Quick questions on Light as a wave: WACE Year 12 Physics Unit 4

Diffraction is the spreading of a wave as it passes an edge or through a gap. The effect is significant when the gap width is similar to or smaller than the wavelength: a wide gap gives little spreading, a narrow gap gives a lot. Because visible light has a tiny wavelength (hundreds of nanometres), noticeable diffraction needs very narrow slits, which is why everyday light appears to travel in straight lines.

In Young's experiment, monochromatic light illuminates two narrow, closely spaced slits that act as coherent sources. The diffracted light from each slit overlaps and superposes, producing alternating bright and dark fringes on a distant screen. Bright fringes (constructive interference) occur where the path difference is a whole number of wavelengths,

For small angles and a screen a distance $L$ away, the spacing between adjacent bright fringes is

Light is one part of a continuous spectrum of electromagnetic waves, all travelling in vacuum at $c=3.0\times10^{8}\ \text{m s}^{-1}$ and all obeying $c=f\lambda$. In order of increasing frequency (decreasing wavelength) the spectrum runs radio, microwave, infrared, visible, ultraviolet, X-ray and gamma. Visible light occupies only a narrow band from about $400\ \text{nm}$ (violet) to $700\ \text{nm}$ (red). All these waves are transverse oscillations of electric and magnetic fields.