Light Reflection and refraction Notes Class 10
Light Reflection and refraction Notes Class 10: CBSE Notes Class 10 Science Chapter 10 Notes. Light Reflection and refraction Notes PDF. Class 10 Science Chapter 10 Revision Notes – Light Reflection and refraction.
Light Reflection and refraction Notes Class 10: Overview
Light Reflection and refraction Notes Class 10 |
|
Board |
CBSE / NCERT |
Class |
10 |
Subject |
Science |
Chapter Number |
10 |
Chapter Name |
Light Reflection and refraction |
Topic |
Notes |
Light Reflection and refraction Notes Class 10 Science
Light:
Electromagnetic radiation that provides sensation of vision for us is called as light. Light always travels in straight line. The ray of light falls on opaque object and bounces back towards our eyes, which is sensed by eyes and we see the object. This phenomenon is known as reflection of light.
Light can pass through the transparent medium, during this it changes its direction as well as speed, and this is known as refraction of light. Along with this, light consisting of multi colours which can be separated into its component using material like prism, like what we see in rainbow. This separation of white light in to its component colours is called as dispersion of light.
Reflection of light:
When light falls on highly polished and opaque object, it bounces back in the same medium, this is called as reflection of light. We can see the object is nothing but the reflection of light. Rays of light always obeys the following laws during reflection
- Incident ray, reflected ray lies on opposite side of normal and all these three lies in the same plane.
- Angle of incidence is always equal to angle of reflection.
Mirrors are the most commonly used materials to study reflection of light. There are two types of mirror viz. Plane mirror and Spherical mirror.
Plane mirrors gives us virtual and erected image of object, whereas spherical mirrors can produce real, inverted, erect, virtual, magnified or diminished image as per the distance of object from mirror. Depending upon the need we can select the mirror.
Spherical mirrors:
The curved polished surface is called as spherical mirror. There are two types of spherical mirror, 1) Concave mirror, 2) Convex mirror.
Internally bulged and shiny surface is called as concave mirror or converging mirror. Whereas externally bulged and polished surface is known as convex mirror or diverging mirror. Some important definitions are needed to explain the mirrors in detail.
- Pole: The midpoint or central point on the surface of the mirror is called as pole (P). It lies on mirror.
- Centre of curvature: The centre of the imaginary sphere of which mirror is part is called as centre of curvature(C). It does not lie on mirror.
- Radius of curvature: The distance between the pole and centre of curvature is known as radius of curvature (R).
- Principal axis: An imaginary line passing through the centre of curvature and pole is called as principal axis.
- Focus: When parallel beam of ray’s falls on the mirror gets reflected and meets at the same point lie on principal axis, this point is called as focus (F).
- Focal length: The distance between pole and focus is called as focal length (f).
Remember that for spherical mirror,
Convergence of rays:
When number of parallel rays falls on concave mirror after reflection all the rays appears to be collected at the same point on principal axis, this is called convergence of reflected rays hence the mirror is called as converging mirror. Due to this the energy of all rays gets concentrated at the same point. See the figure below.
Divergence of rays:
When number of parallel rays falls on convex mirror after reflection all the rays appears to be move away from the same point on principal axis, this is called divergence of reflected rays hence the mirror is called as diverging mirror. Due to this the energy of all rays gets distributed in different direction. See the figure below.
Rules for image formation due to concave mirror:
- If incident ray passes parallel to principal axis, reflected ray passes through the focus
- If incident ray passes through focus, reflected ray passes parallel to principal axis
- If incident ray passes through the centre of curvature, reflected ray traces same path
Image produced by concave mirrors:
Position of object |
Position of image | Nature of image | Size of image |
At infinity | At the focus (F) | Real and inverted |
Point size image
|
Beyond Centre of curvature |
Between Focus and Centre of curvature | Real and inverted |
Diminished
|
At Centre of curvature |
At Centre of curvature | Real and inverted
|
Same size |
Between Centre of curvature and Focus |
Beyond Centre of curvature. | Real and inverted |
Magnified (bigger than object size)
|
At Focus |
At infinity | Real and inverted |
Highly magnified (larger than object size)
|
Between Pole and Focus |
Behind the mirror | Virtual and erect |
Highly magnified (larger than object size) |
The above mentioned positions can shown using ray diagram.
Applications of concave mirrors:
- For shaving or makeup purpose, concave mirrors are held close to face so that highly magnified image of face can be obtained, which helps us to make clean shave or perfect makeup.
