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CHAPTER: 1st Class: Tenth
LIGHT: Light is a form of energy which produces in us the sensation of sight. Light travels in a straight
line and has dual nature i.e. it is emitted or absorbed as a particle but it propagates as a wave. Light enables us to see objects from which it comes or from which it is reflected. We detect light with our eyes.
Source of light: A source of light is an object from which light is given out. There are two types of
sources of light: (1) Self-luminous: These sources of light possess light of their own e.g. Sun, stars, electric lamps, candle etc. (2) Non luminous: These sources of light do not possess light of their own, but they receive light from an external source and scatter it to the surroundings. e.g. Book, chair, bus, moon etc.
Medium: A medium is a substance through which light travels or tries to do so. The mediums are of
three types: (1) Transparent: It is the medium through which light passes easily e.g. air, water, glass etc. (2) Translucent: It is a medium through which light passes partially e.g. paper, grounded glass etc. (3) Opaque: It is a medium through which light cannot travel e.g. wood, wall, metal etc. RAY OF LIGHT: Straight line drawn in the direction of propagation of light is termed as ray of light. It is represented by an arrow head on the straight line (→). BEAM OF LIGHT: A number of rays combined together form a beam of light.
PENCIL OF LIGHT: A narrow beam of light is called pencil of light. It is of three types:
(1) Convergent pencil: It is that pencil of light in which rays of light go to meet at a particular point. The width of the pencil goes on decreasing as the rays move forward. ( 2 ) Divergent pencil: It is that pencil of light in which the rays of light come out of a point source. The width of the pencil goes on increasing as the move forward. ( 3 ) Parallel pencil: It is that pencil of light in which all the rays move parallel to one another. The width of the pencil remains the same.
CHAPTER: 1st Class: Tenth
Nature of Light: There are two theories about the nature of light: wave theory of light and particle
theory of light. According to wave theory: Light consists of electromagnetic waves which do not require a material medium like solid, liquid or gas for their propagation. The wavelength of visible light waves is very small (being only about 4 × 10 –^7 m to 8 × 10 –^7 m). The speed of light waves is very high (being about 3 × 10^8 meters per second in vacuum). According to particle theory: Light is composed of particles which travel in straight lines at a very high speed. These elementary particles of light are called the ‘photons’. Some of the phenomena of light can be explained only if light is considered to be made up of waves whereas others can be explained only if light is thought to be made up of particles. For example, the phenomena of diffraction, bending of light around the corners of tiny objects, interference and polarization of light can only be explained if light is considered to be of wave nature. On the other hand, the phenomena of reflection, refraction of light, and casting of shadows of objects by light, can be explained only if light is thought to be made of particles. Thus, there is evidence for the wave nature of light as well as for particle nature of light.
Real Images and Virtual Images:
Real images: The image which can be obtained on a screen is called a real image. The images of actors or actresses formed on a cinema screen are example of real images. A real image is formed when light rays coming from an object actually meet at a point after reflection from a mirror or refraction through a lens. Real images can be formed by a concave mirror or a convex lens and are always inverted.. Virtual image: The image which cannot be obtained on a screen is called a virtual image. A virtual image can be seen only by looking into a mirror or a lens. The image of our face formed by a plane mirror is a virtual image. Virtual images are also called unreal images. A virtual image is just an illusion. A virtual image is formed when light rays coming from an object only appear to meet at a point when produced backwards but do not actually meet after reflection from a mirror or refraction through a lens. A plane mirror and a convex mirror always form virtual images. A concave mirror can form a virtual image only when the object is kept within its focus.
