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# The laser light beamed towards moon from earth takes $2.56s$ to return after reflection at moon's surface. How much is the radius of lunar orbit around earth?

$(a)\;7.84 \times 10^8 m\quad (b)\;6.84 \times 10^8 m \quad (c)\;2.84 \times 10^8 m\quad (d)\;3.84 \times 10^8 m$

speed of laser light$=c=3 \times 10 ^8m/s$
Total distance travelled by light$=2s$
Therefore $2s=c \times t$
$s=\large\frac{c \times t}{2}$
$\quad=\large\frac{3 \times 10^8 \times 2.56}{2}$
$\quad=3.84 \times 10^8m$
Hence d is the correct answer.
edited May 30, 2014