Back to: PHYSICS SS2
Welcome to class!
In today’s class, we will be talking about the light wave. Enjoy the class!
Light is a form of energy, which gives us a sensation of vision. Though light exhibits some particle nature, it is also a wave (electromagnetic wave). The speed of light in a vacuum is 3 x 108m/s.
SOURCES OF LIGHT
An object that emits light is said to be luminous. A lot of light is obtained from the sun. The stars are also sources of light. The moon, however, does not emit its own light; it only reflects the light of the sun. Hence, the moon is not a luminous object. When metals are heated to a very high temperature, they also emit light. These are called incandescent objects e.g. filament of an electric lamp. Burning woods, kerosene, petrol and gases can also produce light. Some objects produce light when fast-moving electrons hit them. They are called fluorescent materials. Fluorescent tubes used in domestic lighting are examples.
INTERACTION OF LIGHT WITH MATTER
When light falls on the matter, either it is absorbed, reflected or transmitted. In practical, however, more than one of these occur at the same time. For instance, we can have both reflection and refraction from a surface. We can thus classify materials according to whether they absorb, reflect or transmit light that falls on them as transparent, translucent and opaque materials.
These allow light to pass through them. That is they transmit light. Examples are plane glass, clean water, etc. note that we can see through a transparent material.
These allow light to pass through but we cannot see through them.
These do not transmit light at all. They absorb all light that falls on them. Examples include wall, wood etc.
When the direction of light changes as it moves from one medium to another medium, we call this refraction. Polished surfaces are known to turn back (reflect) light falling on their surfaces. An example is a mirror. We shall discuss more of this later.
RECTILINEAR PROPAGATION OF LIGHT
Light travels in a straight line. Rays of light represent the motion of light. A collection of rays of light is called beam of light.
Evidence of Rectilinear Propagation of Light
Formation of shadows:
Formation of shadows is evidence that light travels in a straight line. In the diagram above, a beam of light falls on an opaque object arranged in a line with a screen. The opaque object prevents the rays from reaching the screen. Hence, a shadow of the opaque object is formed on the screen. If the light does not travel in a straight line, it would have bend around the opaque object to reach the screen. The opaque object stands in its way to stop its motion towards the screen, thereby producing dark region (shadow) on the screen.
Note that there are two regions of the shadow. If the light source is small such that the opaque object effectively blocks all rays of light, we have total darkness (umbra). If however, we have extended source such that some rays graze the edge of the opaque object to reach the screen, we have another region of partial darkness (penumbra) around the main region of total darkness.
Many times, people on earth have experienced the sighting of the moon in the afternoon or ‘the day turning into the night’. The local people have attributed this to so many things. Some say the gods are angry. These postulations have no scientific basis, however. The scientific explanations for the eclipse are as follows: the sun is the centre of the solar system. The earth and other planets move in orbits around the sun. it takes the earth about 365 ¼ days to move round the sun. as the earth moves, it rotates (spins) about its own axis. Hence a side of the earth will face the sun at a time. This causes day and night. The spinning of the earth about its own axis takes 24hours. Hence, we have 24hours in a day. That is, the three bodies are constantly moving relatives to each other. Now, the moon also moves round the earth. It sometimes happens that the sun, the moon and the earth fall on a straight line. When this happens, the eclipse takes place. Since the position of the sun is fixed, either the moon or the earth will be in the middle when the three bodies are on a straight line. Hence there are two types of eclipse viz:
When the moon is in between the sun and the earth, the moon will not allow some rays of light coming from the sun to reach the earth. That portion of the earth’s surface will then experience darkness for some time even during daytime. This is called the eclipse of the sun in the moon (solar eclipse). Note that the sun is an extended source and the moon cannot completely block all the rays coming to the earth. Hence, some portion of the earth surface experiences total darkness while some other experience partial darkness.
Eclipse of the sun by the moon
- At point X, total darkness on earth
- At point Y and Z, partial darkness.
A special type of solar eclipse occurs when the tip of the umbra come of the moon’s shadow does not reach the surface of the earth. This occurs when the moon is relatively farther from the earth than in normal solar eclipse. The people at such point as B on earth see the sun like a dark disc with a ring of light around it (annular shape). This is called the Annular Eclipse.
One can further demonstrate that light travels in a straight line with the following experiment
Make a hole in three cardboard papers. Arrange the cardboard so that all the three holes are in a straight line. You can pass a string through the holes to do this. Ensure the cardboard are well spaced from each other. Place a source of light (e.g. torchlight at one end close to the first hole and position the eye at the other end (close to the third hole). You will see the ray of light from the source passing through the holes in a straight line.
Now, displace the cardboard in the middle to one side so that its hole is no longer in a straight line with the others. Repeat the experiment as stated above. You will observe that the ray of light has been cut-off by the second cardboard. The eye cannot see any ray of light through the third hole. This shows that light travels in a straight line.
Rays of light cannot pass through the displaced hole.
In our next class, we will be talking about the Reflection of Light. We hope you enjoyed the class.
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