Light is a form of energy made of photons. Photons are the smallest unit of visible light. Do you know the light is the fastest thing in the universe? As light consists of massless particles called photons that allows it to be the fastest thing in the universe. Light travels in a vacuum at 300,000 km per second. Light is unique as it exists in two very different forms at the same time. One form is tiny particles called photons. The other form is waves. The easiest way to think about light is as waves.
Light will travel in a completely straight line until it hits an object that will bend it or reflect it. This property of light traveling in a straight line is called rectilinear propagation of light.
Have you ever noticed the beam of light that enters your darkroom through a small hole? Yes, you will see the beam of light traveling in a straight line.
Let us conduct a few basic experiments to prove that light travels in a straight line.
Experiment 1:
Objects needed: Candle, match stick, and a small hollow straight pipe, and a small hollow bent pipe.
Light the candle and look at the flame of the candle through the straight pipe. Here the candlelight is visible.
Now, let us try to look at the flame through the bent pipe. The flame will now not be visible to us.
Observation: Flame is not visible through the curved pipe. This experiment proves that light travels in a straight line and gets blocked if it meets any obstacle in its way.
Experiment 2:
Objects needed: Three identical screens and a candle.
Punch holes in the middle of these three screens and mount them on a table one behind the other as shown in the diagram below. Keep the candle on one side of these screens and look through the other side of the three screens.
1.Try to look at the candle flame. Adjust the screen holes and candle so that you can see the flame passing through these screens.
2. Now slide any of the screens sideways and try to see the candle flame. It will not be visible.
Observation: When all three holes and the flame are in the same alignment, then the flame is visible to us. As soon as one of the screens breaks the alignment, the flame is not visible. This is possible only when light traveled in a straight path and not in a zigzag way.
1. Formation of shadow: Whenever light is obstructed by an opaque substance a shadow is formed. It is of the same shape as the object although it might be of a different size.
2. Formation of the eclipse: Another consequence of light traveling in a straight path is the existence of the solar and the lunar eclipse. Do you see why Earth, the Moon, and the Sun must be in a straight line to cause a solar eclipse? The light from the Sun cannot bend around the Moon. If sunlight cannot reach your eyes, then you cannot see the Sun.
3. Formation of day and night: If the sunrays do not follow a rectilinear path then the light would have curved around the earth and there would have been sunlight during the night.
4. Pinhole camera: Pinhole camera is based on the rectilinear propagation of light. Let us see how it works. A pinhole camera is a simple device that consists of a small box that is painted black on the inside and has a tiny hole of the size of a pin at one end. On the other end, there is a screen of tracing paper or frosted glass for viewing. If you turn the side with the hole towards a distant tree or a candle perhaps and then look at the screen on the opposite end, you will notice an image. The image may be blurry depending on how far away or near the object is. But the interesting thing about the image is that it is upside down, or as we say in physics, the image is inverted. This inversion is what is proof that light travels in straight lines.
Light travels from the top of the tree (point A) and falls on point X on the screen. Rays from the bottom of the tree (point B) fall on Y. Thus XY is the diminished inverted image of the tree on the screen. If the screen is replaced by a photographic film, a photograph of the tree can be taken.
How can the path of light be changed?
Below are the reasons for the light to deviate from its straight path:
(i) It hits the surface and bounces back. This is called reflection.
(ii) It passes from one transparent medium to another and changes its direction, for example when light passes from air into water it changes speed, which makes the light bend. This is called refraction.
(iii) Light enters an object but does not pass through. Black surfaces absorb almost all light. This is called absorption.
