Chapter 23: Light and Reflection

Electromagnetic Spectrum

The range of electromagnetic waves, arranged by either increasing frequency or wavelength.

Incandescence

An object is heated until it glows, emitting light.

Gas-Discharge Tubes

Sealed glass tubes with electrodes and a gas inside that emit light when a large potential difference produces a current in the tube.

Laser

A device that produces coherent light, combining multiple identical in-phase waveforms to produce a single energetic electromagnetic wave.

Light-Emitting Diode (LED)

A semiconductor device that emits light when a potential difference is established across it.

Cold Light

Light produced by chemical reactions involving a minimum of heat, such as bioluminescence.

Light as Energy

• Light is a form of energy that can do work and be converted to other forms of energy.
• All electromagnetic waves travel at the same speed in a vacuum: 3.00 × 10⁸ m/s.
• Radio waves are the longest waves, and gamma rays are the shortest waves.
• Waves with shorter wavelengths have higher frequencies and greater energies for a given amplitude.

Types of Electromagnetic Energy

• Radio Waves: Produced by accelerating charges in a conductor or conducting medium. The highest frequency radio waves are called microwaves.
• Infrared Waves: Have less energy than the red light of the visible spectrum. All matter emits infrared waves.
• Visible Light Waves: Found in the narrow band from 4.0 × 10¹⁴ to 7.7 × 10¹⁴ Hz (750–390 nm). There are millions of colors from deep red to deep violet.
• Ultraviolet Waves: Have more energy than the violet light of the visible spectrum. There are three bands of UV light.
• X-Rays: Able to penetrate solid matter. Most solar and cosmic x-rays are absorbed by the atmosphere.
• Gamma Rays: Produced by high-energy changes in subatomic particles. These can be very damaging to human tissue.

The Speed of Light

• Galileo tried to determine the speed of light using lanterns, but it didn’t work.
• Roemer studied eclipses of the moons of Jupiter to get a reasonable estimate.
• Fizeau used a notched wheel and a mirror five miles away, with an error of only 4%.
• Foucault was the first to determine the speed of light in metric terms (in 1862) using a terrestrial method.
• Hertz made a device that could produce and receive radio waves. All electromagnetic waves propagate with the same speed, c, in a vacuum: c = 3.00 × 10⁸ m/s.
• Electromagnetic waves have their highest speed in a vacuum. A medium’s magnetic permeability (μ) and electric permittivity (ε) affect its speed through the medium: v_med = 1 / √(με).

Example: Determining the Speed of Light

A beam of orange light travels through pure water at 20 °C. Water at this temperature has an electric permittivity of 1.77ε₀ and a magnetic permeability of 1.00μ₀ for this color of light. What is the speed of light through water under these conditions?

ε = 1.77ε₀

μ = 1.00μ₀

c = 3.00 × 10⁸ m/s

v_med = 1 / √(με)

v_med = 1 / √(1.00μ₀ × 1.77ε₀)

v_med = 1 / √(1.77) × c

v_med = 1 / √(1.77) × 3.00 × 10⁸ m/s

v_med = 2.25 × 10⁸ m/s

Questions for Students

1. Define the electromagnetic spectrum and its significance.
2. Explain the different types of electromagnetic energy and their characteristics.
3. Describe the methods used to determine the speed of light.
4. Calculate the speed of light in a medium given its electric permittivity and magnetic permeability.
5. Discuss the various sources of light and their applications.