Reflection: mirror, mirror on the wall
Reflection of light is how we ‘see’ things at all, regardless of color. Light waves from the sun--which, as it emanates its own light, is known as a luminous source—are reflected by all the objects in their path. All objects, that is, except those that allow light to pass through them, like glass, for example. These objects are called transparent, but the vast majority of matter obstructs light waves, and is called opaque. As these objects obstruct the path of light, it is forced to deviate, i.e. it is forced to change its path as it bounces of the obstruction. This “bouncing off” is known as reflection, and when light bounces off opaque objects and enters our eyes, which are then able to “see” things once the light sensed is processed and analyzed by the brain.
For this demonstration of reflection of light, you will need a laser pointer, a room with no windows with a mirror on the wall and any type of spraying device (ask mom for some room freshener or window cleaner or use deodorant!). Turn off the lights in the rooms. It is very important to make sure no light enters the room from outside. Once it is nice and dark and creepy, take the pointer in your hand and point it towards the mirror.
You will find that you can only see the red dot of the pointer at two places—where it hits the mirror, and where it is reflected onto the wall. Note that these are the two opaque objects the light has encountered.
Now equip yourself with the spray can and spray away into the darkness, keeping the laser pointed at the mirror. What do you see?
Magic? Not at all. You have just proved that sight depends on the reflection of light. The spray in the air obstructed the light from the laser, enabling you to see the path of the beam. This is exactly how we see our surroundings! The wall behind you is obstructing sun rays and reflecting them back towards your eyes, and eureka! You can see the wall! (Although that may not seem such a brilliant achievement on your resume)
Reflection, however, does follow a set of principles, which explains why you and I when standing in the same position see exactly the same image (given that our eye lenses are properly functional!). It is easiest to demonstrate these laws using a plane mirror, as it is a leveled, flat surface.
There are a few terms we use to describe reflection of light in optical physics. To make things simpler to understand, we choose to demonstrate the reflection of light by a single light ray, although there are thousands in reality. This ray of light which falls on a surface (in this case, the mirror) is known as the incident ray. At the point where the incident ray hits the surface, a perpendicular line to the surface is drawn, which we call the normal. The angle formed between the incident ray and the normal is called the angle of incidence. When the incident ray of light hits the mirror, it will be reflected in a specific direction. The resultant ray is known as the reflected ray. The angle that this particular ray forms with the normal is known as the angle of reflection.
Now that we’ve enhanced our vocabularies, let’s get down to the hands on part—we’re now going to try and prove the law of reflection.
You’ll need a few supplies for this experiment. First, of course, a plane mirror, then a black surface, a dark room, a lamp, a few pins and a piece of black paper with a white colored pencil to write on it. First take the mirror and place it on top of the black paper, near the edge. You may need a stand for the mirror if it is not thick enough to support itself. With the white pencil, mark the position of the mirror carefully.
Set up the lamp opposite the mirror, and use thick paper to create a shield around the lamp. Cut a slit in the shield so that only a thin line of light escapes. Now turn off the lights and turn on the lamp. Carefully mark the incident ray’s path with two pins on the black paper, and do the same for the reflected ray. Then turn on the lights, remove the mirror and the lamp and use the white pencil to connect the pins so that two lines represent the incident and reflected ray. At the point where the incident ray intersects the line marking the position of the mirror, draw a perpendicular with a dotted line. This is your normal. Identify the angle of incidence and the angle of reflection, and measure them with a protractor. What do you find?
Precisely. The angle of incidence is always of the same magnitude as the angle of reflection. This is the law of reflection, and at each point where light is obstructed and reflected, this law is obeyed.
For our convenience here, we have used a flat, regular surface. However, most objects are not regular, so how do we see them?
If you experiment further, you will find that when using a plane mirror, all the rays of incidence are parallel to each other, as are the rays of reflection. However, when an irregular object is used, irregular or diffuse reflection takes place, where the reflected rays are not parallel to each other. But you will still find that at each point, the law of reflection is obeyed, although the reflected rays are irregular.
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