Final answer:
Bright and dark lines on a screen formed by red light passing through two slits are explained by the phenomenon of diffraction and interference. These lines are a result of constructive and destructive interference between waves from slits close to the light's wavelength in size.
Step-by-step explanation:
The phenomenon primarily responsible for the pattern of bright and dark lines on a screen when red light is passed through two narrow slits is known as diffraction and interference. In Young's double-slit experiment, the light of a single wavelength that passes through the pair of vertical slits produces a diffraction pattern on the screen. This pattern consists of multiple vertical light and dark lines that spread out horizontally. If only diffraction or only interference occurred, the light would display a different pattern on the screen: diffraction alone would not create the evenly spaced bright spots, and interference alone would not account for the intensity variation.
However, when combined, these two phenomena produce the characteristic pattern seen in the experiment. The intensity of the fringes falls off with angle, and the bright fringes are the result of constructive interference, where the waves from the two slits arrive in phase and reinforce each other. The dark fringes represent destructive interference, where the waves arrive out of phase and cancel each other out. The width and separation of the slits play a crucial role in the diffraction pattern, as they need to be similar in size to the wavelength of light for a noticeable pattern to form.