Sources of Light, Form of Energy
Sources of Light, Transmission, and Reflection of Light. Light is a form of energy called luminous energy. This energy causes a sensation of vision, enabling us to see. There are various sources of light, for example, the sun and the stars are natural sources of light. Artificial sources of light are the candle, electric torch, electric lamp, incandescent, and arc lights, and fluorescent light.
Self-luminous or luminous sources of light are those that generate and emit light by themselves. Examples are the sun, stars, fireflies and some deep-sea fishes, and artificial light sources.
Non-luminous bodies depend on natural or artificial light sources to illuminate them. They are seen only when they reflect the light from a luminous body. For example, light from a car headlamp falling on a road sign at the night, causes the sign to throw back part of this light into the eyes of the car driver thereby enabling the road sign to be seen. Moreover, the road sign is a non-luminous body, the head-lamp is a luminous body, an artificial luminous body.
Examples of non-luminous bodies are a page of a book, a person’s face, a brick, and the moon. The sun’s rays illuminate the moon and make it appear luminous at night.
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Transmission of Light
Light is an electromagnetic wave. It can pass through a vacuum and through a material medium. If light shines on a body, part of the light is transmitted through the body, and the rest is reflected. The amount of light passing through a body depends on the nature of the body. If a large percentage of light falling on a body passes through it, the body is said to be transparent.
Examples of transparent bodies are glass and water. Because light is easily transmitted through these transparent bodies, we can see objects through them easily. Some objects like frosted glass and tissue paper allow some small amount of light to pass through them. Such objects are called translucent objects.
Because the amount of light passing through translucent bodies is small, objects cannot be seen clearly through them. Some other bodies do not allow any light to pass through them. These are called opaque objects. Examples of opaque objects are wood, bricks, walls, and metal sheets.
Rays and Beams of Light
A light ray is a direction or path along which light energy flows. Such rays are indicated in diagrams by thin lines with arrowheads which indicate the direction of travel of the light. A collection of rays is called a beam. There are three
Types of Beams
There are three types of beams;
- Parallel Beam
- Convergent Beam
- Divergent Beam
A parallel beam is one in which the light rays are parallel to one another. Searchlights give off parallel beams of light.
A convergent beam is one in which the rays converge or meet at a point. A hand lens can be used to produce such a beam.
A divergent beam is one in which the light rays all come from a point and spread out or diverge from the source. Lamps produce a divergent beam of light.
The Ray Box
Rays of light are produced in the laboratory by means of a ray box. A simple laboratory ray box consists of a box made of wood or cardboard which is a source of light. For example, a candle or an electric lamp. On one face of the box is a hole or a slit or several slits through which light rays can travel out of the box.
Rectilinear of Light Propagation
Light travels in straight lines as can be shown by the following simple demonstration using two cardboard pipes one straight and the other bent. If we place a candle flame on one end of each pipe and view through the other end, light from the candle flame will be seen clearly on the end of the straight pipe but no light will be seen at the end of the bent pipe.
The phenomenon of light traveling in a straight line is known as rectilinear propagation of light.
Another demonstration of rectilinear propagation of light is the following simple experiment. We punch a tiny hole in the center of each of three equally sized cardboards which are then arranged in a straight line in front of each other. A string is passed through the holes and pulled taut to ensure that the holes are aligned in a straight line.
Light from a candle is then placed at one end of the row of cards. When we look through the other end of the row, we will see the light from the candle flame through the holes. If either cardboard is slightly shifted out of the straight line position, the light is cut off.
Two natural effects that result from the rectilinear propagation of light are the formation of shadows and eclipses. The principle of operation of the pin-hole camera also depends on the fact that light travels in straight lines.
A shadow is an area in which light rays from a source cannot reach. A shadow is produced by the obstruction of light by an opaque object. The shade under a tree or a canopy, on a bright sunny day, is a shadow produced by the opaque tree or canopy which obstructs light rays from the sun and prevents them from getting to the area of the shade.
We can use a small light source known as a point source of light. If an opaque object, For example, a coin or a solid ball is placed between the point source and a screen, a circular shadow is cast on the screen.
The shadow takes the shape of the opaque object producing it. The shadow cast by a point source has sharp edges.
If the light source is large, or extended the shadow formed consists of two parts, a completely dark area known as the Umbra and an outer light or grey area known as the Penumbra or partial shadow. In the Umbra region, the light from the source is completely blocked by the opaque body. In the Penumbra region the light is partially blocked by the opaque object. The inner region of the shadow receives less light than its outer parts. Thus the penumbra becomes brighter from the umbra end outwards.
An eclipse is a result of a shadow cast by one heavenly body on another. While the Sun is a luminous body, that means, it produces its own light; the Moon and the Earth are both non-luminous. The Moon revolves around the Earth, and the Earth also revolves around the Sun. At certain times during these movements, the three bodies all lie in a straight line. If the Moon is between the Sun and the Earth, the shadow of the Moon will be cast on the Earth’s surface. If we are living in the part of the Earth that is in the Moon’s shadow, the light from the Sun is cut off from view and we are in darkness. Since we cannot see the Sun, we call this an Eclipse of the Sun or Solar Eclipse.
This usually lasts for just a few minutes. During this period, a portion of the earth is in the shadow cast by the Moon. It is a total eclipse for people living in the umbra region of the shadow, and for those in the penumbra regions of shadow, it is a partial eclipse. For them, the Sun is only partially covered.
The Earth and the Moon may be in positions where the extreme rays at the Moon’s edge intersect before reaching the Earth. Under such conditions, an annular eclipse is formed. A ring of light is formed around the shadow of the Moon.
When the Sun, the Earth, and the Moon are in line during their movements, with the Earth lying between the Sun and the Moon, the earth being an opaque object casts a shadow on the moon and an eclipse of the Moon or Lunar Eclipse occurs. In the eclipse of the Moon, the Moon is in the Earth’s shadow. As in the solar eclipse, the lunar eclipse can be total or partial.
The Pinhole Camera
The Pinhole Camera makes use of the fact that light travels in straight lines. It consists of a light-proof box, one end of which has a small hole made with a pin or needlepoint. The opposite end has a screen made of tracing paper or ground glass. Light from an object in front of the pinhole passes through it and forms an image on the screen. If the screen is replaced with photographic paper or film, a picture of the object can be taken with the pinhole camera.
The camera forms an image of an illuminated object AB, in the following way: Each point in the illuminated object AB acts as a point source of light. Light rays from point A pass through the hole.
Similarly, light rays from all other points on the object will fall on corresponding points between A’ and B’, forming a complete image of AB. This image is inverted. If the pinhole is small, the image is sharp, though not very bright since the amount of light entering the camera is small. A brighter image can be obtained by having a larger hole. More light then enters the box to form the bright image but this image is blurred. The wide hole acts like a large number of pinholes, each forming its own image. These images overlap and are seen as a single blurred image.
If the distance between the pin-hole and the screen is increased the image is enlarged but less bright. The decrease in brightness is as a result of the distribution of the same amount of light over the greater area of the enlarged image.