1. Waves: Light

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

3. 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.

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

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

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

7. 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

8. 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

9. 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.

10. 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.

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

12. 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

13. 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

14. Electromagnetic Spectrum:

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

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

17. 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

18. 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

19. 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.

20. 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.

21. 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.

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

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

24. 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.

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

26. 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