]]>
LearnNext
Get a free home demo of LearnNext

Available for CBSE, ICSE and State Board syllabus.
Call our LearnNext Expert on 1800 419 1234 (tollfree)
OR submit details below for a call back

clear

Reflection

4,56,046 Views
Have a doubt? Clear it now.
live_help Have a doubt, Ask our Expert Ask Now
format_list_bulleted Take this Lesson Test Start Test

Reflection - Lesson Summary



Light

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.



Comments(0)

Feel the LearnNext Experience on App

Download app, watch sample animated video lessons and get a free trial.

Desktop Download Now
Tablet
Mobile
Try LearnNext at home

Get a free home demo. Book an appointment now!

GET DEMO AT HOME