Question

How long would it take light from the sun to reach Neptune?

Answer 1

Hint: The light emitted by the Sun reaches the different planets of the galaxy in different time durations due to the distance of the planet from the Sun. Neptune being the farthest planet receives very little amount of sunlight but even that affects the atmosphere of Neptune drastically.

Complete solution:
Neptune orbits the sun at a distance of 4.495 billion kilometer and the light travels at a speed of $3\times {10}^{6}m{s}^{-1}$. Dividing the two quantities as speed is equal to distance divided by time.
Hence,
 $\begin{array}{rl}& speed={\displaystyle \frac{\text{distance}}{time}}\\ & \Rightarrow time={\displaystyle \frac{\text{distance}}{speed}}\end{array}$

Distance of Neptune from Sun = 4.495 billion km = $4.495\times {10}^{12}m$ $\{\because 1km=1000m\}$
$\Rightarrow time={\displaystyle \frac{4.495\times {10}^{12}m}{3\times {10}^{8}m{s}^{-1}}}=14983.33s$

Also, 1 hour = 3600 seconds
$\begin{array}{rl}& \Rightarrow time=14983.33s\times {\displaystyle \frac{1h}{60\times 60s}}\\ & \Rightarrow time=4.16hours\end{array}$
$\begin{array}{rl}& \Rightarrow time=4.16hours\\ & \Rightarrow time=4.16hours\times {\displaystyle \frac{60min}{1hour}}\\ & \Rightarrow time=249.6min\end{array}$

Therefore, light approximately takes 4.16 hours or 250 minutes to reach Neptune.

Photons (light) are created by fusion reactions inside the Sun’s core. They start off as gamma radiation and then are emitted and absorbed countless times wandering around inside the sun before they finally reach the surface. These photons striking your eyeballs were actually created tens of thousands of years ago and it took them that long to be emitted by the Sun.

Once they escape the surface, it takes 250 minutes for these photons to cross this vast distance from the Sun to Neptune and to reach the surface of Neptune.

Note:
As you look outward into space, you are actually looking backward into time. The light we see from our computer screens is only nanoseconds old, the light reflected from the surface of the moon takes only one second to reach the surface of earth but if light from a star takes 4 years to reach us then we are seeing that star four years in the past!