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Reflection Light Light - Reflection and Refraction Light is the form of energy, which enables us to interact with our surroundings in a most effective way. Light causes the sensation of vision. There are
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Reflection Light Light - Reflection and Refraction Light is the form of energy, which enables us to interact with our surroundings in a most effective way. Light causes the sensation of vision. There are two major phenomena of light that takes place in the process of seeing. They are reflection and refraction. In general reflection is the process where the incident light on an object is bounced back into the same medium. Mirror Most of all the objects reflect the light incident on them to different extent. Some of the objects which have a smooth surface reflect the incident light to the maximum extent. An object that reflects 100% of the incident light is called a mirror. If the surface of the mirror is plane, it is referred to as a plane mirror. Other types of mirrors are curved mirrors. We find our images in proper proportions in plane mirrors due to regular reflection. We cannot observe our images formed by plane mirrors on a screen as they are virtual images unlike those which are formed on a screen and termed as real images. The reflection that takes place on other surfaces other than plane mirrors is irregular reflection. Irrespective of the type of reflection, the light ray (which is the path of light) follows two laws of reflection. Laws of Reflection The first law of reflection states that the angle of incidence (i.e., the angle between the incident ray and the normal at the point of incidence) is equal to the angle of reflection (i.e., the angle between the normal and the reflected ray). The second law of reflection states that the incident ray, reflected ray and the corresponding normal, all lie in the same plane. Rear View Mirror The rear view mirror in a car is a plane mirror as it helps us to estimate the distances of the vehicles that are behind our car in motion. In a plane mirror, the object distance is equal to the image distance. If we stand at any distance in front of one whose length is half your height we can observe our full size image in a plane mirror. If we rotate a plane mirror maintaining the object position, through certain angle we find that the reflected ray rotates twice the angle. Regular and Diffuse Reflection The phenomenon of bouncing back of light after falling on the surfaces of the objects is called reflection of light. The reflection of light is of of two types, they are 1. Regular reflection or Specular reflection 2. Diffused or irregular reflection. In general, reflection is the process where the light incident on an object bounces back into the same medium. This happens when light is incident on a translucent or an opaque medium. When light is incident on a transparent medium, all the incident light passes through the medium, and reflection does not take place. In the case of translucent medium, a part of the incident light is reflected, and the rest is transmitted through the medium. We get light from a luminous object, which we refer to as a source of light. If the size of the source of light is very small, then we call it a point source of light. If the size of the source of light is considerable, then we say it is an extended source of light. Light rays from a point source of light travel in all directions, moving away with time. Such a beam of light is called a divergent beam of light. If the light source is an extended source, then we get a parallel beam of light from it. Consider a parallel beam of light from an extended source, incident on a plane surface like a plane mirror. As the beam of light is parallel, and the surface on which the beam is incident is a plane surface, the angle made by each ray with the normal at the point of incidence on the surface is equal, which implies that the angle of incidence of all the rays is equal. Each ray of light follows the laws of reflection irrespective of whether it is from a parallel beam or not. According to the laws of reflection, the angle of reflection is equal to the angle of incidence. Thus, for every ray of light incident on the mirror, the angle of reflection is equal to its angle of incidence. Regular Reflection As the angles of incidence of a parallel beam of t all the ligh rays are equal for a smooth plane like a plane mirror,the angles of reflection of all the rays equal. This implies that all the reflected rays are parallel. When all the reflected rays, reflected from a given surface, are parallel then it is called regular reflection. Diffuse Reflection If the surface is not a plane surface, then the reflected rays are not parallel to each other. In such a case, the reflection is called diffused reflection. The diffused reflection occurs at the rough or un polished or the slightly polished non smooth or rough surfaces. Figure