REFLECTION AND ITS LAWS:
Reflection of light: It is the process of sending back or bouncing back of light in the same medium on striking the surface any object. Reflection is of two types: (1) Regular reflection: When the reflecting surface is smooth and well-polished, the parallel rays falling on it are reflected parallel to one another. This is called regular reflection. A plane mirror produces regular reflection of light. Images are formed by regular reflection of light. Some more examples of regular
CHAPTER: 1st Class: Tenth
MIRROR: A smooth and well-polished surface which reflects most of the light falling on it is called a
mirror. They are of two main types: (1) Plane mirror: A highly polished plane (flat) surface which reflects almost all the light rays falling on it is known plane mirror. In plane mirror opposite sides are parallel to each other. In a plane mirror, the image of a real object is always virtual, erect, of the same size as that of object and laterally inverted. Characteristics of image formed by a plane mirror: (1) The image of a real object is always virtual i.e. the image cannot be taken on the screen. (2) The image formed is always erect. (3) The size of the image in a plane mirror is always the same as the size of the object. (4) The image is formed as far behind the mirror as the object is in front of the mirror. (5) The image formed in a plane mirror is laterally inverted i.e. the left side of the object becomes the right side of image and vice versa. (2) Spherical Mirrors: A spherical mirror is that mirror whose reflecting surface is part of a hollow sphere of the glass. One side of this mirror is well polished and reflecting and the other side of the mirror is opaque and often painted. Spherical mirrors are of two types: (1) Concave mirror: A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface i.e., bent-in surface. The other surface of concave surface is opaque and non- reflecting. (2) Convex mirror: A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface i.e. bulging out surface. The other surface of the convex mirror is opaque and non- reflecting.
CHAPTER: 1st Class: Tenth DEFINITIONS RELATED TO SPHERICAL MIRRORS: (1) Centre of curvature : It is the centre of the hollow sphere of the glass of which the spherical mirror is a part. It is represented by the letter “C”. It is not the part of the mirror. (2) Pole of mirror: The middle point or the centre of the spherical mirror is called the pole of the mirror. It is not the centre of the sphere. It is represented by letter “p”. (3) Radius of curvature: the radius of hollow sphere of glass of which spherical mirror is a part is called radius of curvature. It is denoted by “R”. It is the distance between the centre of curvature and the pole of the spherical mirror. (4) Principal axis: The principal axis of a spherical mirror is the straight line passing through the centre of curvature “c” and the pole of the spherical mirror “p” produced on both sides of this mirror. (5) Aperture: It is the diameter of the reflecting area of the mirror. It is also called linear aperture of the mirror. (6) Principal focus: The principal focus is a point where a beam of light initially parallel to the principal axis converge to a point after reflection or appear to diverge after reflection from the mirror. It is denoted by “F”.
CHAPTER: 1st Class: Tenth (2) Object is taken on the left side of the mirror which means that light from the object falls on the mirror from the left hand side. (3) All the distances parallel to principal axis are measured from the pole of the mirror. ( 4) 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. (5) Distances measured upwards and perpendiculars to the principal axis are taken as positive. (6) Distances measured downwards and perpendiculars to the principal are taken as negative. RULES FOR TRACING IMAGES FORMED BY CONCAVE MIRROR: Rule(1): A ray of light falling on a concave mirror in a direction parallel to the principal axis, after reflection passes actually through the principal focus of the mirror. Figure28. Rule ( 2 ): A ray of light incident on a concave mirror on passing through centre of curvature of the mirror, retraces its path after reflection. Figure 29 Rule (3): A ray of light incident on a concave mirror on passing through the focus of the mirror after reflection becomes parallel to the principal axis. Figure 30. Rule (4): A ray of light incident obliquely towards the pole of the Concave mirror is reflected obliquely as per the law of reflection. i.e. ∠𝒊 = ∠ r Figure31.
CHAPTER: 1st Class: Tenth IMAGE FORMATION BY A CONCAVE MIRROR IN DIFFERENT POSITIONS OF OBJECT: Case (1) When object is at infinity: When object is at infinity i.e. at a very large distance from the concave mirror, the image is formed at the focus. Two parallel rays incident on a concave mirror gets reflected and both Meet at a point of principal focus. Thus image is formed at focus. The image so formed is real, inverted and much smaller than the object.
CHAPTER: 1st Class: Tenth Case (4) When object is placed between centre of curvature and focus: When object is placed between centre of curvature and focus, image is formed beyond centre of curvature. The image so formed is real, inverted and magnified. Two incident rays one parallel to the principal axis and another passing through centre of curvature, after reflection meet at point beyond centre of curvature. Thus image is formed beyond the centre of curvature. Case (5) When object is placed at focus: When object is placed at focus, image is formed at infinity. The image so formed is real, inverted and highly magnified. Two incident rays, one parallel to the principal axis and another passing through centre of curvature after reflection meet at infinity.
