CHAPTER – 10 LIGHT REFLECTION & REFRACTION

CHAPTER – 10
LIGHT REFLECTION & REFRACTION

Light is a form of energy, which gives us the power of vision.

In this chapter we will study the phenomena of reflection and refraction using the property of light i.e. straight line propagation (Light wave travel from one point to another, along a straight line).

Ray of light : It is a line in the direction of movement of light.
Beam of light : It is bucnh of rays of light.
Parallel beam : All the rays are parallel.

Reflection of Light
When the light is allowed to fall on highly polished surface, such as mirror, most of the light gets reflected.

Laws of Reflection

1. The angle of incidence is always equal to angle of reflection.

2. The incident ray, reflected ray and the normal to the reflecting surface at the point of incidence lie in the same plane.

Image formed by Plane Mirror (Plane reflecting surface)

1. Virtual (imaginary) & Erect : The image that do not form on screen.
Real images can be recorded on the screen.
2. Laterally inverted (The left side of object appear on right side of image)
3. The size of image is equal to that of object.
4. The image formed is as far behind the mirror as the object is in front of it.

Reflection of light by spherical Mirrors
Mirrors, whose reflecting surface are curved inward or outward spherically are called spherical mirror.
For example – Spoon  The curved surface of shinning spoon can be considered as curved mirror.

If it is curved inward Act as concave mirror
If it is curved outward  Act as a convex mirror.

Few Basic terms related to Spherial Mirror

1. Principal axis : Line joining the pole and centre of curvature of the spherical mirror.
2. Pole : The geometrical central point of the reflecting spherical surface. (aperture), denoted by (P).
3. Aperture : The width of reflecting spherical surface.
4. Centre of curvature : The centre of the hollow glass sphere of which the spherical mirror is a part is called as centre of curvature.
5. Radius of curvature : The distance between the pole and the centre of curvature. i.e.
   PC = R or The radius of the hollow sphere of which the mirror is a part.
6. Focus point : The point on the principal axis, where all parallel rays meet after reflection is called as Principal Focus or Focus. It is denoted by letter ‘F’.
7. Focal length : The distance between the pole and focus point i.e. PF = f
8. Relationship between focal length and Radius of curvature. 

Image Formation by Spherical Mirror

Before we learn the formation of image or ray diagram, let us go through few tips

(a) Remember, a ray of light which is parallel to principle axis always pass through focus (meet at focus) or vice-versa.

(b) A ray of light which passes through centre of curvature (it is also known as normal at the point of incidence on spherical mirror) will retrace their path after reflection.

(c) A ray of light falling on pole get reflected at the same angle on the other side of principal axis.

Note : A ray of light passes through centre of curvature of reflecting spherical surface always act as normal at the point of incidence. If we know the normal we can draw angle of incidence and angle of reflection

Note 1 : The image will only form when two or more rays meets at a point. Image formation by a concave mirror for different position of the object.
The ray diagrams given in NCERT Books are also good to be followed.

Image Formation by Concave Mirror

Image formation by Convex Mirror

  

  

Uses of Concave Mirror
1 Used in torches, search light and headlight of vehicle.
2. Used to see large image of face as shaving mirror
3. Used by dentist to see large images of the teeth
4. Large concave mirror used to focus sunlight (heat) in solar furnaces.

Uses of Convex Mirror
Used as rear-view mirror in vehicles because it gives erect image. Italso helps the driver to view large area.

Sign Convention for Reflection by Spherical Mirror

1. The object is always placed to the left side of mirror.
2. All distance should be measured from pole (P); parallel to principal axis.
3. Take ‘P’ as origin. Distances measured

Right of the origin (+x-Axis) are taken positive
Left of the origin (–x-Axis) are taken negative

Perpendicular to and above principal axis (+y-Axis) are taken positive
Perpendicular to and below principal axis (–y-Axis) are taken negative

Mirror Formula

f  distance between F and Pole
v  distance of image from Pole
u  distance of object from Pole
R  distance between centre of curvature and pole.

Where 

Magnification

It is expressed as the ratio of the height of the image to height of the object.

 From 1 and 2 equation

 When  image height from principle axis.

 Object height from principle axis

Few tips to remember sign convention for Spherical mirror

	f	u	v
CONCAVE	-ve(real)	-ve(real)	-ve(real) +ve(virtual)
CONVEX	+ve	+ve	+ve

h – is always +ve
h´ – is +ve for virtual , –ve for Real.

