SUBJECT:-PHYSICS CLASS :-Xth (C.B.S.E)
Chapter :- Light – Reflection and Refraction
LIGHT: Light is a form of energy that enables us to see things.
Reflection of Light: The phenomenon of bouncing back of light into the same medium by the smooth surface is called reflection.
Incident Ray: Ray which falls on the surface is called incident light.
Reflected Ray: Light which goes back after reflection is called reflected light.
Angle of incidence: The angle between the incident ray and the normal is called angle of incidence.It is denoted by i.
Angle of reflection: The angle between the reflected ray and the normal is called angle of reflection.It is denoted by r.
LAWS OF REFLECTION : There are two laws of reflection:
1.) The angle of incidence is equal to the angle of reflection
∠i = ∠r
2.) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
IMAGE: Image is a point where two or more rays actually meet or appear to meet. Image is are of two types:
1.) Real image: Image formed when the light rays are actually met is called real image.
2.) Virtual image: Image formed when the light rays appears to meet is called virtual image.
Difference between real and virtual image
Real image Virtual image 1.) It formed when light rays actually
met.
1.) It formed when light rays appeared to meet.
2.) It can be obtain on screen . 2.) It can’t be obtained on screen.
3.) Real image is always inverted 3.) It is always erect.
4.) Ex: image formed on cinema screen and an image formed by a concave mirror.
4.) Ex: Image formed by plane mirror and convex mirror.
Image formed by Plane Mirror (Plane reflecting surface) Following are the characteristics of image formed by a plane mirror:
1.) Virtual (imaginary) & Erect
2.) The size of the object is equal to the size of the image.
3.) Image will be laterally inverted.(Right side appears left and left side appears right)
4.) The image formed is as far behind the mirror as the object is in front of it.
Plane Mirror: If the reflecting surface is a plane then the mirror is plane.Image formed by a plane mirror is always virtual and erect,size of the image is same as object. The image formed is as far behind the mirror as the object is in front of it. Further, the image
is laterally inverted.
Spherical Mirror: If the reflecting surface is part of the hollow sphere then the mirror is a spherical mirror. The spherical mirror is of two types:
Concave mirror: A spherical mirror, whose reflecting surface is curved inwards,that is, faces towards the centre of the sphere, is called a concave mirror. It converges the light so it is also called converging mirror.Ex: automobile headlights,reflecting
telescopes,torch light etc.
Convex mirror: A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.It diverges the light so it is also called a diverging mirror. Ex: rear side mirrors of the vehicle, optical instruments etc.
Some terms related to Convex and Concave 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. denoted by (P).
3.)Centre of curvature : The centre of the hollow glass sphere of which the spherical mirror is a part is called as centre of curvature.
4.) 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.
5.) 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 the letter ‘F’.
6.) Focal length : The distance between the pole and focus point i.e. PF = f
7.) The relation between f and R is: R=2f
Rays incident on a spherical mirror exhibits the following behaviour:
1.) A ray parallel to the principal axis will pass through focus after reflection from a concave or convex mirror .
2.) A ray passing through focus of a concave/convex mirror after reflection through
mirror it will go parallel to the principal axis.
3.) A ray passing through the centre of curvature of concave/convex mirror then after
reflection it will go back to
the same path or retrace the path.
4.) A ray incident obliquely in the principle axis at pole of concave/convex mirror ,then incident and reflected ray follow the law of reflection at point of incidence
(P),and make equal
angles with the principle axis.
RAY DIAGRAM FOR IMAGE FORMED BY CONCAVE MIRROR
1.) When object is at infinity :
Image position : Image formed at “F”
Nature of Image: Real and inverted
Size: Point size or highly diminished
2.) When object is beyond “C”
Image Position: Image formed between F and C
Nature of Image: Real and Inverted
Size: Diminished
3.) When object is at “C”
Image position: Image will formed at “C”
Nature of image: Real and inverted
Size: Same size as object
4.) When object is placed between “F” and “C”
Image Position: Image will formed Beyond “C”
Nature of Image: Real and Inverted
Size: Enlarged
5.) When object is at “F”
Image Position: Image will form at “infinity”
Nature of Image : Real and Inverted
Size : Highly enlarged
6.)When object is between “P” and “F”
Position of Image: Image will formed behind the mirror
Nature of Image : Virtual and Erect
Size: Enlarged
USES OF CONCAVE MIRROR: 1.) Uses in torches,searchlights and vehicle headlights to form powerful parallel beams of light.
