Light

Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye.

In the past, scientists debated about light, is it wave or particle.

Light as particle
Einstein's explained by imagining photoelectric effect, which light striking a metal surface ejected electrons from the surface.

Light as a wave
Thomas Young proof that the wave is the nature of light by doing double-slit experiment.
A beam of light(laser) is shone on a pair of 2 parallel slits. Light diffracts from each slit and projected on screen. In the space after the light difracts, it overlaps and formed interference pattern(light and dark) on the screen, it's called 'fringes'.

λ
In the double slit experiment, we determine λ with this equation

a, is the slit separation(distance between the centres of the 2 slits)
x, is the distance between the centres of 2 close bright/dark fringes
D, is the distance between slits's midpoint and central fringe on screen

Note:
To observe interference fringes with white light instead of laser, it's necessary use a single slit behind/before the double slit. it's to make a clearer fringes.
To see and doing double-slit experiment, check this site