The Wave Nature of Light

Waves So far we have learned about mechanical waves in slinkies, strings, and air. Sound waves are mechanical waves. Mechanical waves need a medium in order to support the wave propagation. –There can be no sound in a vacuum. The spaceship movies have sound effects- but outer space is QUIET

Light is a wave? If light is a wave, how can it get through the vacuum of space to earth without a medium? At one point, scientists thought that there was an ether – an invisible, undetectable medium, that supported electromagnetic radiation. Our understanding - EM radiation can propagate through a vacuum

Electromagnetic Waves We will learn about electricity and magnetism later in year. EM Waves are Transverse

Source of all waves is an object moving Slinky – Hand moves end Sound – A mechanical vibration Electromagnetic – A moving charge –The frequency that the charge moves is the same as the frequency of the electromagnetic wave –In an antenna charges are made to move forward and backward along the wire with the frequency of the radio waves

Production of Electromagnetic Waves We will learn that: A changing electric field produces a magnetic field, and a changing magnetic field produces an electric field, once sinusoidal fields are created they can propagate on their own. These propagating fields are called electromagnetic waves. Propagation of electromagnetic waves is analogous to the propagation of mechanical waves. However no medium is required!

22.3 Light as an Electromagnetic Wave and the Electromagnetic Spectrum Electromagnetic waves can have any wavelength; we have given different names to different parts of the wavelength spectrum.

Types of electromagnetic radiation All electromagnetic waves have the same speeds in vacuum

Speed of light: c

Electromagnetic Waves All types of EM waves follow same basic principles in a vacuum However, because of size effects, when EM radiation interacts with matter, you can get very different physical effects Geometric optics –When the wavelength of the EM radiation is much smaller then the size of the object- shadow effects only Wave optics –When the wavelength of the EM radiation is ~comparable to the size of the object –In wave optics – interference effects play a very important role

Diffraction The spreading out of a wave around a corner or when passing through a hole It is as if the hole acts as a source of the wave Why can we hear someone walking down the hall through the door, but we can not see her until she is in front of the door?

24.1 Waves Versus Particles; Huygens’ Principle and Diffraction Huygens’ principle: Every point on a wave front acts as a point source; the wavefront as it develops is tangent to their envelope

24.1 Waves Versus Particles; Huygens’ Principle and Diffraction Huygens’ principle is consistent with diffraction:

What happens when light goes through a pair of slits? Particle Model Would Say: Two Bright Spots

What happens when light goes through a pair of slits? Actually Observed Numerous Bright Spots

24.3 Interference – Young’s Double-Slit Experiment If light is a wave, interference effects will be seen, where one part of wavefront can interact with another part. One way to study this is to do a double-slit experiment:

24.3 Interference – Young’s Double-Slit Experiment If light is a wave, there should be an interference pattern.

24.3 Interference – Young’s Double-Slit Experiment The interference occurs because each point on the screen is not the same distance from both slits. Depending on the path length difference, the wave can interfere constructively (bright spot) or destructively (dark spot).

Calculating the phase difference Since d << L, the angles from slit 1 and slit 2 are essentially the same.

24.3 Interference – Young’s Double-Slit Experiment We can use geometry to find the conditions for constructive and destructive interference: (24-2a) (24-2b)

24.3 Interference – Young’s Double-Slit Experiment Between the maxima and the minima, the interference varies smoothly.

24.3 Interference – Young’s Double-Slit Experiment Since the position of the maxima (except the central one) depends on wavelength, the first- and higher-order fringes contain a spectrum of colors.

Summary In the double-slit experiment, constructive interference occurs when and destructive interference when Light bends around obstacles and openings in its path, yielding diffraction patterns

Summary Visible spectrum of light ranges from 400 nm to 750 nm (approximately) Diffraction grating has many small slits or lines, and the same condition for constructive interference