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Reflection of light


Reflection is one of the basic phenomena of light.

 

Example

When will the reflection of light occur?

Solution

A highly polished surface, such as a mirror, reflects most of the light falling on it. When light falls on the surface of an object, it may be absorbed, transmitted, or reflected. If an object absorbs all the light, which falls on it, it will appear perfectly black. For example, a blackboard appears black because most of the light falling on the blackboard is absorbed and hence, very little light is reflected from it. If an object transmits the light, that is, if the object allows the light to pass through itself, the object is said to be transparent.



 

Most of the objects, however, send back some of the light, which falls on their surface.
 

Example :  polished mirror sends back more than 95 percent of the light falling on it. The process of sending back the light rays falling on the surface of an object is called reflection of light.


Silver metal is one of the best reflectors of light. A polished block of silver metal reflects almost all the light falling on it and does not transmit or absorb any of the light rays. But the smooth and polished surface of the silver metal can get easily scratched and may turn rough. In order to avoid this, ordinary mirrors are made by coating a thin layer of silver metal or mercury on one side of a plane glass sheet. The silver/mercury layer is protected by applying a coat of red paint over it. We use plane mirrors to see what we look like and adore ourselves. We need to study a set of laws to understand the reflection of light.


Laws of reflection

 

The angle of incidence is equal to the angle of reflection


Incident Ray

Take a plane mirror MM’. The ray of light, which falls on the mirror surface, is called the incident ray. In the figure, AO is the incident ray of light. The incident ray gives the direction in which light falls on the mirror. The point at which the incident ray falls on the mirror is called point of incidence. In the same figure, the point O on the surface of the mirror is the point of incidence.


Reflected Ray
We know that when a ray of light falls on a mirror, the mirror sends it back in another direction. When this happens, we say that the mirror has reflected the ray of light. The ray of light, which is sent back by the mirror, is called the reflected ray. In the same figure, OB is the reflected ray of light. The reflected ray of light shows the direction in which the light travels after reflection from the mirror.


Normal
Another term that is commonly used in association with a plane mirror is the ‘normal’. The ‘normal’ is a line drawn at the right angle to the mirror surface at the point of incidence. In other words, ‘normal’ is a line which is perpendicular to the mirror at the point of incidence. In the figure, the mirror is MM’ and the point of incidence is O. So the line ON is the normal to the mirror surface at point O. The normal has been represented by a dotted line to distinguish it from the incident ray and the reflected ray. Note that ‘normal’ is just a ‘perpendicular line’ to the mirror and it should not be called the ‘normal ray’ like the incident ray or reflected ray.


Angle of Incidence and Angle of Reflection

Let us now discuss the angle of incidence and the angle of reflection. The angle of incidence is the angle made by the incident ray with the normal at the point of incidence. In other words, the angle between the incident ray and normal is called the angle of incidence. In the figure, the incident ray is AO and the normal is ON, so the angle AON is the angle of incidence. The angle of incidence is denoted by the letter i.

 

The angle of reflection is the angle made by the reflected ray with the normal at the point of incidence. In other words, the angle between the reflected ray and normal is called the angle of reflection. In the figure, the reflected ray is OB and the normal is ON, so the angle NOB is the angle of reflection. The angle of reflection is denoted by the letter r.
 

The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.


These laws are applicable to all types of reflections including the reflection from spherical surfaces.

 

Image formed by plane mirror is always virtual and erect. The size of image is equal to that of the object. The image formed is as far behind the mirror as the object is in front of it. Further, the image is laterally inverted.

 




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