A yet-to-be-built spacecraft starts from Earth moving at constant speed to the yet-tobe-discovered planet Retah, which is 20 lighthours away from Earth. It takes 25 h (according to an Earth observer) for a spacecraft to reach this planet. Assuming that the clocks are synchronized at the beginning of the journey, compare the time elapsed in the spacecraft’s frame for this one-way journey with the time elapsed as measured by an Earth-based clock.

The  time elapsed at the spacecraft’s frame is less that the time elapsed at earth's  frame

Explanation:

From the question we are told that

The distance between earth and Retah is  

Here c is the peed of light with value

The time taken to reach Retah from earth is  

The velocity of the spacecraft is mathematically evaluated  as

substituting values

The time elapsed in the spacecraft’s frame is mathematically evaluated as

substituting value

=>    

So  The  time elapsed at the spacecraft’s frame is less that the time elapsed at earth's  frame

Suppose that the wave equation is:

f(x) = A*cos(k*x)

The wave is in phase for two points x₁ and x₂ if:

f(x₁) = f(x₂)

And if these points are consecutive (for example, two consecutive peaks) then the wavelength of the wave is:

wavelength = x₂ - x₁

In this case, we know that the wave is in phase for two points that are 2 m apart.

Then we can use the points:

x₁ and x₂ such that:

x₂ = x₁ + 2m

We also know that these are the nearest points such that this happens, then we can assume that the points are consecutive.

Then the wavelength will be:

wavelength = x₂ - x₁ = (x₁ + 2m) - (x₁) = 2m

Then the wavelength is 2m.

The correct option is C.