Refraction of Light

Refraction of Light : Happens in Transparent medium when a light travels from one medium to another, refraction takes place.
A ray of light bends as it moves from one medium to another Refraction is due to change in the speed of light as it enters from one transparent medium to another.

Speed of light decreases as the beam of light travel from rarer medium to the denser medium.

Some Commonly observed phenomenon due to Refraction

• Your eyes.
• Rainbows.
• Light bending in a glass of water.
• Glasses.
• Camera lenses.
• Object dislocation in water.
• Binoculars..

Refraction through a Rectangular Glass Slab

When a incident ray of light AO passes from a rarer medium (air) to adenser medium (glass) at point. O on interface KL, it will bends towards the normal. At ptO1, on interface NM the light ray entered from denser medium(glass) to rarer medium (air) here the light ray will bend away from normal OO1 is a refracted ray OB is an emergent ray. If the incident ray is extended to C, we will observe that emergent ray O1B I parallel to incident ray. The ray will slightly displaced laterally after refraction.

Note : When a ray of light is incident normally to the interface of two media it will go straight, without any deviation.

Laws of Refraction of Light

The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

• The ratio of sine of angle of incidence to the sine of angle of refraction is a constant i.e.
  

For given colour and pair of media, this law is also known as Snell’s Law

Constant n is the refractive index for a given pair of medium. It is the refractive index of the second medium with repect to first medium.

Refractive Index

The refractive index of glass with respect to air is given by ratio of speed of light in air to the speed of light in glass.

c Speed of light in vacuum =  speed of light in air is marginally less, compared to that in vacuum.

Refractive index of air with respect to glass is given by

Refractive index of water 

Refractive index of water 

Spherical Lens

A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens.

Convex lens Concave lens
1. Bulging outwards 1. Bulging inwards.
2. Converging lens. 2. Diverging lens.
 

Concave Lens
A lens bounded by two spherical surfaces, curved inwards is known asdouble concave lens (or simply concave lens)
It is also known as diverging lens because it diverges the light.

Few Basic Terms Related to Spherical Lens

1. Centre of curvature : A lens, either a convex lens or a concave lens is combination of two spherical surfaces. Each of these surfaces forma part of sphere. The centre of these two spheres are called centre of curvature represented by C1 and C2.
2. Principal axis :Imaginary straight line passing through the two centres of curvature
3. Optical Centre : The central point of lens is its optical centre (O). A ray of light, when passes through ‘O’ it remains undeviated i.e. it goes straight.
4. Aperture : The effective diameter of the circular outline of a spherical lens.
5. Focus of lens : Beam of light parallel to principal axis, after refraction from

1. Convex lens, converge to the point on principal axis, denoted by F, known as Principal focus

2. Concave lens, appear to diverge from a point on the principal axis known as principal focus.

The distance OF2 and OF1 is called as focal length

Tips for Drawaomg Ray Diagram

1. After refraction, a ray parallel to principal axis will pass through F.
2. A ray passes through F, after refraction will emerge parallel to principal axis
   

Image formation by a convex lens for various position of object

Image Formation by Concave Lens

Sign Convention for Refraction by Spherical Lens

Similar to that of spherical mirror, only the difference is that all themeasurement are made from optical centre ‘O’

Lens formula

Magnification

It is defined as the ratio of the height of image to the height of object.

It is also related to ‘u’ & ‘v’

From equation (1) & (2)
If magnification
m > 1,then image is magnified
m = 1 ,image is of same size
m < 1, image is diminished

Few Tips to Remember Sign Convention for Spherical Lens

	f	u	v
CONCAVE	-ve	-ve	-ve(virtual image always)
CONVEX	+ve	-ve	+ve(real) -ve(virtual)

h is always +ve
h´  –ve for Real and +ve for Virtual &Errect.

Power of Lens

The degree of convergence or divergence of light ray achieved by a lensis known as power of a lens.
It is defined as the reciprocal of its focal length Represented by P.
If F is given in meter, then

If F is given in cm , then

SI unit of power of a lens is “diopter” denoted by ‘D’
I diopter or ID  It is the power of lens whose focal length is I m

Power of convave lens or diverging lens is always negative

If any optical instrument has many lens, then net power will be

<1>

Heredity and Evolution

Important Question for Class 10 Science Heredity and Evolution 

Take diagramms from NCERT BOOK

VERY SHORT ANSWER TYPE QUESTION [1 Mark] -Year 2007

1. What is a gene? [Delhi]
Answer. Gene is the unit of inheritance. Gene is the part of a chromosome which controls the appearance of a set of hereditary characteristics.

