Question

The planet Pluto is at an average distance of 38.5 a.u from Earth. So, it takes for light to reach Pluto from Earth.

• A. 8.785 hr
• B. 6.083 hr
• C. 5.351 hr

Answer 1

The correct option is C 5.351 hr

One astronomical unit (a.u) = $1.496\times {10}^{11}m$

Distance between Pluto and Earth $d=38.5\times 1.496\times {10}^{11}m$ $=57.596\times {10}^{11}m$

1 light second (ls) = $2.99\times {10}^{8}m$

$⟹$ Light travels $2.99\times {10}^{8}m$ in a second.
So, the time taken by light to travel a distance of $57.596\times {10}^{11}m$ is: $\frac{57.596\times {10}^{11}m}{2.99\times {10}^{8}m/s}=19.2628\times {10}^{3}s$
$=19262.8s$.

1 hour = 3600 s
$⟹$1 s = $\frac{1}{3600}$ hr
$⟹$ 19262.8 s = $\frac{19262.8}{3600}\approx 5.351hr$.