Waves: Light

What is a wave? A wave is a disturbance involving the transfer of energy from place to place.

What is a medium? The material through which a wave travels is called a medium. Most kinds of waves travel through a medium (these are also called mechanical waves). For example, sound waves travel through air. Not all waves require a medium though. For example, light from the sun can be carried through empty space. Waves that can travel without a medium are called electromagnetic waves.

What causes waves? Mechanical waves (the ones that require a medium) are produced when a source of energy causes a medium to vibrate.

What is a vibration? A repeated back-and-forth or up-and-down motion.

Types of waves: Mechanical waves are classified by how they move There are two types of mechanical waves: transverse waves and longitudinal waves

Transverse Waves: Waves that move the medium at right angles to the direction in which the waves travel are called transverse waves As a transverse wave moves, the particles of the medium move at a right angle to the direction of the wave

Transverse Waves: For example, when you make a wave on a rope, the wave moves from one end of the rope to another. The rope itself moves up and down at right angles to the direction in which the wave travels. The high part of a transverse wave is called a crest, while the low part of a transverse wave is called a trough. crest trough

Longitudinal Waves: move the medium parallel to the direction in which the waves travel For example, when you stretch out a spring toy and push and pull one end, you can produce a longitudinal wave. The coils in the spring move back and forth parallel to the wave motion. The parts where the coils are close together are called compressions while the parts where the coils are spread out are called rarefactions.

Longitudinal Waves: Sound is a longitudinal wave In air, sound waves cause air particles to move back and forth In areas where the particles are pushed together, compressions form In between the compressions, particles are spread out. These are rarefactions.

Basic Properties of Waves: Amplitude Wavelength Frequency Speed at a certain speed In certain amount of time is frequency

In certain amount of time is frequency Amplitude-the maximum distance the medium moves up or down from its rest position (from rest to crest or from rest to trough) Wavelength-distance between two corresponding parts of a wave (from crest to crest or trough to trough) Frequency-number of complete waves that pass a given point in a certain amount of time Speed- wavelength x frequency at a certain speed In certain amount of time is frequency The speed, wavelength, and frequency are all related

Electromagnetic Waves: Electromagnetic waves are transverse waves that transfer electrical and magnetic energy All electromagnetic waves travel at the same speed in a vacuum (the speed of light), but they have different wavelengths and different frequencies. Turn to page 579 and 584

Electromagnetic Spectrum:

Radio waves: Electromagnetic waves with the longest wavelengths and lowest frequencies. For example, broadcast waves (AM and FM radio) and microwaves

Infrared waves: Electromagnetic waves with wavelengths shorter than those of radio waves For example, heat lamps and infrared cameras

What is visible light? Radiation in the wavelengths that your eyes can see is called visible light Only a small portion of electromagnetic radiation is visible light The rest of the wavelengths are invisible

Visible light The Sun is the dominant source for visible light waves our eyes receive. What appears as white light from the sun is actually a mixture of many colors These can be separated by a prism

Visible light When waves enter a new medium, the waves bend or refract. The prism refracts different wavelengths of visible light by different amounts and separates the colors. Red light waves refract (bend) the least. Violet light waves refract the most.

Visible Light: Rainbows When white sunlight hits a collection of raindrops at a fairly low angle, you can see the component colors red, orange, yellow, green, blue, indigo and violet -- a rainbow.

How is it possible to see an object? For an object to be seen, light must come directly from the object or from an external source (luminous objects like the sun or illuminated objects like the moon) The light must be reflected from the object and enter the eye. It is impossible to see an object in the absence of light.

Why do we see different colors? Objects absorb some colors and reflect others.

Visible Light: Spectroscope An instrument used to observe the spectra of light sources.

Ultraviolet Rays (UV): Electromagnetic waves with wavelengths shorter than those of visible light Can be healthy in small doses: skin cells use UV rays to produce Vitamin D; Hospitals use UV rays to sterilize hospital equipment and kill bacteria & viruses. Overexposure can be harmful: kills healthy cells, leading to skin cancer. We are protected by the ozone layer.

X-Rays: Electromagnetic waves with wavelengths shorter than those of ultraviolet rays Have higher energy and greater penetrating power. For example: dental x-rays

Gamma rays: Electromagnetic waves with the shortest wavelengths and the highest frequencies These have the highest penetrating power Uses: to kill cancer cells in the body, to examine body’s internal structure