Question

Write a short note on the speed of light.

Answer 1

Light can be considered as matter and as a wave. The speed of light refers to the speed at which light propagates. Unlike sound waves, light does not require a medium to propagate and so it can travel in a vacuum as well in a medium.

Formulas used:
-The circumference of a circle is given by, $C = 2\pi R$ where $R$ is the radius of the circle.
-The time taken to cover some distance is given by, $t = \dfrac{D}{v}$ where $D$ is the distance covered and $v$ is the speed of the body.

Complete step by step answer.
Step 1: Try to explain the propagation of light.
Light can travel in vacuum as well as in a medium. The speed of light will be maximum in a vacuum and it is represented by $c$ and its value is known to be $c = 299792{\text{m}}{{\text{s}}^{ - 1}} \cong 3 \times {10^8}{\text{m}}{{\text{s}}^{ - 1}}$ .
To get a better understanding of how fast the light travels, we can find how much time it takes for light to cover the circumference of the Earth.
The radius of the Earth is known to be $R \cong 6400{\text{km}}$ .
Assuming that the Earth has a circular perimeter, the circumference of the Earth will be $C = 2\pi R = 2\pi \times 6400 = 40192{\text{km}} = 4 \cdot 02 \times {10^7}{\text{m}}$ .
Now the time taken to cover this distance will be $t = \dfrac{C}{c} = \dfrac{{4 \cdot 02 \times {{10}^7}}}{{3 \times {{10}^8}}} = 0 \cdot 134{\text{s}}$ .
So a single second light can cover the circumference of the Earth about seven times.
However, in a medium such as glass, air, water, the speed of light will decrease and will be less than the speed of light in a vacuum i.e., $v < c$. The ratio of the speed of light in a vacuum to its speed in a medium is referred to as the refractive index of the medium i.e., $n = \dfrac{c}{v}$ .
The distance covered by light in one year is referred to as light year.

Note: In a medium, the particles of the medium will scatter light and cause the light to travel slowly which means the speed of light decreases. The refractive index of a material essentially gives an idea about the optical density of the medium. The speed of light in a vacuum is considered to be the upper limit of the speed of light.