Question

why red colour has longest wavelength? ​

Answer 1

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Objects appear different colours because they absorb some colours (wavelengths) and reflected or transmit other colours.

Red light has a longer wavelength than blue light, simply because it does. Different coloured light has to have different wavelengths (that's how our eyes discern colour).

The wavelength interval of red colour is ~ 700–635 nm.

The frequency interval of red colour is ~ 430–480 THz.

Answer 2

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Red light has the longest wavelength and, therefore, the least amount of energy in the visible spectrum. Wavelength decreases and energy increases as you move from red to violet light across the spectrum in the following order: red, orange, yellow, green, blue, and violet.