Mirrors and Reflection
Reflection is the phenomenon in which light incident on an object bounces back into the same medium through which it was propagating while incident on the object, following some laws. The path along which light propagates is called a light ray. The light ray that strikes an object is called the incident ray. The ray that bounces back from the object into the same medium is called the reflected ray. Types of 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. Diffused 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. 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. Mirror A smooth plane surface that reflects the entire light incident on it is called a plane mirror. Light incident on an object is reflected by that object. That reflected light is incident on the mirror, and is reflected to our eyes. It is because of this reflection that we see images in a plane mirror. An image is the likeness of an object carried and formed by light in a mirror. In other words, your image is a reflection of yourself  in the mirror. In ancient times, people mostly used pools of still water, or water collected in a vessel as mirrors. The first man-made mirrors were in the form of a polished stone made by using obsidian, a naturally occurring volcanic glass.  Mirrors change the direction of light incident on them. The image in a plane mirror is of the same size and colour as that of the object. Moreover, the distance between the image and the mirror is the same as the distance between the mirror and the object. A periscope is an instrument used for observing objects from a hidden position. This instrument is mostly used in submarines, and works on the concept of reflection in plane mirrors. Using mirrors, it enables people in a submarine to view objects on the surface of the water. A glass that acts as a mirror on one side and plain glass on the other is called a two-way mirror. A two-way mirror is used when you want to observe people without letting them know that they are being watched, for example, by police or in psychological institutions. Differences between Images Formed by A Pinhole Camera and A Plane Mirror Images formed by a pinhole camera are entirely different from the images formed by a mirror. The image in a plane mirror is virtual, erect and is of the same size as the object, whereas the image formed by a pinhole camera is real, inverted and is smaller in size when compared to the object. Moreover, the distance between the image and the mirror is the same as the distance between the mirror and the object whereas the distance between the object and the pinhole is not the same as the distance between the pinhole and the image. Comparison of Shadow and Mirror Image The similarity between a shadow and an image is that neither of them can exist without light. However, there are some differences between shadows and images. Shadows are black, whereas images are colourful. Shadows of objects placed in the sun change in length with the position of the sun through the day. Your shadow will be at its longest at dawn and dusk, whereas it will be at its shortest at noon. An image in a plane mirror doesn’t change in size at all; it is always of the same size as that of the object. An image in a mirror can be seen without a screen, whereas it is mandatory to have a screen to form a shadow.

#### Summary

Reflection is the phenomenon in which light incident on an object bounces back into the same medium through which it was propagating while incident on the object, following some laws. The path along which light propagates is called a light ray. The light ray that strikes an object is called the incident ray. The ray that bounces back from the object into the same medium is called the reflected ray. Types of 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. Diffused 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. 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. Mirror A smooth plane surface that reflects the entire light incident on it is called a plane mirror. Light incident on an object is reflected by that object. That reflected light is incident on the mirror, and is reflected to our eyes. It is because of this reflection that we see images in a plane mirror. An image is the likeness of an object carried and formed by light in a mirror. In other words, your image is a reflection of yourself  in the mirror. In ancient times, people mostly used pools of still water, or water collected in a vessel as mirrors. The first man-made mirrors were in the form of a polished stone made by using obsidian, a naturally occurring volcanic glass.  Mirrors change the direction of light incident on them. The image in a plane mirror is of the same size and colour as that of the object. Moreover, the distance between the image and the mirror is the same as the distance between the mirror and the object. A periscope is an instrument used for observing objects from a hidden position. This instrument is mostly used in submarines, and works on the concept of reflection in plane mirrors. Using mirrors, it enables people in a submarine to view objects on the surface of the water. A glass that acts as a mirror on one side and plain glass on the other is called a two-way mirror. A two-way mirror is used when you want to observe people without letting them know that they are being watched, for example, by police or in psychological institutions. Differences between Images Formed by A Pinhole Camera and A Plane Mirror Images formed by a pinhole camera are entirely different from the images formed by a mirror. The image in a plane mirror is virtual, erect and is of the same size as the object, whereas the image formed by a pinhole camera is real, inverted and is smaller in size when compared to the object. Moreover, the distance between the image and the mirror is the same as the distance between the mirror and the object whereas the distance between the object and the pinhole is not the same as the distance between the pinhole and the image. Comparison of Shadow and Mirror Image The similarity between a shadow and an image is that neither of them can exist without light. However, there are some differences between shadows and images. Shadows are black, whereas images are colourful. Shadows of objects placed in the sun change in length with the position of the sun through the day. Your shadow will be at its longest at dawn and dusk, whereas it will be at its shortest at noon. An image in a plane mirror doesn’t change in size at all; it is always of the same size as that of the object. An image in a mirror can be seen without a screen, whereas it is mandatory to have a screen to form a shadow.

#### Activities

 Activity 1 Sciencekids has deveoped an interactive online activity in which one can choose and drag a plane mirror in the path of light to illuminate various objects. By changing the position of the mirror the angle of incidence and hence the angle of reflection can be changed. Two mirrors can be used simultaneously too. Go to Activity Activity 2 Pearson.com.au has created an interative simulation in which one can change the position of the light source or the angle of the mirror to see how the angle of reflection changes. This simulation gives a virtual experimental set up to verify the law of reflection for different angles of incidence. One can see that the law is valid for all colours of light. Also there is a fun game to test the understanding of reflection of light. Go to Activity

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