In Music, Wavelength (λ) Is Pitch

Low Pitch: Long Waves High Pitch: Short Waves

demo: tin whistle

In light, Wavelength (λ) Is Color

The length of light waves determines their color Red Light: Long Waves (λ=700 nm) Blue Light: Short Waves (λ=400 nm)

(1 nm=10-9 m)

c = λ * f

The Electromagnetic Spectrum

Radio waves: f ranges from 106 Hz to 109 Hz. (1-1000 MHz; λ = 30 centimeters-meters). Lowest-frequency EM waves. Transmit Radio & TV broadcasts & Cell phone, wireless internet, Bluetooth.

Microwaves: f = 109 Hz - 1012 Hz (λ = 1 mm to 30 cm) Cooking, RADAR & telecommunications.

Infrared: f =1012 Hz - 1014 Hz. (λ = 700 nm- 1mm) are felt as heat by humans. Remote controls, night vision

Visible: f=[4.3 - 7.5] x 1014 Hz (λ = 400 nm {violet} - 700nm {red})

Photobooth TVremote

Long Wavelengths:

Video cameras can detect IR light Check it out w/ Skype or Zoom! https://www.youtube.com/watch?v=Fg2P6nHcOWw

Ultraviolet: f= 7.5 x 1014 Hz to 1017 Hz. (λ = 10 nm-400 nm) sunburn, skin cancer. Insects & flowers. “Black light”

X-rays: f=1017 Hz to 1020 Hz. (λ = 0.01 - 10 nm) medical imaging. Imaging crystals.

Gamma rays: highest frequencies:above 1020 Hz. (λ<0.01 nm) Very high energy, produced by nuclear reactions. Can kill living cells & are used to kill cancer cells & harmful bacteria in food.

Short Wavelength:

Our Sun produces mostly “visible” light.

But some animals can detect non-viable light. Eg:

Snakes sense IR, Bees see UV.

Visible Light UV Light

Doppler Effect

Doppler Effect• Since light is a wave, it shows wave phenomena

• The Doppler Effect is a wave phenomenon

• Example: Sound waves are compressed to higher freq. when a car drives toward you. (lower pitch when away)

• The Doppler Effect depends on the relative speed of the observer and emitter. (u)

• The frequency of light we receive from (f’) is:

• u = speed of emitter

• f = original frequency

• +- = toward or away from the observer.

doppler ballhttps://www.youtube.com/watch?v=JX_A99Bq9AI

• Example: Frequencies of light coming from the Andromeda galaxy are found to be 0.1 % higher than normal.

• How fast is Andromeda moving? Is it moving away?

Δf = f’ - f = +- f (u/c)

*detected by comparing a spectral line of Hydrogen seen in Andromeda’s

•NOTE: Doppler effect only measures motion toward or away from the observer.

Doppler Effect causes a change in frequency of waves:

(f’ - f ) / f = + - (u/c)

If we divide by the original frequency, f, we get the fractional change:

A “Doppler Gun” measures speed.

Light is emitted at frequency: fIt is ‘seen’ & reflected by a moving object (car, baseball..) at frequency: f’It returns to the gun at frequency: f’’

EXAMPLE [Ch. 25 #24]: A state highway patrol car unit uses a frequency of 5 x 109 Hz. What frequency difference will the unit detect from a car receding at speed 30.0 m/s?

Another Doppler Application: Spacecraft Communication

https://www.esa.int/ESA_Multimedia/Videos/2014/07/Doppler_effect_-_classroom_demonstration_video_VP05

Since the light is reflected back to the receiver, we must apply the Doppler Effect twice.

Optional topic: Polarized Light (not on exam)

ONE wave of light.

Copyright © 2010 Pearson Education, Inc.

Chapter 26

Geometrical Optics

A rock dropped in a lake makes circular waves, or “fronts”, moving outward.We can draw lines, or “Rays”, perpendicular to these ripples. These Rays show the direction of wave motion.

A small source sends out light rays in all directions.

At a long distance from the source of light, the wave fronts appear as flat planes (not curved)

The rays are straight lines.

Geometric Optics: analyzing light as rays.

(c) C. McCarthy

(c) C. McCarthy

An OBJECT emits light, which strikes a Mirror, smooth water, etc. The rays will reflect parallel to each other.

Looking at these rays, we will see an IMAGE

Goal of Geometric Optics: find the image.

Two Types of Reflection

Figure 26-4B

Most surfaces reflect light diffusely, eg. a piece of paper. (no image)

The eye interprets rays as having had a straight-line path, and sees an image behind the mirror.

Since no actual light exists at this location, the image is said to be

“virtual.”

Images in a Plane Mirror

imageobject

The Angle of Incidence = Angle of Reflection

Note: Angles are measured with respect to the normal to the surface.

An object is a distance do from the mirror.At what distance (di) does the Image Appear? Use the Law of Reflection and some geometry.

Therefore,do = di

Object Image

Full Length Mirrors are not needed. The bottom part is never used.