- If you have toothache, and you are really in trouble, you need to visit a Dentist. He can diagnose your problem by observing your mouth by holding a concave mirrors close to your mouth
- For safe driving during night time we need sharp and focused beam of light, which can be obtained by fixing a bulb at centre of concave shaped silvered polished mirror
Image produced by concave mirrors:
When object kept at different position in front convex mirror it always produces virtual and erected image. See the following table for more explanation.
Position of object |
Position of image | Nature of image | Size of image |
At infinity | At the focus (F) but behind the mirror | Virtual and erected |
Point size image. |
At any point between infinity and pole |
Between pole and focus but behind mirror | Virtual and erected |
Diminished. |
The above mentioned positions can shown using ray diagram.
Applications of concave mirrors:
- Rear view mirror of vehicle: To see the passage of road and vehicles running behind our vehicles, we need to fix a convex mirror of suitable size outside the vehicle.
- To increase the width of surveillance of CCTV’s they come with convex mirrors so that more area can be covered.
Sign conventions used for spherical mirrors:
For performing any experiments using mirrors we need to follow the sign conventions viz.
- Pole (P) of mirror is taken as origin X-axis.
- Principal axis is considered as X- axis.
- Objects are always kept on left side.
- Distances parallel to the principal axis are measured from the pole of the mirror.
- Distances measured on left side are taken as negative (- X axis), whereas the distances measured towards right side are taken as positive (+ X axis).
- Height of object above and perpendicular to principal axis is taken as negative(- Y axis), and height of object below and perpendicular to principal axis is taken as positive(+ Y axis).
- The focus of concave mirror appears on left side; hence the focal length of concave mirror is taken as negative.
- The focus of convex mirror appears on right side; hence the focal length of concave mirror is taken as positive.
Magnification by mirror (M):
Magnification is defined as the ratio of height of image (h2) to the height of object (h1) or the ratio of image distance (v) to object distance (u).
Mirror formula:
If u = object distance,v=image distance and
f = focal length of mirror,then the mirror formula is given by
According to sign conventions we need to assume the distance as follows,
Object distance, u= -ve , Image distance, v = -ve/+ve (depends upon the position of image) and Focal length, f= -ve for concave mirror/+ve for convex mirror.
Refraction of light:
Light ray has ability to penetrate through the transparent medium when it strikes on it. During this the ray gets bend slightly and changes its direction causes refraction of light. The change in direction of light when it passes from one transparent medium to other is called refraction of light.
Along with direction, speed of light also gets changed when it travels from different media; this is also nothing but the refraction of light. It’s due to refraction that we can enjoy beautiful scenario in nature like twinkling of stars, rainbow etc.
Following laws are always obeyed b ray of light during refraction viz.
- Incident ray, refracted ray and normal lies in same plane at the point of incidence, and incident ray and refracted rays lies on opposite side of normal.
- For given pair of media, the ratio of sine of angle of incidence to the sine of angle of refraction is constant.
The constant is known as refractive index of the medium, which is nothing but the extent of the change in direction/change in speed that takes place in a given pair of media.
On the basis of refractive index, medium is said to be optically rarer if its refractive index is less, and optically denser if the refractive index is more.
Refractive index is also studied on the basis of speed of light in given transparent media. Light travels fastest in vacuum with speed equal to, .
When rays of light travels from one transparent medium to another then the speed of light changes, if v1 is speed of light in medium 1, and v2 is the speed of light in medium 2, then the refractive index further can be define as ratio of speed of light in medium 1 to speed of light in medium 2.
- Refractive index of medium 1 with respect to medium 2 is defined as the ratio of speed of light in medium 2 to the speed of light in medium 1, written as
- Refractive index of medium 2 with respect to medium 1 is defined as the ratio of speed of light in medium 1 to the speed of light in medium 2, written as,
- Absolute refractive index of medium is defined as the ratio of speed of light in vacuum (or air) to the speed of light in the given medium, written as,
(Where ‘c’ is speed of light in vacuum or air and ‘v’ is speed of light in given medium).
Refraction through glass slab:
When a ray of light is allowed to strike obliquely on a glass slab of thickness ‘t’suffers refraction twice; once from rarer medium to denser medium, and then again from denser medium to rarer medium and finally emerges out the glass slab. It is observed that the incident ray and emergent ray remain parallel to each other, as well as the angle of incidence is equal to angle of emergence i.e., shown in fig below.
Refraction by spherical lenses:
Lenses are the transparent material bounded by two surfaces out of which at least one should be spherical. There are two types of the lenses 1) Convex lens, 2) Concave lens.
