Interference and diffraction of light
When waves of equal frequency and constant phase difference (coherent waves) meet, they strengthen each other at some points and weaken at others: this is interference. If the path difference Δd = d₂ – d₁ equals an even number of half-waves, there is a maximum: Δd = 2k·λ/2 = kλ; if it equals an odd number there is a minimum: Δd = (2k + 1)·λ/2 (k = 0, 1, 2, …). Two separate lamps are not coherent, because atoms emit light independently and at random; so the light of one source is artificially split in two: in Young's experiment (1801) by two narrow slits, in a thin film (a soap bubble, an oil layer) by reflection from the upper and lower surfaces, in Newton's rings by reflection from the air layer between a lens and a plate. For slit separation d and screen distance L the distance between neighbouring bright fringes is Δy = λL/d. Diffraction is the bending of a wave around an obstacle or the edge of a slit, clearly noticeable when the obstacle's size is comparable to the wavelength. For a diffraction grating of period d the bright maxima are seen when d·sinφ = nλ (n = 0, 1, 2, …); in white light the centre is white and spectra appear on the sides. These phenomena confirm the wave nature of light.
A safe experiment: make a soap film on a wire ring and watch coloured fringes in white lamp light and how they move downward. See the spectra from a lamp or phone flashlight reflected from the surface of a compact disc (CD). Do not look at the Sun and never point a laser at anyone's eyes.