above represents the light falling over the rough surface. As the rays of light falls on a rough surface at any angle of incidence then the angle of reflection is equal to the angle of incidence. Note: Laws of reflection are valid for both regular and irregular reflections of liht. Spherical Mirrors Terms Associated with Spherical Mirrors Centre of curvature (C) is the centre of the sphere, of which the mirror is a part. Radius of curvature (R) is the radius of the sphere, of which the mirror is a part. Pole (P) is the geometric centre of the spherical mirror. Principal axis is the line joining the pole and the centre of curvature. Principal focus (F) is the point on the principal axis, where a parallel beam of light, parallel to the principal axis after reflection converges in the case of a concave mirror and appears to diverge from in the case of a convex mirror. Focal length (f) is the distance of the principal focus from the pole of the mirror. There are two types of images: real and virtual. Real images are those that can be caught on a screen while virtual images are those that cannot be caught on a screen. Concave Mirror If a part of a hollow glass sphere is cut and the cut part of the sphere is coated outside with silver or similar material, then its inner surface reflects the entire light incident on it, and thus, forms a mirror. Since the inner surface is a concave surface, the mirror so formed is called a concave mirror. Concave mirrors converge the light incident on them and hence are called converging mirrors. We can observe ourselves magnified when the mirror is placed close to our face. This is due the position of the object between the focus and the pole. As the object moves away from the mirror, the size of its image reduces along with its distance from the mirror. If an object is placed close to a concave mirror such that the distance between the mirror and the object is less than its focal length, then a magnified and virtual image is formed. Due to this property, concave mirrors are used as shaving mirrors, and by dentists to view clearly the inner parts of the mouth. Convex Mirror If the cut part of the glass sphere is coated from inside with silver or a similar material, then its outer surface reflects the entire light incident on it, and thus forms a mirror. Since the outer surface is a convex surface, the mirror so formed is called a convex mirror. Convex mirrors diverge the light incident on them and hence they are called the diverging mirrors. Due to this they always form diminished, virtual and erect images irrespective of the position of the object in front of them. Thus, the magnification produced by these mirrors is always less than one. The field of view for a convex mirror is greater than that for a plane mirror, the aperture being the same. Hence, convex mirrors are used as rear-view mirrors in vehicles. It is also installed behind automated teller machines as a security measure. Rules for Construction of Ray Diagrams for Spherical Mirrors Rule 1: A light ray incident parallel to the principal axis, after reflection, either actually passes through the principal focus or appears to pass through the principal focus. Rule 2: A light ray which first passes through the principal focus or appears to pass through the principal focus, after reflection, will travell parallel to the principal axis. Rule 3: A light ray which first passes through the centre of curvature or appears to pass through the centre of curvature, after reflection, retraces its initial path. Image Formation by Concave Mirror Depending on the position of the object in front of the concave mirror, the position, size and the nature of the image varies. 1. Object at infinity: A real, inverted, highly diminished image is formed at the focal point F, in front of the concave mirror. 2. Object beyond C: A real, inverted, diminished image is formed between C and F, in front of the concave mirror. 3. Object at C: A real, inverted, same sized image is formed at C, in front of the concave mirror. 4. Object between C and F: A real, inverted, enlarged image is formed beyond C, in front of the concave mirror. 5. Object at F: A real, inverted, highly enlarged image is formed at infinity, in front of the concave mirror. 