LIGHT

We are able to see things because of light. It enables us to see beautiful landscapes, buildings, stars, moon and everything else around us. How we are able to see these objects? We are able to see these objects when light from these objects enters our eyes. This light may have been emitted by object or reflected by it.

We use mirrors in barber shop. We use mirror while getting ready. Mirrors are used to create illusion.

We use mirror while driving. Mirrors are used in shops.

Based on the nature of the reflecting surface, the mirrors are classified into two categories. Plane mirror (ii) Curved mirror

REFLECTION OF LIGHT AT A PLANE SURFACE

Learning outcome Terms related with reflection. Laws of reflection. Image formation by plane mirror and its characteristics. Uses of plane mirrors.

The return of light into the same medium after striking a surface is called reflection. Depending upon the nature of the polished surface, the reflections are classified into two categories Regular reflection (ii) Irregular reflection

Regular reflection It occurs when a beam of light falls on a smooth surface. Incident rays as well as reflected rays are parallel to each other

Irregular reflection It occurs when a beam of light falls on a rough surface. Incident rays are parallel but reflected rays are not parallel.

Terms related with reflection Incident ray: The light ray striking a reflecting surface is called the incident ray.

Terms related with reflection (ii) Point of incidence: The point at which the incident ray strikes the reflecting surface, is called the point of incidence.

Terms related with reflection Normal: The perpendicular drawn to the surface at the point of incidence, is called the normal.

Terms related with reflection (iv) Reflected ray: The light ray obtained after reflection from the surface, in the same medium in which the incident ray is travelling, is called the reflected ray.

Terms related with reflection Angle of incidence: The angle between the normal and the incident ray is called the angle of incidence. i

Terms related with reflection (vi) Angle of reflection: The angle between the normal and the reflected ray is called the angle of reflection. i r

Terms related with reflection Plane of incidence: The plane containing the incident ray and the normal, is called the plane of incidence.

Terms related with reflection (viii) Plane of reflection: The plane containing the reflected ray and the normal, is called the plane of reflection.

Laws of Reflection The angle of incidence i is equal to the angle of reflection r. The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane.

Image Virtual image Real image Image formation Image Virtual image Real image Virtual Image Real Image 1. The rays of light after reflection or refraction appear to meet at some other point. 1. The rays of light after reflection or refraction actually meet at some other point. 2. It cannot be taken on screen. 2. It can be taken on screen. 3. It is always erect. 3. It is always inverted.

Image of a point object formed by a plane mirror

Distance between the image and mirror is same as the distance between the object and mirror By law of reflection ABN =NBC (i =r) But FBA =ABN (Alternate angle) FAB = i NBC = FA1B (Corresponding angle) FA1B = r = i AFB =A1FB (Perpendicular angle) And FB is common side. N F

The triangle AFB and A1FB are congruent. Hence AF= A1F Thus the perpendicular distance of image from mirror is equal to perpendicular distance of object from mirror. N F

Image of an extended object formed by a plane mirror

Lateral inversion The interchange of the left and right sides in the image of an object in a plane mirror is called the lateral inversion.

Lateral inversion The letters on the front of an ambulance are written laterally inverted like   because the driver of the vehicle moving on road ahead of the ambulance reads these words laterally inverted as AMBULANCE in his rear view mirror and so he gives side to pass the ambulance first.

Characteristics of the image formed by a plane mirror Erect Virtual Of same size as the object, and Laterally inverted In addition to this, The image distance from the mirror is same as the distance of object from the mirror. If the object is displaced through a distance ‘x’ away from the mirror, the image is also displaced by a distance x away from the mirror. If the mirror is moved through a distance ‘x’ towards or away from the object, the image moves a distance ‘2x’ towards or away from the object. If the mirror is moved with a velocity ‘v’ towards or away from the object, the image is appear to move with velocity ‘2v’ toward or away with respect to the object.

Images formed in a pair of mirrors If you stand between two parallel mirrors, you can find infinite no. of images of yours with diminishing clarity.

Images formed in a pair of mirrors When two mirrors are kept inclined at certain angle, they form a number of images for a single object. Actually the image formed by one of the mirrors act as an object for the second mirror.

Images formed in a pair of mirrors The number of images formed, depend on the angle between the two mirrors. If Θ is the angle between the two mirrors, the no. of images formed due to multiple reflection is: or Where n= no. of images formed, which is always odd. If the value of n is even value then no. of images formed is (n-1).

Images formed in a pair of mirrors Eg. If Θ =450 , then = 8 So, the no. of images will be 8-1 = 7. If Θ =400 , then = 9 So, the no. of images is 9.

Images formed in a pair of mirrors placed parallel to each other If pair of mirrors are parallel, then Θ =00. n= = infinity. Here, infinite no. of images formed, but due to absorption of light. Last few images become dim.

Images formed by two mirrors placed perpendicular to each other If mirrors are placed perpendicular to each other (i.e. Θ =900), then = 4. So, no. of images will be 4-1 = 3 

Periscope This is the device to observe objects behind obstacles based on the principle of multiple reflections. It helps us to observe the surrounding objects from lower level.

Periscope Working: Light from the object strikes the upper mirror at an angle of nearly 450 and reflects along the axis of the tube obeying the laws of reflection. The reflected light strikes the second mirror at an angle 450 and reaches the observer. The final image seen by the observer is the same way up as the original object.

Periscope Use: By soldiers to see the movement of the enemy from inside the trenches. By the captain of the submarine to see the movement of the other ships and aircrafts above the water surface.