CBSE Class 10 Science Light Reflection And Refraction Notes

LIGHT-REFLECTION & REFRACTION

Light is a form of energy, which enable us to see the object. 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).

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. i = r

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)

Reflection of light by spherical Mirrors

Mirrors, whose reflecting surface are curved inward or outward spherically are called spherical 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 reflecting surface of a spherical mirror form a part of sphere. It has a centre, which is known as centre of curvature, denoted by (C)

5. Radius of curvature : The separation between the pole and the centre of curvature. ie. PC = R

6. Focus point : The point on the principal axis, where all parallel rays meet after reflection, denoted by (F)

7. Focal length : The length between the pole and focus point i.e. PF = f

8. Relationship between focal length and Radius of curvature. F= R/2

Image formation by spherical Mirror

Before we learn the formation of image or ray diagram, let us go through few tips
a) Remember, A say 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 cus-valerie reflecting spherical surface is 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 : The image will only form when two or more rays meets at apoint. Image formation by a concave mirror for different position of the object 

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
1. Used as rear-view mirror in vehicles because it gives erect image. It also 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 

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

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
1. The stone at the bottom of water tub appear to be raised.
2. A fish kept in aquarium appear to be bigger than its actual size.
3. A pencil partially immersed in water appears to be displaced at the interface of air and water. 

Refraction through a Rectangular Glass Slab 

When a incident ray of light AO passes from a rarer medium (air) to a denser medium (glass) at point. O on interface AB, it will bends towards the normal. At pt O1, on interface DC the light ray entered from denser medium (glass) to rarer medium (air) here the light ray will bend away from normal OO1is a refracted ray OB is an emergent ray. If the incident ray is extended to C, we will observe that emergent ray O1B is 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-
1. 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.
2. The ratio of sine of angle of incidence to the sine of angle of refraction is a constant ie. 

for given colour and pair of media, this law is also known as Snells Law
Constant n is the refractive index for a given pair of medium. It is the refractive index of the second medium with respect to first medium. 

Refractive Index
The refractive index of glass with respect is air is given by ratio of speed of light in air to the speed of light in glass. 
n_ga = n_g/n_a = Speed of light in air/Speed of light in glass = c/v
C → Speed of light in vacuum = 3•10⁸ m/s
speed of light in air is marginally less, compared to that in vacuum.
Refractive index of air with respect to glass is given by 

The absolute refractive index of a medium is simply called refractive index
nm = Speed of light in air/Speed of light in the medium = c/v
Refractive index of water (n_w) = 1.33 w
Refractive index of glass (n_g) = 1.52

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

CONVEX LENS
A lens may have two spherical surfaces, bulging outwards, is called double convex lens (or simply convex lens.
It is also known as converging lens because it converges the light. 

CONCAVE LENS
A lens bounded by two spherical surfaces, curved inwards is known as double 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 has two spherical surfaces. Each of these surfaces form a part of sphere. The centre of these two spheres are called centre of curvature represented by C and C . 1 2
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 is 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 OF and OF is called as focal length

Tips for drawing Ray diagram

a) After refraction, a ray parallel to principal axis will pass through F. 

b) A ray passes through F, after refraction will emerge parallel to principal axis. 

c) A ray passes through optical centre 'O', paeses without any deviation. 

Image formation by a convex lens for various position of object

Sign Convention for Refraction by spherical lens
Similar to that of spherical mirror, only the difference is that all the measurement are made from optical centre 'O' 

Few tips to remember sign convention for spherical lens 

Power of Lens
The degree of convergence or divergence of light ray achieved by a lens is known as power of a lens.
It is difined as the reciprocal of its focal length Represented by P 

Please click the link below to download pdf file for CBSE Class 10 Science Light Reflection And Refraction Notes Set A

Key learnings:

1. When light falls on a body, it may be absorbed, may be transmitted or light may come back to the same medium.

2. Reflection of a light means light waves are neither transmitted nor absorbed but are deflected from the surface of the medium back into the same medium.

3. Laws of Reflection:

First law of reflection: The incident ray, the normal to the surface at the point of incidence and the reflected ray, all lie in the same plane.

Second law of reflection: The angle of incidence is equal to the angle of reflection.

4. Real image is obtained when the rays of light after reflection, actually converge at a point. It can be obtained on the screen and can be seen with the eye.

5. Virtual image forms when rays of light do not actually meet, but appear to meet when produced backwards. It cannot be obtained on the screen.

