Answer:
The tension in the cable when the craft was being lowered to the seafloor is 4700 N.
Explanation:
Given that,
When the craft was stationary, the tension in the cable was 6500 N.
When the craft was lowered or raised at a steady rate, the motion through the water added an 1800 N.
The drag force of 1800 N will act in the upward direction. As it was lowered or raised at a steady rate, so its acceleration is 0. As a result, net force is 0. So,
T + F = W
Here, T is tension
F = 1800 N
W = 6500 N
Tension becomes :

So, the tension in the cable when the craft was being lowered to the seafloor is 4700 N.
C.) A thunderstorm
That’s the Answer
Because the refractive indexes of the immersion oil and the glass microscope slide are the same; the oil prevents the refraction of the incoming light rays.
An invisible space is present between the slide and the lens that contains air.
When the light rays fall on the slide and lens they might get refracted due to air present between them.
If the rays get refracted then the diameter of the objective lens will expand further.
So in order to prevent the refraction of light, it is very important to lubricate the surface of the lens because without oil the light rays refract from their original path.
If you need to learn more about the refractive index click here
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The shortest frequency is present in the wave with the highest energy.
Therefore, the answer is X-Rays.