Externally bulged transparent material (thick towards centre and thinner at the edges) is known as convex lens. Whereas, internally bulged transparent material (thinner towards centre and thick at the edges) is known as concave lens.
Some important concepts:
Centre of curvature: The centre of imaginary sphere of which lens is formed is called as centre of curvature. For lens there are two centres of curvatures, denoted as C1 and C2.
Optical centre: The central point lying on the lens is called as optical centre. It is denoted as O.
Principal Focus: The point on principal axis where all the refracted rays gets converges or diverges is called as focus. For lenses, two foci are considered and denoted as F1 and F2.
Principal focus of convex lens is obtained behind lens, whereas for concave lens it appears in front of the lens.
Like mirrors, lenses also have ability to converge or diverge the refracted rays of light.
Convergence of rays by convex lens:
When a parallel beam of light falls on convex lens, after refraction all the rays meet a same point which lies on the principal axis. This point of intersection of all the refracted rays is called as principal focus of lens and the phenomenon is known as convergence of the ray. See the figure.
Divergence of rays by convex lens:
When a parallel beam of light falls on concave lens, after refraction all the rays appears to move away from the same point which lies on the principal axis. This point is called as principal focus of lens and the phenomenon is known as divergence of the ray. See the figure.
Rules for formation of images due to convex/concave lens:
- If incident ray passes parallel to principal axis, refracted ray passes through the principal focus.
- If incident ray passes through focus, refracted ray passes parallel to principal axis
- If incident ray passes through the optical centre, refracted ray traces same path.
Image produced by convex lens:
When object kept at different position in front convex lens real, inverted, erected, virtual images can be formed. Go through the table for position, size and nature of images due to concave mirror.
Position of object |
Position of image | Nature of image | Size of image |
At infinity | At the focus (F2) | Real and inverted |
Point size image |
Beyond 2F1 |
Between F2 and
2F2 |
Real and inverted |
Diminished
|
At 2F1 |
At 2F2 | Real and inverted |
Same size |
Between F1 and 2F1 | Beyond 2F2. | Real and inverted | Magnified (bigger than object size)
|
At F1 |
At infinity | Real and inverted |
Highly magnified (larger than object size) |
Between F1 and optical centre |
On same side of lens | Virtual and erect |
Highly magnified (larger than object size) |
The above mentioned positions can shown using ray diagram.
Image produced by concave lens:
When object kept at different position in front concave lens, it always produces virtual and erected image. See the following table for more explanation.
Position of object |
Position of image | Nature of image | Size of image |
At infinity | At the focus (F1) on the same side | Virtual and erected |
Point size image. |
At any point between infinity and optical centre |
Between optical centre and focus (F1) on same side | Virtual and erected |
Diminished. |
The above mentioned positions can shown using ray diagram.
Sign conventions used for spherical lenses:
For performing any experiments using lenses we need to follow the all the sign conventions which we used for spherical mirror, out of which few important changes should be noted as follows.
- Principal focus of convex lens appears behind the lens; hence the focal length of convex lens is taken as positive.
- Principal focus of concave lens appears on left side of the lens; hence the focal length of concave lens is taken as negative.
Rest all the conventions used for mirrors are also applicable here. Using those we need to decide the signs of u, v and f appropriately.
Some important applications of concave lens:
- Peep hole of door: This is safety device kept in door. If any unknown person/or any threat is observed through peep hole, we can be alert before opening the door.
- Spectacles: Person those are suffering from the disease in which he/she can see nearby objects clearly but unable to see distinct objects clearly is known as nearsightedness. For correcting the vision of such person doctors prescribe concave lens of suitable power
Magnification by lenses (M):
Magnification is defined as the ratio of height of image (h2) to the height of object (h1) or the ratio of image distance (v) to object distance (u).
Lens formula:
If u=object distance,v=image distance and
f=focal length of lens,then the lens formula is given by
Power of lens:
Ability of lens to converge or diverge the refracted rays is known as power of the lens. Note that the, smaller is the focal length of lens; more is converging/diverging ability Hence power of lens is defined the reciprocal of focal length of lens. Denoted as P.
SI unit of power of lens is 1/meter (per meter) or dioptre (D).
Power of convex lens is positive, while that o concave lens is negative.
If ‘n’ number of lenses of different power P1, P2, P3,….. are arranged such that there optical centre align on same principal axis, then the total power of combination is sum of individual powers, given as
P=P1+P2+ P3+……….
Such type of combination is very useful in manufacturing in instruments like compound microscope, telescope etc.
If focal length of convex/concave lens is 1 m, then the power of lens is said to 1 diopter.