. Therefore we can say the zygote is actually a fertilised egg or ovum. 

All the multicellular animals start their life from a single cell called zygote through sexual reproduction.

There are two types of fertilisation process known as internal fertilisation and external fertilisation.

INTERNAL FERTILISATION:

The fertilisation takes place inside the female body is called internal fertilisation. The process of fertilization in humans the fusion of male gamete of sperm and female gamete or a occurs inside the body of a female animal is called internal fertilisation. 

EXTERNAL FERTILISATION:

The fertilization which takes place outside the female body is called external fertilisation. In external fertilisation, the male and female animals released their sperms and eggs in water where fertilisation takes place by collisions between sperms and eggs. For example, the males and females of frogs and fishes release their sperms and eggs in water in which they live. The sperms then collide with the eggs and fertilised them outside the body of female frog or fish. 

External fertilisation is very common in aquatic animals such as frog fish and starfish etc. 

Although the animals like frog and fish which undergo external fertilisation, lay hundreds of eggs, all the eggs do not get fertilised and hence do not develop into new frogs or fishes. 

This is because of the following reasons.

(i) frog of fish lay eggs or release sperms in water. Many of these eggs and sperms are carried away by the movement of water, wind or rainfall and hence all the eggs do not get fertilised. 

(ii) many of the eggs of frogs are eaten by other animals which live in water. 

(iii) many of the larvae of frog and fish which are hatched from the fertilized eggs are eaten up by other animals in water and fail to develop into adult frogs or fishes.

SEXUAL REPRODUCTION IN ANIMALS

The whole process of sexual reproduction in animals involves the formation of sperms and eggs, joining together of sperm and egg to form a fertilized egg called zygote and growth and development of zygote to form a baby animal. 

The sexual reproduction in animals takes place in the following steps:

(a) the male parent produces male gametes called sperms.

(b) the female parent produces female gametes called eggs or ova.

(c) The sperm enters into egg. The nucleus of sperm fuses with the nucleus of egg cell to form a new cell called zygote. 

(d) The zygote divides repeatedly to form a hollow ball of hundreds of cells which is called embryo. 

(e) Embryo grows and becomes a foetus (in which all the main body features of the baby animal have formed).

(f) Foetus grows and develop to form a new baby animal. 

SEXUAL REPRODUCTION IN HUMANS

In human beings, there are special reproductive organs to make sperms and eggs; to bring together sperms and eggs for fertilisation and make a zygote; and for the growth and development of a zygote into a baby. 

THE MALE REPRODUCTIVE SYSTEM

The human male reproductive system consists of the following organs: Testes, Scrotal sacs, Epididymis, Sperm ducts, Seminal vesicles and Penis. 

Testes are the real reproductive organs in men. Testes make male gametes called sperms. Millions of sperms are produced in the testes. The testes are enclosed in two small bags of skin called scrotal sacs. 

The sperms formed in testes come out and go into a coiled tube called Epididymis. From epididymis, the sperms are carried by a long tube called sperm duct into organs called seminal vesicles. The seminal vesicles join to another tube called urethra coming from the bladder. Urethra carries the sperms to an organ called penis which opens outside the body. These sperms are carried in a liquid called semen. When the penis introduces sperms into the woman, one of the sperms be able to fuse with an egg cell and bring about fertilisation. 

THE FEMALE REPRODUCTIVE SYSTEM

The human female reproductive system consists of the following organs: Ovaries, Oviducts, Uterus and Vagina. 

Ovaries are the real reproductive organs in a woman. Ovaries make the female gametes called eggs or ova. Just above the ovaries, there are tubes called oviducts or fallopian tubes. In human beings, one mature egg is released into oviduct every month by one of the ovaries. The fertilisation of egg by a sperm takes place in the oviduct. 

The process of reproduction in humans takes place in two steps:

1. Fertilisation, and

2. Development of embryo.