2.) Concave mirrors are used by dentists to see a large image of teeth of patients.
3.) It can be used as shaving mirrors.
4.) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnace
Image formation by a Convex Mirror
1.) When object is placed at infinity:
Image position: image will formed at focus
Nature of image : virtual and erect
Size: Diminished/point size
2.) When object is placed between pole and infinity:
Image position : Between P and F
Nature of image: Virtual and erect
Size: Diminished
Uses of convex mirrors:
1.) Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles
Sign Convention for Reflection by Spherical Mirrors
While dealing with the reflection of light by spherical mirrors, we shall follow a set of sign conventions called the New Cartesian Sign Convention. In this convention, the pole (P) of the mirror is taken as the origin . The principal axis of the mirror is taken as the x-axis (X’X) of the coordinate system. The conventions are as follows –
(i) The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
(ii) All distances parallel to the principal axis are measured from the pole of the mirror.
(iii) All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative.
(iv) Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive.
(v) Distances measured perpendicular to and below the principal axis (along –y-axis) are taken as negative.
1.) u = Always negative
2.) Focal length for concave mirror = Negative
3.) Focal length of convex mirror is = Positive
Mirror Formula and Magnification
This is called mirror formula.
Magnification: It is the ratio of the height of the image to the height of the object. It is usually represented by the letter “m”.
m = Height of the image (h’ ) /Height of the object (h)
The magnification m is also related to the object distance (u) and image distance (v). It can be expressed as:Magnification (m) = ′ = (h’/ h) =−v/ u
REFRACTION: This phenomenon of light bending in a different medium is called refraction.
Laws of Refraction : 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 the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.Sin i /Sin r = constant. This constant value is called the refractive index of the second medium with respect to the first.
Refraction through a Rectangular Glass Slab :
When the light is incident on a rectangular glass slab, it emerges parallel to the incident ray and is laterally displaced. It moves from rarer to denser medium and then again to the rarer medium.
Refractive Index : Refractive index of medium 2 with respect to medium 1 is equal to the ratio of speed of light in medium 1 to the speed of light in medium 2.
n21= Speed of light in medium 1/ Speed of light in medium 2 = v1/ v 2
By the same argument, the refractive index of medium 1 with respect to medium 2 is represented as n12.It is given by: n12= Speed of light in medium 2/ Speed of light in medium 1 = v2/v1
If medium 1 is vacuum or air, then the refractive index of medium 2 is considered with respect to vacuum. This is called the absolute refractive index of the medium. It is simply represented as n2 .
If c is the speed of light in air and v is the speed of light in the medium, then, the refractive index of the medium n(m) is given by n(m) = Speed of light in air /Speed of light in the medium = c / v
Refraction by Spherical Lenses:
LENS: A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens. Lens are of two types: 1.) Convex lens: A lens may have two spherical surfaces, bulging outwards. Such a lens is called a convex lens. Convex lenses converge light rays, hence convex lenses are also called converging lenses.
2.) Concave lens: A concave lens has two spherical surfaces ,bulging inwards .It diverges light so it
is also known as a diverging lens.
PARTS OF A LENS.
1.) Centre of curvature : A lens, either a convex lens or a concave lens, has two spherical surfaces. Each of these surfaces forms a part of a sphere. The centres of these spheres are called centres of curvature of the lens. The centre of curvature of a lens is usually
represented by the letter C(2f). Since there are two centres of curvature, we may represent them as C1(2f1) and C2(2f2) .
2.) Principle axis: An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.
3.) Optical centre: The central point of a lens is its optical centre. It is usually represented by the letter O.
4.) Aperture: The effective diameter of the circular outline of a spherical lens is called its aperture
5.) Focus:
Image Formation by Lenses
