The Dual Nature of Light: Waves and Particles
Light: A Wave and a Particle
The Particle Nature of Light
Energy of light is contained in small packets called photons. This explains how light can travel through the vacuum of space and the phenomenon known as the Photoelectric Effect.
The Photoelectric Effect describes how light shining on a metal plate can give electrons enough energy to escape. Bright light has more energy than dim light, but interestingly, dim blue light can dislodge electrons while bright red light cannot.
- Low-frequency light (red/orange) cannot eject electrons.
- High-frequency light (blue/violet) can eject electrons.
Thinking of Light as Rays
Instead of individual particles, we can also think of light as rays. This model helps visualize the detection of light and its straight-line nature.
Waves: Mechanical & Electromagnetic
Waves can be categorized as either mechanical or electromagnetic. They can also be:
- Longitudinal: Particle and wave motion are parallel.
- Transverse: Particle and wave motion are perpendicular.
The Wave Nature of Light
Light behaves like a transverse, electromagnetic wave. It exhibits properties such as reflection, diffraction, and refraction.
Wave-Particle Duality
Light exhibits a fascinating duality: it behaves as both a wave AND a particle. This concept, theorized in 1905, has been consistently confirmed through experimentation.
The Electromagnetic Spectrum
Particles, such as electrons and photons, create changing electric and magnetic fields as they move. These changes generate electromagnetic (EM) waves, which we can perceive as light.
- Warmer objects emit more energy.
- Colder objects emit less energy.
The Speed of Light
The speed of light (c) is a fundamental constant, approximately 3 x 108 m/s (300,000,000 m/s). While constant in a vacuum, the speed of light can change depending on the medium it travels through.
We can use the following equations:
- c = d/t (speed = distance/time)
- c = fλ (speed = frequency * wavelength)
Energy and Light
At a constant speed, as frequency increases, wavelength decreases. This relationship is tied to energy:
- E = hf (Energy = Planck’s Constant * frequency)
- h = Planck’s Constant (6.626 x 10-34 Js)
Energy Levels of Light
We can categorize light based on its energy levels:
Low Energy
- Radio Waves (including Microwaves) have such low energy that it’s often not even reported.
Introducing the Electron Volt (eV)
A new unit of energy, the Electron Volt (eV), is often used for light: 1 eV = 1.60218 x 10-19 J
Even with this new unit, low-energy light contains less than 1 meV (10-3 eV).
Medium Energy
- Microwaves
- Infrared
- Visible Light
- Ultraviolet
- Range from 0.001 eV to 10 eV (1.6 x 10-22 J to 1.6 x 10-18 J)
High Energy
- Ultraviolet
- X-Rays
- Gamma Rays
- Energies greater than 10 eV
- Also known as ionizing radiation, with enough energy to rip electrons from atoms and molecules.
The Effect of Mediums on Light
Just like sound waves, light waves change speed in different mediums. Light travels fastest through a vacuum and slowest in a dense medium like diamond due to the amount of refraction.
Interactions of Light Waves
Superposition of Light
Multiple light waves can interact with each other in a phenomenon called superposition. When two waves occupy the same space, they add together.
- Above equilibrium (crest) = positive
- Below equilibrium (trough) = negative
Superposition plays a crucial role in reflection, diffraction, and refraction, shaping how we perceive light.
Reflection
Waves hit a boundary and bounce back. Bubbles, for example, exhibit multiple reflections interfering with each other (superposition).
Diffraction
Waves hit a corner of a boundary and bend around it. This is demonstrated in Young’s slit experiment. Diffraction gratings produce rainbows because different wavelengths of light bend (diffract) at different angles. Longer wavelengths bend more than shorter wavelengths.
Polarization
Polarization is related to diffraction and relies on the perpendicular nature of the electric and magnetic fields of light. Polarized glasses, for example, only allow vertically oriented light waves to pass through.
Refraction
Waves hit a new medium and either speed up or slow down. Light bends at a larger angle when it travels faster in a medium.
Mirages
Mirages occur due to the refraction of light in different air densities. Light travels faster in cold air than in warm air. There are two types of mirages:
- Inferior Mirage: Appears below the actual object.
- Superior Mirage: Appears above and reflected.
Dispersion
Refraction is also wavelength-dependent. Blue light refracts more than red light because of its shorter wavelength, causing it to bend at a greater angle.
Rainbows
Rainbows form when sunlight enters raindrops and refracts back into our eyes at a specific angle. Each raindrop reflects a different wavelength of light, creating the spectrum of colors we see.
Fiber Optic Cables
Fiber optic cables utilize both reflection and refraction. Light follows the curve of the cable due to total internal reflection, bouncing off the interface between the core and the cladding.
Key Concepts Summarized
- Shorter Wavelength = Higher Frequency
- Longer Wavelength = Lower Frequency
- Violet light has the highest energy level.
- As wavelength increases, energy decreases.
- As frequency increases, energy increases.
- Light can be transmitted, reflected, absorbed, or passed through completely (transparent).
- Opaque objects block light.
- Luminous objects emit their own light, while illuminated objects reflect light.
- Black absorbs all colors, while white reflects all colors.
- Lenses have limited resolution due to dispersion.
- Rough surfaces cause diffuse reflection.
- Perpendicular surfaces block light most effectively.
- Cell phones and digital communication use short bursts of radio waves, which are more efficient than the long, continuous waves used for traditional phone calls.