Now we all know that an object is visible to us only if the light reflected or emitted by it reaches our eye. Let us understand the phenomenon of reflection of light.
In this lesson we will learn:
Have you observed a torchlight in a dark room striking a plane mirror or wall? What happens to light. You will see some of the light rays bounce back. This phenomenon is called reflection of light.
When a light ray strikes the boundary of two media, like air and glass, a part of the light is turned back into the same medium. This is called "Reflection of light". A highly polished surface such as mirror reflects most of the light falling on it.
Consider a light-ray incident on the surface of a plane mirror,
Incident ray is the ray of light falling on the surface.
Reflected ray is the incident ray bouncing back to the same medium after striking the reflected surface.
Point of Incidence, which is 'P' here is the point on the reflected surface where incident ray strikes and reflected ray bounces off.
Normal is the line drawn perpendicular to the reflecting surface at the point on incidence.
Angle of incidence (i) is the angle between the normal and the incident ray.
Angle of reflection (r) is the angle between the normal and the reflected ray.
Laws of reflection of light state that,
Regular Reflection: If the parallel incident rays are reflected in such a way that all the reflected rays are also parallel to each other then such a reflection is called regular reflection or specular reflection. For example, reflection from a polished smooth surface like plane mirror shows regular reflection. The angle of incidence for all the parallel rays of light falling on a smooth surface is the same and the angle of reflection for all the reflected rays of light is also same, therefore the parallel rays of light falling on a smooth surface are reflected as the beam of parallel rays in one direction only. Due to this property mirror, polished metal surface and still water form images. We can also turn the sunlight towards dark places by the regular reflection of light using shiny surface.
Irregular Reflection: When a parallel beam of incident light falls on an irregular or rough surface they are reflected in different directions such a reflection is called an irregular reflection or diffused reflection. The incident rays do not remain parallel after reflection, they are reflected in different directions. Why? The answer to this is, the particles of rough surface are all facing in different directions due to this the angles of incidence for all the parallel rays of light are different and thus the angle of reflection for all rays are also different. For example, rays falling on rough surface like wall and floor. We see non-luminous objects due to diffused reflection. A book lying on the table is seen from all parts of the room due to the diffuse reflection of light from its surface. The surface of the book being rough reflects light in all directions, hence the book is seen from all the parts of the room.
Note: The brightness of an object depends on the intensity of the incident light rays and also on the reflectivity of the object.
Question: Why does a mirror form an image but the wall doesn't?
Answer: In the mirror, the reflective part is very flat, so after reflecting off the mirror the light has the same pattern as before and can form an image. But the wall surface is rough, so the light rays would reflect off in all different directions and get jumbled up in the process. It is the same as seeing your reflection on a smooth sheet of aluminum foil. Crumple up the foil and then try to see your image, it won't be visible anymore.
Now as we understand what is reflection, let us learn more about plane mirrors and how they form images.
Plane mirrors are made by putting a thin layer of silver nitrate or aluminum behind a flat piece of glass. They are mirrors with a flat reflective surface.
The ray diagram below shows how we see an image in a plane mirror. Light rays from the object strike the mirror and reflect according to the law of reflection. When some of the light rays enter our eye, our eye and brain interpret these rays as having traveled in a straight line path. Therefore, our eyes and brain track the light rays backward to a position from which they appear to have come. At this position, we see an image.
The incident rays 1(starting from the tip of the candle) and 2(starting from the candle-end) strike the mirror surface, follow the laws of reflection bounces back and reach the observer's eye. If the reflected rays are extended backwards behind the mirror (see dashed lines 5 and 6), they seem to originate from points A' and B'. By forming images of all points of the object, we obtain an upright image of the object behind the mirror.
The plane mirror forms a virtual image. Here we see that the light rays diverge or spread apart after reflection, therefore when light rays from a source do not cross to form an image. Instead, they can be ‘traced back’ to a point behind the mirror. Virtual images can be seen directly without using a screen for projection. Virtual images are formed behind the mirror where light never reaches. Virtual images are upright images.
It happens on a very simple concept that in a plane mirror the distance of the object from the mirror will be equal to the distance of the image from the mirror so when you have 'IF' written and the image formation happens, the distance of F from the mirror is same as the F formed in the mirror.
The invention of plane mirror is indeed the greatest contribution to mankind. We know now that plane mirrors are primarily used to see the reflection of an object. Some of the uses of plane mirror are:
1. Plane mirrors are used as looking glass.
2. They are used in solar cookers to reflect most of the sunlight to trap and concentrate the power of the sun to cook food.
3. They are also used in constructing periscope which is used in submarines. The plane mirrors used in periscope reflect the image of all the ships that are present on the surface of the water. The below diagram shows the principle on which the periscope is designed.
4. They are also used to make a kaleidoscope, a toy that produces beautiful patterns. Interested to make one for yourself?
5. They are also used in various scientific instruments like microscopes.
6. Used in Automobiles for reflecting a powerful beam of parallel light, vehicles widely use mirrors in their headlights.
7. Used in torchlights - Plane mirrors are used in torchlights and flashlights for reflecting the light beams.
8. Used by dentists to see the images of teeth and examine them.
Experiment for you to try on - Let's see how we can create beautiful images using the reflection of light by a plane mirror by making a kaleidoscope.
Materials Required:
Three small mirrors that are approximately the same size. Thin cardboard. Overhead transparency or plastic page protector, colored glass pieces, Tape.
What to do:
1. Tape the long edges of the mirrors together so that they form a pyramid shape, with the reflecting sides of the mirrors all facing inward.
2. Next, cut out a triangle of thin cardboard to fit one end of the kaleidoscope and tape it on. Use a sharp pencil to poke a hole in the center of the cardboard, to serve as a peephole.
3. Cut two triangles of a transparent substance, like a plastic overhead transparency, to fit the other end; tape up two of the edges to form a three-sided envelope, and put bits of colored glass pieces inside. Tape the third side closed, then use tape to attach the envelope to the end of the kaleidoscope.
4. Now, look through the end that has the peephole and aim the kaleidoscope at a light source. The colored objects on the other end will reflect off of the mirrors into star-shaped patterns.
Challenge: What is the minimum length of a plane mirror required to view the full image of a boy who is 4 feet tall?
Solution: In order to see the full image of a person, the minimum size of the mirror should be one-half the person's height. Let us try to prove this by using a ray diagram.
The ray from the foot strikes the mirror at point Y, bounces back, and reaches your eyes. Light ray starting from your head strikes the mirror at point X and reflects your eyes. The minimum length of the mirror required to view the entire image of the boy is XY.
In order to make the angle of incidence equal to the angle of reflection, the normal line N must sit exactly halfway between the observation point and the feet. Therefore, XY = Half of the Boy's height.
Answer: Mirror minimum height required is 2 feet.