6. Characteristics of the image formed by the plane mirror:

i. The image formed by a plane mirror is always virtual and erect.

ii. Size of image = Size of object and the image is laterally inverted.

iii. The image formed by the plane mirror is as far behind the mirror as the object is in front.

7. Lateral Inversion: The phenomenon due to which the right side of the object appears as left and the left side of the object appears as right. That is the image is inverted sideways.

8. A spherical mirror whose reflecting surface is curved outwards and polished on the inner spherical surface is convex mirror.

9. A spherical mirror whose reflecting surface is curved inwards and polished on the outer spherical surface is concave mirror.

10. Concave mirror mostly forms real images, which can be received on the screen.

11. Convex mirror forms virtual images, which cannot be received on the screen.

12. Differentiating between a plane mirror, a concave mirror and a convex mirror, without touching them:

i) If the image formed is erect and is of same size as in reality then it is a plane mirror.

ii) If the image formed is still erect but smaller in size then it is a convex mirror.

iii) If the image is erect but magnified when mirror is close to the object then it is a concave mirror.

13. Solar concentrators use huge concave mirrors to focus large amounts of solar energy thereby producing high temperature conditions in a solar power plant.

14. Concave mirrors are used as shaving and make up mirrors.

15. The centre of the reflecting surface of a spherical mirror is a point called the pole of the mirror and usually represented by P.

16. The horizontal line passing through the centre of curvature and pole of the spherical mirror is known as Principal axis.

17. The centre of curvature of a spherical mirror is the centre of the hollow sphere of glass, of which the spherical mirror is a part and usually represented by C.

18. The radius of curvature of a spherical mirror is the radius of the hollow sphere of glass, of which the spherical mirror is a part and usually represented by R.

19. The diameter of the reflecting surface that is twice the radius is called its aperture.

20. Radius of curvature (R) = 2 x focal length (f)

21. Rules for ray diagrami.

i. The path of the reflected light ray depends upon how the incident ray is oriented with respect to the principal axis.

ii. A ray of light parallel to the principal axis, after reflection pass through the principal focus.

iii. A ray of light passing through the principal focus of a mirror becomes parallel to the principal axis of the mirror, on reflection

iv. A ray of light incident obliquely towards the pole of mirror is reflected obliquely as per the laws of reflection

v. A ray of light passing through centre of curvature of a mirror is reflected back along the same path

22. Image formation by a concave mirror for different positions of the object

23. Nature, position and relative size of the image formed by a convex mirror:

24. The ratio of the height of the image to the height of the object is known as the magnification.

25. Magnification is positive for virtual image and negative for real image.

26. The phenomenon of change in the path of light from one medium to another is called refraction of light.

27. The angle formed between the incident ray and the normal is called angle of incidence and the angle formed between the refracted ray and the normal is called angle of refraction.

28. The cause of refraction is the change in the speed of light as it goes from one medium to another medium.

29. Larger the difference in speeds of light, the greater will be the angle of bending and vice-versa.

30. When a ray of light passes from a rarer medium to a denser medium, it bends towards the normal. Also, the angle of incidence is greater than the angle of refraction.

31. When a ray of light passes from a denser medium to a rarer medium, it bends away from the normal. Also, the angle of incidence is less than the angle of refraction

32. Laws of refraction: The first law of refraction: The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.

Second law of refraction: The ratio of the sine of angle of incidence in the first medium to the sine of angle of refraction in the second medium is a constant for a given pair of medium and for a given wavelength of light.

n=sini/sinr 
where n is a constant known as refractive index of the second medium with respect to the first medium. This law is also called as Snell’s Law

33. The phenomenon of change in path of light in going from one medium to another is called refraction of light.

34. The ratio of speed of light in vacuum to the speed of light in a medium is called the refractive index of the medium. It has no unit.

35. When a ray of light traveling from a rarer to a denser medium slows down and bends towards the normal.

36. When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.

37. The shifting of emergent ray sideways from the direction of original incident ray is called lateral displacement or lateral shift

38. The extend of the lateral shift depends on

- the angle of incidence,

- the thickness of slab, and

- Refractive index of the glass slab

39. Lateral displacement is produced during refraction through a glass slab and not through a glass prism.

40. The angle through which a ray of light deviates on passing through a prism is called the angle of deviation.

41. Convex lens is thicker at the middle than at the edges.

42. Concave lens is thicker at the edges than at the middle.

43. Convex lens converge the rays of light while concave lens diverges.

44. As the object moves towards the optical centre of convex lens, the image moves away from the optical centre except when the object is placed between focus and optical centre of the lens.