SHORT ANSWER TYPE QUESTIONS [I] [2 Marks] -Year 2007

2. What is meant by analogous organs? Taking a suitable example, explain how they support the theory of Organic Evolution. [Delhi]
Answer. Analogous organs are those organs which I have different basic structural design and developmental origin but have similar appearance and perform similar functions.
Example: The wings of birds and bats look similar but have different design in their structure. They have a common function of flying but their origins are not common. So, birds and bats are not closely related.

CBSE Class 10 Science – More Resources

3. What is a sex chromosome? [Foreign]
Answer. Sex chromosome is a chromosome that operates in the sex-determining mechanism of a species. Many animals have two different types of sex chromosomes. For example, in human there is a large X chromosome and a much smaller Y chromosome.

SHORT ANSWER TYPE QUESTIONS [II] [3 Marks] -Year 2007

4.Define ‘evolution’. Describe Darwin’s theory of evolution. [All India]
Answer. Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years and new species are produced. Since, the evolution is of the living organisms, so it is called ‘Organic Evolution’.
Darwin’s theory of Evolution: Charles Robert Darwin gave the theory of evolution in his famous book, ‘The Origin of Species’. The theory of evolution proposed by Darwin is known as ‘The Theory of Natural Selection’. It is also called ‘Darwinism’.
According to Darwin’s theory of evolution:

1. There is natural variation within any population and some individuals have more favourable variations than others.
2. Population remains fairly constant even though all species produce a large number of off springs.
3. This is due to ‘competition’ or struggle for existence between same and different species.
4. The struggle for survival within population eliminates the unfit individuals and those with ‘favourable variations’ survive and pass on these variations to their progeny to continue. This is called natural selection.
5. The favourable variations are accumulated over a long time period leading to the origin of a new species.

SHORT ANSWER TYPE QUESTIONS [I] [2 Marks] -Year 2008

5.”The sex of the children is determined by what they inherit from their father and not their mother.” Justify. [Delhi(C)]
Answer. It is because a child who inherits an X chromosome from her father will be a girl and one who inherits a Y chromosome from his father will be a boy. But all children inherit a X chromosome from their mother regardless of whether they are boys or girls.

6. Define variation in relation to a species. Why is variation beneficial to the species? [Delhi]
Answer. Variation refers to the differences in the characters or traits among the individuals of a species.
Variations are beneficial to the species because:

1. They enable the organisms to adapt themselves in changing environment.
2. Variations form the basis of heredity.
3. They form the raw materials for evolution and development of new species.

7. Describe briefly four ways in which individuals with a particular trait may increase in a population. [Foreign]
Answer. The four ways in which individuals with a particular trait may increase in a population are as follows:

1.  Sexual reproduction results into variations.
2. The individuals with special traits survive the attack of their predators and multiply while the other will perish.
3.  Genetic drift provides diversity without any adaptation.
4. Variations in the species may lead to increased survival of the individuals.

8. What are fossils? What do they tell us about the process of evolution? [All India(C): All India 2011]
Answer. The remains of dead plants and animals which were buried under the rocks millions of years ago are called fossils.

Fossils tell us about the process of evolution. The fossils of different organisms have some features similar to one species while some features are similar to the other species. In this way, they show the link between two species. They tell us that one species evolves from the other.

SHORT ANSWER TYPE QUESTIONS [II] [3 Marks] -Year 2009

9. Distinguish between acquired and inherited traits by giving one example of each. Why are traits acquired during the lifetime of an individual not inherited? [Delhi(C)]
Answer. Acquired trait is a particular characteristic that is developed during the lifetime of an individual. Such characteristics are not genetically controlled and cannot be passed on to the next generation. Example: Loss of weight due to starvation.
Inherited trait is the transmission of particular characteristics from parents to their off springs, generation to generation. Such traits are genetically determined characteristic that distinguishes a person.
Example: Colour of skin.
The acquired traits cannot change the DNA of the germ cells. Therefore, the acquired traits cannot be inherited over generations during the lifetime of an individual.

10. The human beings who look so different from each other in terms of colour, size and looks are said to belong to the same species. Why? Justify your answer. [All India(C)]
Answer. Human beings are said to belong to the same species because of the following reasons:

1. DNA studies.
2. Number of chromosome is same,
3. All have a common ancestor.
4. They interbreed among themselves to produce fertile young ones of their own kind.

