The unit of length most suitable for measuring the thickness of a cell phone is a . the unit of length most suitable for measuring the height of a backyard tree is a .

Millimeter and meter

Explanation:

Cellphones are only a few millimeters in thickness.

It is easier to say 5 millimeter than 0.0005 meter.

The unit we use must help others easily visualize and understand the size of the object.

a 5 millimeter is easy to visualize than a 0.0005 meter or a 5,000,000 nano meter

In the same way, a tree in our backyard usually grow as tall as a few meters. ( remember trees in amazon forest might grow as tall as a few kilometers). so in the case of trees in our home the right unit to measure them is meters.

double the distance = divide strength by 4

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.