CHAPTER: 1st Class: Tenth Case (6) When object is placed between focus and pole of concave mirror: When object is held between focus and pole of the concave mirror, the image is formed behind the mirror. The image so formed is virtual, erect and magnified. Two incident rays, one parallel to the principal axis and another passing through the centre of curvature after reflection appear to meet at a point which is behind the mirror. RULES FOR IMAGE FORMATION BY CONVEX MIRROR: Rule (1): A ray of light falling on the convex mirror in a direction parallel to principal axis of a convex mirror after reflection appears to be coming from its focus. Rule (2): A ray of light falling on the convex mirror appearing to pass through the focus of the mirror, after reflection becomes parallel to principal axis of the convex mirror.
CHAPTER: 1st Class: Tenth IMAGE FORMATION BY A CONVEX MIRROR: There are two ways for image formation through a convex mirror, which are as: Case (1) When the object is at infinity: Two rays parallel to each other and inclined to the principal axis of the convex mirror coming from the object gets reflected at pole of the mirror and at another point on the convex mirror. The two reflected rays when produced backwards intersect at a point near focus (F).The image so formed is virtual, erect, behind the mirror and highly diminished. OR Case (2): When the object lies anywhere between pole of mirror and infinity: Two incident rays coming from the object, one parallel to the principal axis gets reflected and appear to pass through focus. Another ray of light appearing to pass through the centre of curvature after reflection retraces its path. The two reflected rays intersect each other at a point which lies between pole and focus of the convex mirror. The image so formed is erect, virtual, behind the mirror and diminished.
CHAPTER: 1st Class: Tenth
Mirror formula: The relation between the distance of the object ‘u’ from the pole of the mirror ,
distance of image “v” from the pole of the mirror and focal length “f” of the mirror is called mirror formula. It is written as: 𝟏 𝒇 =
+
If values of any two of these with signs are known, the third would be calculated easily. We know that the radius of the curvature (R) of a spherical mirror is equal to twice the focal length. i.e. R = 2f or f =
+
=
=
It is used for both convex and concave mirrors. It is used to locate position and nature of images formed by spherical mirrors. EXPLAIN LINEAR MAGNIFICATION PRODUCED BY THE SPHERICAL MIRRORS: The linear magnification produced by a spherical mirror (concave or convex) is defined as the ratio of height of the image (h 2 ) to the height of the object (h 1 ). The size of the image depends on the position of the object. Images can be bigger, smaller or of the same size as that of object. The relative size of the image to that of object is called linear magnification. It is denoted by m. Linear magnification = Image height(Image size) object height(object size) m = ℎ 2 𝒉𝟏 If m is greater than 1, the image formed is magnified or enlarged than the object. If m is equal to 1, the image formed is of the same size as that of object, while as if m is less than 1, the image formed is smaller or diminished than the object. When the image is real, it is inverted i.e. the image lies below the principal axis .Therefore height of image is taken as negative.
Hence m =
𝒉𝟐 𝒉𝟏
= Negative
CHAPTER: 1st Class: Tenth (3) It is used by dentists to observe large images of the teeth of patients. (4) It is used as doctor’s head mirror to focus light on body parts like eyes, ears, nose, throat etc. (5) It is also used in reflecting type telescopes. (B) Convex mirror: (1) It is used as a reflector in street lamps. (2) It is used in automobiles like cars, trucks etc. (3) Convex mirrors have a wider field of view as they are curved out-words.
Medium: Optical medium is a transparent material medium through which light travels e.g., air, glass
water etc. Medium is of two types: Rare medium: A rare medium is the optical medium in which light travels fast e.g., air. Denser medium: It is the optical medium in which light travels slow e.g., water, glass etc.