45. As the object moves towards the optical centre of the convex lens, the size of image increases and it’s highly enlarged when object is at focus.

46. Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is Dioptre.

Very Short Answer Questions

Question. What is the unit of refractive index?
Answer : It has no unit.

Question. What is the relation between optical density, refractive index and speed of light?
Answer :The medium with higher refractive index in which speed of light is less is known as optically denser medium and the medium with lower refractive index in which the speed of light is more is known as optically rarer medium.

Question. Define magnification of lens.
Answer : Magnification (m) = Height of image ( h') / Height of object (h ) = v/u
For convex lens ‘m’ can be more than, less than or equal to one.
For concave lens ‘m’ is less than one.

Question. What is the S.I. unit of power? Define it.

Answer : The S.I. unit of power is dioptre denoted by “D” 1 dioptre is the power of a lens whose focal length is 1 metre.
• Power of a convex lens is +ve
• Power of a concave lens –ve

Question. What is the radius of curvature of plane mirror?
Answer : Infinity.

Question. If a convex lens is used to focus sunlight on a paper, where the paper should be placed so that it catches fire.
Answer : At the Principal focus.

Question. Where should be an object placed in front of convex lens so as to use it as a magnifier?
Answer : Between the pole and the focal length.

Short Answer Questions

Question. The power of a lens is –4.0 D. What is the nature of this lens?
Answer : Negative power is associated with only concave lens.

Question. A convex lens has a focal length of 10 cm. At what distance from the lens should the object be placed so that it gives a real and inverted image 20 cm away from the lens?
What would be the size of the image formed if the object is 2 cm high? With the help of a ray diagram show the formation of the image by the lens in this case.
Answer : f = +10 cm, ν = +20 cm as image is real and inverted. Height of the object
= 2 cm (say +ve)
Using
1/f = 1/ν  – 1/u
, we get
1/u = 1/ ν – 1/f
= 1/+20 – 1/10
=   +1 – 2/20
= –1/20
u = – 20 cm (= 2f)
Image will be of the same size as that of object (as u = ν) and hence, the height of the image will be 2 cm.

Question. Why does a ray of light bend when it travels from one medium into another?
Answer : Due to change in velocity in the medium and to reduce the time taken to travel the same, a ray of light bends when it travels from one medium to another.

Question. What kind of mirrors are used in big shopping stores to watch activities of customers?
Answer : Convex mirror as the image is independent of position of the object.

Key concepts and terms:

1) Light:light is a form of energy. It brings the sensation of sight. It is a form of electromagnetic radiation. It also provides us means of communication (fiber-optics).

2) Light wave: light wave travels with a speed of 3 x 108 ms-1 in free space. Its speed depends on the medium.

3) Ray and beam: the straight line indicating the path of the light (arrow- direction is called a ray. A bundle of rays originating from the same source of light in a particular direction is called a beam of light.

4) Reflection: when light falls on a surface and gets back the same medium, it is called reflection.

5) Image: the point of convergence or the point form where the light appears to diverge after reflection or refraction is called image.

6) Angle of incidence: the angle between the incident ray and the normal at the point of incidence is called angle of incidence.

7) Angle of reflection: the angle between the reflected ray and the normal at the point of reflection is called angle of reflection.

8) Laws of reflection:

1) the incidence ray the reflected ray and the normal at the point of incidence, all lie in the same plane.

2) The angle of reflection and the angle of incidence are equal.

9) Aperture: the width of the reflecting surface is called aperture.

10) Focus: the point on the principle axis where all parallel rays meet after reflection is called principle focus.

11) focal length: the length or separation between the pole and the focus is called focal length (PF = f)

12) In order to draw ray diagram, two rules are used:

1) The rays of light passing parallel to the principle axis will coverage at the focus after reflection.

2) The rays of light passing through the focus will emerge parallel to the principle axis after reflection.

3) The rays of light passing through the center of curvature will all retrace their both after reflection.(as it is normal at the point of incidence)

4) The rays of light falling at the pole get reflected at the same angle on the other side of principle axis.(Laws of reflection)

13) Relation between radius of curvature and focal length It is two times the focal length i.e. R=2f.

14) Mirror formula: 1/f= 1/v +1/u    where f, v and u are the focal length, image distance and object distance.

15) Lens formula: if u, v and f      are the object distance, image distance and focal length respectively then 1/f=1/v-1/u

16) Magnification of a lens:        M= size of image (h1)/ size of object (h0) also m= (h1)/ (h0) = v/u.

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