SHORT ANSWER TYPE QUESTIONS [I] [2 Marks] -Year 2010

11. Give one example each of characters that are inherited and the ones that are acquired in humans. Mention the difference between the inherited and the acquired characters. [Delhi]
Answer. Eye colour or hair colour of a person is an example of inherited character whereas, body weight is an example of acquired character.
The basic difference between inherited and acquired character is that inherited character is passed on from parent to offspring and acquired characters are acquired by an individual during his lifetime depending upon his lifestyle.

SHORT ANSWER TYPE QUESTIONS [II] [3 Marks] -Year 2010

12. Explain the mechanism of sex determination in humans. [All India]
Or
With the help of a flow chart explain in brief how the sex of a newborn is genetically determined in human beings. Which of the two parents, the mother or the father, is responsible for determination of sex of a child? [Foreign]
Answer. Mechanism of Sex Determination in Human Beings:
In human beings, the sex of the individual is genetically determined.

• Sex determination is the process by which sex of a new born individual can be determined.
•  Human beings have 1 unpaired sex chromosome. Sex chromosome of male is XY and of female is XX.
•  Sex of a child depends on what happens at fertilisation.
  
  Thus, father is responsible for the determination of the sex of a child.

13. With the help of suitable examples explain natural selection. [Foreign]
Answer. Let us assume that there exist a group of red beetles in some green bushes. Due to variation during sexual reproduction one green beetle evolved among them. This green beetle can pass the colour to its progeny which are green beetles.
Crows cannot see green beetles in green bushes and therefore, cannot eat them. The progeny of green beetles are not eaten while the progeny of red beetles continued to be eaten. As a result there are more and more green beetles in the beetle population. The progeny of green beetles increase due to natural selection which gives survival advantage.

SHORT ANSWER TYPE QUESTIONS [I] [2 Marks] -Year 2011

14. How is the equal genetic contribution of male and female parents ensured in the progeny? [Delhi]
Answer. During sexual reproduction, a female gamete or egg cell fuses with a male gamete or sperm cell which are haploid to form zygote. Zygote is diploid which contains 23 chromosomes from mother and 23 from father. In this way, an equal genetic contribution of male and female parents is ensured in the progeny.

15. What evidence do we have for the origin of life from inanimate matter? [All India]
Answer. Stanley L. Miller and Harold C. Urey provided evidence regarding origin of life from inanimate matter. They assembled an atmosphere similar to that existed on early earth. The atmosphere had molecules like ammonia, methane and hydrogen sulphide, but no oxygen and kept over water at temperature just below 100°C. Sparks were passed through the mixture of gases. At the end of a week, 15% carbon from methane had been converted to simple compounds of carbon like amino acids which make-up protein molecules. So, life arose afresh on earth.

SHORT ANSWER TYPE QUESTIONS [II] [3 Marks] -Year 2011

16. Explain the terms: (i) Speciation (ii) Natural selection [Delhi]
Answer. (i) Speciation is the evolution of reproductive isolation among once-interbreeding populations, i.e. the development of one or more species from an existing species.
(ii) Natural selection is the process, according to Darwin, which brings about the evolution of new species of animals and plants.

17. Explain with examples how the following are evidences in favour of evolution in organisms. (i) Homologous organs (ii) Analogous organs (iii) Fossils [Delhi]
Answer. (i) Forelimb of human and bird are homologous organs. They have same structural design and developmental origin but they have different functions and appearance. Homologous organs help us to understand that the organism has evolved from a common ancestor. The more common characteristics the two species have, the more closely they are related.
(ii) Analogous organs are those organs design and developmental origin but have similar appearance and perform similar functions.
Example: The wings of birds and bats look similar but have different design in their structure. They have a common function of flying but their origins are not common. So, birds and bats are not closely related.
(iii) Fossils and their study is useful in knowing about the species which are no longer alive. They provide evidence and missing links between two classes. They are helpful in forming a sequence of organisms in the pathway of evolution. Thus, fossils have an importance in deciding evolutionary relationship. Archaeopteryx is a fossil bird. It had feathers, fused bones and beak which are exclusively bird structures. It also had some features which are found in reptiles, e.g. teeth in jaw, claws on free fingers and a long tail. This fossil provides a clue that birds have evolved from reptiles.

18. Give an example of body characteristics used to determine how close two species are in terms of evolution and explain it. [All India]
Answer. Homologous organs help to identify the relationship between organisms. These characteristics in different organisms would be similar because they have inherited from a common ancestor. For example, forelimbs of human and wings of birds show closeness between the two species because the organs have similar basic structural design of limbs though it has been modified to perform different functions.

