Light - Definition & Properties

Introduction to Light

The eyes are a precious part of our body, as we all know. It’s because of our eyes that we can see the beauty of nature and all the wonderful things in our surroundings.

Have you ever thought about what makes it possible to see anything? The light, our eyes, and our brain are what helps us to view the beautiful world.

You can also learn about the human eye and the human brain.

What is Light?

Light is a form of energy that enables us to see our surroundings. Light is made of photons which is the smallest unit of the visible light spectrum.

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What is the visible light spectrum? The visible light spectrum is a small part of the electromagnetic radiation that the human eye can view. In simple words, visible light is a mixture of all 7 colors of the rainbow.

Natural and Man-Made Source of Light

There are two sources of light; natural and man-made sources of light.

Natural lights are given off by nature. For example, the sun, stars, fireflies, lightning, etc.

Man-made lights are artificially made. For example, bulbs, candles, flashlights, table lamps, television, etc.

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If the object gets hot when emitting light they are called hot sources of light. It is also known as incandescence. For example, the sun, a burning lamp, an electric bulb, etc.

If the object doesn’t get hot when producing light it is known as a cold source of light. It is also called luminescence. For example, fireflies, LEDs, and some algae.

Luminous and Non-luminous Object

Luminous objects are objects that emit their light and are capable of reflecting light to our eyes. For example, the sun, stars, and electric lights are all luminous objects.

Non-luminous objects are objects that can not emit light by themselves. We can see the non-luminous objects because light from other sources bounces/reflects off them and back into our eyes enabling us to see. For example, the moon, paper, toys, cars, books, etc.

Transparent, Translucent and Opaque Objects

Opaque object

The objects that don’t allow light to pass through them are called opaque objects.
We can’t see anything that is kept on the other side of an opaque object.
Since these types of objects completely block the light, there are dark shadow forms behind the object. For example, wooden doors, stone, metals, concrete, etc.

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Transparent Object

Objects that allow light to pass through them are called transparent objects. We can see things on the other side of a transparent object by looking through it.

Since this object doesn’t block the light, there is no shadow formation. Sometimes it refers to see-through objects.

For example clear glass windowpane, water, clean air, lense, clear plastics, prism, diamond, etc.

Translucent Object

The objects that allow light to pass through partially are called translucent objects. We can’t see anything clearly that is kept on the other side of a translucent object.

Since this object doesn’t block the light completely, there are faint shadow forms. Sometimes it refers to partially see-through objects. For example, frosted glass, butter paper, tinted glass, wax paper, vegetable oil etc.

Rays and Beams of light

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Ray of Light: Ray is a very narrow path of a light which is represented by a straight line with an arrow that shows the direction in which the light is travelling. The straight line from X to Y is called ray of light.

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Parallel Beam of Light: A bundle of these rays is known as beams of light. When a bundle of rays are parallel to each other it is called a parallel beam of light. It is also called a pencil of light.
Convergent beam of light: When a ray of light coming from different directions meets at a point, it is known as a convergent beam of light. The distance between the rays goes on decreasing till all the rays meet at a point. For example, light rays from various directions enter our eyes and converge on to the retina to form an image.
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Divergent beam of light: When the rays of light starting from a point travel in different directions are called divergent beams of light. The distance between the rays goes on increasing.
For example, light rays coming out from a candle or a car headlight are divergent beams of light.

Rectilinear Propagation of Light (Light travel in straight line)

Have you ever seen the sun rays entering a dark room through a small opening? Or have ever seen the laser light show?
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One common thing you must have noticed if you have experienced these events is that the sunrays and laserlight will be traveling in a straight line. It won’t be in wavy form.

Rectilinear means ‘straight’ and Propagation means ‘travelling of light’.

Light travels along a straight line path in a homogeneous medium (air or water) because light always tries to travel between two points in the shortest possible time. This property of ‘light travel in a straight line’ is called rectilinear propagation of light.

Let’s take an example to verify this property of light.

Step 1. Take three identical cardboards with a hole in the middle and place it on a table.

Step 2. Light up a candle and place it on the other side of the cardboard deck.

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Step 3. Now peek through the deck of cards (make sure the holes are aligned).

Step 4. Now move the middle card to the left/right, and try to see the flame again.

What did you observe in step 3 and 4 respectively? Can you see the flame of the candle in step 4?

Did you notice that when the holes are aligned, you can see the flame of the candle. But on the other hand, if the middle cardboard is moved, the flame is no longer visible.

This is because of the rectilinear propagation of light ( because light travels in a straight line).

Formation of shadows, pinhole cameras and occurrence of eclipse are also examples of the rectilinear propagation of light.

Properties of light

  • Light is a form of energy
  • Light can behave as a wave or a particle
  • Light always travels along a straight line. This property is known as the rectilinear propagation of light.
  • Light does not require any medium for its propagation. It can even travel through a vacuum.
  • It travels with a very high velocity of about 3 × 108 m s–1 in air or vacuum.
  • Light does not change the properties of the medium through which it passes.
  • The velocity of light decreases as it travels from a rarer medium to a denser medium.
  • The colors we see in the rainbow, called visible light, are just a small portion of the electromagnetic spectrum.

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