REFRACTION: When light travels from one medium to another medium, it changes its direction. This
change in direction of light or bending of light when it goes from one medium to another medium is called refraction. This refraction takes place at the boundary of the two media. This refraction takes place due to the change in the speed of light in different mediums. When light travels from rarer into denser medium, it bends towards the normal and when it travels from denser to rarer medium, it bends away from the normal.
CHAPTER: 1st Class: Tenth
LAWS OF REFRACTION:
The refraction of light in going from one medium to another takes place according to two laws which are known as the laws of refraction, which are: (1) First Law: The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane. (2) Second Law: When a light goes from one medium to another medium, the velocity (speed) and wave length of light changes but the frequency of the light does not change. (3) The ratio of sin of angle of incidence to the ratio of sin of angle of refraction for the pair of given media is constant. This law was given by Snell in 1621 and is known as Snell’s law of refraction. Thus ratio of sin of angle of incidence to the sin of angle of refraction is called refractive index of that medium and is denoted by η (Neeta). Refractive index η = 𝐒𝐢𝐧∠𝐢 𝐒𝐢𝐧∠𝐫
= constant
Suppose the angle of incidence (i) for a ray in air is 37^0 and the angle of refraction(r) in glass be 24^0. Thus Then, η =
= 1.50 (sin 370 = 0.60 and sin 24^0 = 0.40)
CHAPTER: 1st Class: Tenth
Relative refractive index: When light travels from one medium say 1 to another medium say 2, the
refractive index of medium 2 with respect to medium 1 is written as 1η 2 and is called relative refractive index i.e. relative refractive index is the ratio of speed of light without vacuum or air as any medium. Refractive index of medium 2 with respect to medium 1 = 1η 2 = 𝑽𝟏 𝑽𝟐
Refractive index of medium 1 with respect to medium 2 = 2η 1 = 𝑽𝟐 𝑽𝟏
Multiply ( 1 ) and ( 2 ), we get 1η 2 x 2η 1 = 𝑽𝟏 𝑽𝟐 x 𝑽𝟐 𝑽𝟏
1η 2 X 2η 1 = 1 Or 1η 2 = 𝟏 𝟐𝜼𝟏
REFRACTION TROUGH A GLASS SLAB: Take a rectangular glass slab KLMN. A ray of
light falls on it on the side KL. As the ray goes from rarer medium (air) into denser medium (glass), it bends towards normal and goes along BC. According to Snell’s law Sin i Sin r =
At point c ray of light leaves the glass slab and entering into air i.e. moves from denser medium to rarer and bends away from normal. Hence first refraction occurs at point B and 2nd^ refraction occurs at C .the ray of light which comes out from the glass slab is called emergent ray ,the ray which enters glass slab is called refracted ray and the ray which falls on glass slab is called incident ray .The angle which the incident ray makes with the normal is called angle of incidence, the angle which the refracted ray makes with the normal is called angle of refraction and the angle which the emergent ray makes with the normal is called angle of emergence. Here angle of incidence is equal to the angle of emergence .This shows that the incident ray is parallel to the emergent ray. The perpendicular distance between the original path of incident ray and the emergent ray coming out of the glass slab is called lateral displacement of the emergent ray of light. Lateral displacement is directly proportional to (1) angle of incidence (2) thickness of glass slab and (3) refractive index of the glass slab. CONDITIONS FOR NO REFRACTION: From Snell’s law of refraction there are two such situations where refraction does not occur:
CHAPTER: 1st Class: Tenth (1) When light is incident normally on a boundary (i.e. from medium 1to medium 2), the angle of incidence here is equal to 0^0. (2) When the refractive index of two mediums are equal i.e. 𝝻 1 = 𝝻2. Then according to Snell’s law the ray passes without deviation when it travels from one medium to another medium of same refractive index i.e. no refraction occurs. *-
SPHERICAL LENSE:
A spherical lens is a transparent refracting medium having either one or two curved surfaces. Spherical lenses are of two types: (1) Convex lens or converging lens: The lens which is thick at the centre and thin near the edges is known as convex lens. It is also called converging lens because it converges the rays falling on it. (2)Concave lens or diverging lens: The lens which is thin at the centre and thick near edges is called concave lens. It is also called diverging lens because it diverges the rays of light falling on it.