6. Object between F and P: A virtual, erect and enlarged image is formed behind the concave mirror. Image Formation by Convex Mirror Irrespective of the position of the object, a virtual, erect and diminished image is formed between F and P, behind the convex mirror. Mirror Formula and Sign Conventions The relation between the focal length (f), object distance (u) and the image distance (v) is given by 1/f = 1/v + 1/u. This is called the mirror formula. All the distances are measured from the pole of the mirror. If we measure the distances in the direction of the incident light, then they are taken positive or else they are taken negative. These constitute the sign conventions. Uses of Concave Mirrors Concave mirrors are used as shaving mirrors to see a larger image of the face. Dentists use concave mirrors to view the back of the tooth. ENT doctors use them for examining the internal parts of the ear, nose and throat. They are used as reflectors in the headlights of vehicles, search lights and in torch lights to produce a strong parallel beam of light. Huge concave mirrors are used to focus sunlight to produce heat in solar furnaces. Uses of Convex Mirrors Used as rear view mirrors in automobiles as it covers wide area behind the driver. Used as reflectors for street light bulbs as it diverges light rays over a wide area. Spherical Mirrors Mirrors are the basic means of viewing our own beauty. Generally we can classify the mirrors into the following two types as i. Plane mirrors ii. Curved mirrors. Generally mirrors refer to plane mirrors. But if the surface of a mirror is curved it is said to be a curved mirror. If the curved mirror is a part of a huge sphere, then the mirror is a spherical mirror. Terms Associated with Spherical Mirrors Centre of curvature (C) is the centre of the sphere, of which the mirror is a part. Radius of curvature (R) is the radius of the sphere, of which the mirror is a part. Pole (P) is the geometric centre of the spherical mirror. Principal axis is the line joining the pole and the centre of curvature. Principal focus (F) is the point on the principal axis, where a parallel beam of light, parallel to the principal axis after reflection converges in the case of a concave mirror and appears to diverge from in the case of a convex mirror. Focal length (f) is the distance of the principal focus from the pole of the mirror. Spherical mirrors can be further classified into the following two types as i. Concave mirrors ii. Convex mirrors. The images formed by the mirrors are of two types they are i. Real images ii. Virtual Images Real images are those that can be caught on a screen while virtual images are those that cannot be caught on a screen. Formation of Images by Spherical Mirrors The image formed by a convex mirror is always erect, virtual, and diminished in size. The location of the object does not affect the characteristics of the image. Thus, as the object approaches the mirror, the image approaches the mirror too but not proportionately. This is why, the rear view mirrors of the cars and bikes are made of convex mirrors. Hence, we have the caution Objects seen in the mirror are closer than they appear printed on the outside rear view mirrors of vehicles. Unlike in a convex mirror, the nature and size of the image in a concave mirror depends on the distance of the object from the mirror. Concave Mirror If a part of a hollow glass sphere is cut and the cut part of the sphere is coated outside with silver or similar material, then its inner surface reflects the entire light incident on it, and thus, forms a mirror. Since the inner surface is a concave surface, the mirror so formed is called a concave mirror. The geometric centre of a concave mirror is called its pole. The centre of the sphere from which concave mirror was cut is called the centre of curvature of the concave mirror. The distance from any point on the concave mirror to its centre of curvature is called the radius of curvature of the concave mirror. An imaginary line passing through the centre of curvature and the pole of the concave mirror is called principal axis of the concave mirror. The area of a concave mirror that is exposed to incident light is called the aperture of the concave mirror. The length along the principal axis from the pole to the principal focus is called the focal length of the concave mirror. If an object is placed close to a concave mirror such that the distance between the mirror and the object is less than its focal length, then a magnified and virtual image is formed. Due to this property, concave mirrors are used in many applications. A concave mirror can be used as a shaving mirror, and by dentists to view clearly the inner parts of the mouth. Concave mirrors converge the light incident on them and hence are called converging mirrors. We can observe ourselves magnified when the mirror is placed close to our face. This is due the position of the object between the focus and the pole. As the object moves away from the mirror, the size of its image reduces along with its distance from the mirror. If an object is placed close to a concave mirror such that the distance between the mirror and the object is less than its focal length, then a magnified and virtual image is formed. Due to this property, concave mirrors are used as shaving mirrors, and by dentists to view clearly the inner parts of the mouth. Reflection by Concave Mirrors Incident Ray Parallel to principal axis Passes through C Passes through focus Strikes the pole at an angle eith principal axis Reflected Ray Passes through focus Retraces its path parallel to principal axis Makes the same angle with principal axis
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