Which wavelength is the longest in the visible spectrum of light?

Answer 1

That's going to be red light. Red light has a wavelength ~ 635-700nm (nanometers).

A nanometer is equal to 1x10^-9 meters. This also explains why red light is the least energetic of the visible spectrum. From the Planck-Einstein relation we know

E=hv=hc/λ .

Where

E ≡ Energy

v ≡ frequency

h ≡ planck's constant

c ≡ speed of light

λ ≡ wavelength

If you notice that wavelength (λ) is in the denominator then you can see why longer wavelength corresponds to less energy.