19. What are homologous organs? Can the wing of a butterfly and the wing of a bat be regarded as homologous? Why? [All India]
Answer. Homologous organs are those organs which have the same basic structural design and developmental origin but have different functions and appearance.
Example: The forelimb of a frog, a lizard, a bird and a man seem to be built from the same basic design of bones, but they perform different functions.

No, the wing of a butterfly and the wing of a bat cannot be considered homologous organs because they have a common function for flying but their origin and structure are not common. So, they are analogous organs.

SHORT ANSWER TYPE QUESTIONS [II] [3 Marks] -Year 2012

20. What is meant by the term speciation? List four factors which could lead to speciation. [Delhi]
Answer. Speciation is the evolution of reproductive isolation among once interbreeding population.
Factors which can lead to speciation are:

1. Genetic drift: Over generations, genetic drift may accumulate which leads to speciation.
2. Natural selection: Natural selection may work differently in different location which may give rise to speciation.
3.  Severe DNA change.
4. A variation may occur which does not allow sexual act between two groups.

21. A blue colour flower plant denoted by BB is cross bred with that of white colour flower plant denoted by bb.
(a) State the colour of flower you would expect in their F1  generation plants.
(b) What must be the percentage of white flower plants in F2  generation if flowers of F1 plants are self-pollinated?
(c) State the expected ratio of the genotypes BB and Bb in the F2 progeny. [Delhi]
Answer.

22. Distinguish between homologous organs and analogous organs. In which category ; would you place wings of a bird and wings of a bat? Justify your answer giving a suitable reason.[Delhi]
Answer.

Wings of a bird and wings of a bat are analogous organs as they have different basic structural design but have similar appearance and perform similar functions.

23. Define the term ‘evolution’. “Evolution cannot be equated with progress”. Justify this statement. [Delhi]
Answer. Evolution is the sequence of gradual change which takes place in primitive organisms over millions of years and new organisms are formed. Evolution cannot be equated to progress from lower forms to higher forms. It seems to have given rise to more complex body designs even while the simpler body designs continue to flourish. For example, human beings have not evolved from chimpanzees, but both have common ancestor.

24. If we cross pure-bred tall (dominant) pea plant with pure-bred dwarf (recessive) pea plant we will get pea plants of F1generation. If we now self-cross the pea plant of F2  generation, then we obtain pea plants of F2  generation.
(a) What do the plants of F2 generation look like?
(b) State the ratio of tall plants to dwarf plants in F2 generation.
(c) State the type of plants not found in F2generation but appeared in F2 generation, mentioning the reason for the same. [All India]
Answer. (a) All plants of F1  generation will be tall plants.
(b) 3:1
(c) Dwarf trait is recessive trait which was not expressed in the F1  generation, the recessive trait gets expressed in the F2generation after self pollination.

25. How are fossils formed? Describe, in brief, two methods of determining the age of fossils. [All India]
Answer. When organisms die, their bodies decompose due to action of micro organisms. However, sometime the body or at least some parts of the body may be in such an environment that does not let it decompose completely. All such preserved traces of living organisms are called fossils.
The age of fossils can be estimated by the following two methods:

1.  If we dig into the earth and start finding fossils, it can be assumed that the fossils closer to the surface are more recent to those found in deeper layers.
2. By detecting the ratios of different isotopes of the same element in the fossil material.

26. State the meaning of inherited traits and acquired traits. Which of the two is not passed on to the next generation? Explain with the help of an example.
Answer. Inherited traits are the characteristics transmitted from parents to their offspring.Acquired traits are characteristics which are developed during the lifetime of an individual.Acquired traits are not passed on to the next generation. For example, if we breed a group of mice, all their progeny will have tails. Now, if the tails of these mice are removed by surgery and allowed to breed, the next generation mice will also have tails. If these tails are also removed and allowed to breed, the progeny of mice will again have tails. Removal of tail by surgery is an acquired trait and do not change the genes of germ cells and hence, are not passed on to the next generation.

27.”An individual cannot pass on to its progeny the experiences of its lifetime.” Justify the statement with the help of an example and also give reason for the same. [Foreign]
Answer. Experience achieved during the lifetime of an individual does not make any change in the gene of the individual.
For example, if a person reads a book on birds, the knowledge he earns by reading the book does not make any change in the gene, hence, this knowledge will not get automatically transmitted to his next generation. Such a trait is called acquired trait.