we will now study some properties of light.a ray of light is an idealization in reality . just like a 'line' in geometry which is not supposed to have breadth. even the narrowest beam of light that we can get is made up of several rays . for simplicity, we use the term ray for a narrow beam of light.
Activity - to study the first law of reflection
fix a sheet of white paper on a table . cut out a thin slit about 1mm wide , in a piece of cardboard . make the cardboard peice stand vertically on the white sheet using supports . darken the room . shine a tourch just on the sheet of paper . alternately you can use a laser torch that gives a thin beam of red light .
keep mirror vertically in the path of light . the light falling on the mirror is called the incident ray. you can clearly see the path this ray takes after being reflected by the mirror . afterreflection it is called the reflected ray . if you rotate yhe mirror , this reflected ray also rotates. mark the position of the mirror MN with a straight line. using a scale and a pencil , mark a path of the incident ray AO and the reflected ray OB . measure the angles that the incident and reflected rays make with the mirror (angle MOA and angle NOB,respectively).
you will find that these angles are equal . this shows that the reflected ray leaves the mirror at the same angle at which the incident ray strikes it.
first law of reflection
now , let us consider two other angles . at O,called the point of incidence , draw a perpendicular to the mirror , OX . this is called the normal at the point of incidence . we call angle AOX the angle of incidence and angle BOX the angle of reflection . measure angle AOX and angle BOX are equal .
therefore , angle of incidence =angle of reflection or angle i = angle r
this is the first law of reflection.
second law of reflection
there is another law of reflection which firmly fixes the reflected ray. once you have drawn the incident ray and the normal., there is only one palne that can pass throw both of these. In this case it si the plane of the paper
in activity 1 , use a bigger sheet of paper and extend it beyond the edge of the table . fold the paper downwards at the edge of the table so that it hangs down . does the reflected ray reach the edge of the paper beyond the table? now straighten the paper so that it is again in the plane of the table. does the reflected ray now reach the edge of the paper? this shows that the reflected ray is also in the plane of the paper.
thus,the second law of reflection states that the incident ray , the normal and the reflected ray all lie in the same plane.
the above activity is , however, too rough to give exact values.in the laboratory, we use pins to mark the incident and reflected rays to get more accurate results , as explained below.
activity 2 - to verify the seconf law of the reflection experimentally
fix a sheet of white paper on a table . draw a line MN and keep the mirror vertical exactly along this line .
draw an incident ray XO which meets the mirror at an angle (say 45degree). fix two pins A and B , about 5cm apart,on this ray .
now look at the reflections of A and B and fix a pin C in such a way that pin C and the reflections of pins A and B appear to be in a straight line. fix another pin D after C, but nearer your eye , such that D,C and the images of A and B appear to be ina straight line. then CD is the reflected ray.
remove the pins and the mirror . join DC and extend it to meet the mirror line exactly at O . draw the normal OT at O, measure angle AOT and angle DOT. you will find that these angles are equal.
if you carry out the activity for different positions of the incident ray , you will find that all cases,
angle of reflection = angle of incidence aslo, the incident ray , the reflected ray and the normal all lie in one plane , that is , on the plane of the paper. this experiment verifies the two laws of reflection.
lets us summarise the definitions os terms you have learnt so far,
* the ray of light from the source that strikes a given surface is called the incident ray.
*the point at which the incident ray strikes the surface is called the point of incidence.
* the perpendicular to the surface at the point of incidence is called the normal.
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