Properties of Light Waves

M. Rocha  

Physics 2B

Refraction, Reflection, Diffraction, Interference and Polarization

Refraction

Wave Refraction

The bending of waves due to change of speed

Refraction

Light rays refract (bend) when they pass from one medium to another at an oblique (not straigth) angle

Refraction is the result of waves changing speed as they cross from one medium to anoter

Direction of Refraction

Waves bend towards the normal when going from fast to slow

 and

away from the normal when going from slow to fast

Refraction of light is frequency dependent

This is because higher frequencies travel slower inside the prism

Slowest

Fastest

Total Internal Reflection

When refraction angle exceeds 90º from the normal the light does not cross the interface

Fiber optics

Total internal reflection causes light to reflect inside a solid glass/plastic tube

Atmospheric Refraction

Wave fronts of light travel faster in the hot air near the ground, thereby bending the rays of light upward

Lenses

Lenses

Curved surface of a convex lens causes light rays to converge, magnifying images

Convex Lenses

Curved surface of a concave lens causes light rays to diverge, shrinking images

Concave Lenses

Lenses

Thin Lens Equation

Checkpoint

o = 10cm

i = 30cm

What is the focal length of the lens in the system below?

Virtual vs. Real Images in Lenses

Real Image

Virtual Image

Reflection

Reflective Materials

In metals, the outer electrons of atoms are not bound to any particular atom. When light shines on metal and sets these free electrons into vibration, their energy is reemitted as light

Law of Reflection

The angle of incidence equals the angle of reflection

This Is the result of Fermat's principle of least time

Diffuse vs. Specular Reflection

Diffuse

Specular

Incoming light scattered in all directions

Incoming light reflected in one direction

Checkpoint 

Which person in the front row sees the guy with the hat (person F) in the mirror?

Reflections from Transparent Surfaces

Transparent surfaces such as water and glass reflect some light

When the reflected light is more than the transmitted light from the other side they look like mirrors

Reflections from Transparent Surfaces

A one way mirror is just a clear glass window

Mirrors

Mirrors are the result of specular reflection

Tracing light rays from original, to mirror, to eye allows us to construct the image

Mirrors

To observer at point B, the light from point A seems to come from point A’, within mirror

Non-Flat Mirrors

Image in a curved mirror is distorted from original

Convex Mirrors

Image from convex mirror is smaller and closer than original

Convex Mirrors

Image from convex mirror is smaller than original

Concave Mirrors

Image from concave mirror is larger and farther than original if object close to mirror

Concave Mirrors

We use concave mirrors to build telescopes in order to focus the light to the detector

Virtual vs. Real Images in Mirrors

Real Image

Virtual Image

Diffraction and Interference 

Wave Diffraction

Diffraction is the bending of a wavefronts as they pass the edge of an object

Huygens' Principle

Every point of a wavefront may be considered the source of secondary wavelets that spread out in all directions

Huygens' principle explains wave diffraction

Wavelength and Diffraction

Diffraction is greater when the wavelength is large compared to the size of the object or aperture

Checkpoint 1

Two light rays of different wavelength (one blue and one red) go through a small slit opening. For which one would you expect stronger diffraction?

Wavelength and Diffraction

Diffraction is greater when the wavelength is large compared to the size of the object or aperture

Long wavelength (> 100 m) radio waves diffract more from mountain tops, reducing shadow regions 

Short wavelength (< 100 m)

 radio  waves diffract less, casting shadow regions. 

f < 3 MHz (AM, VLF, LF, MF )

f > 3 MHz (FM, HF, VHF)

Diffraction Interference Pattern 

Wave Interference in 2D

Diffraction and Interference

Constructive Interference

Destructive Interference

Diffraction and Interference

Interference Patterns are Wavelength Dependent

IF                            THEN constructive interference 

IF                             THEN  destructive interference

Where            is the difference in distance traveled between waves from different sources

and  n = 1, 2, 3, ... is an integer number

Checkpoint 2

By how much should a pair of monochromatic light rays differ in distance traveled to produce destructive interference?

Interference Pattern as a function of wavelength

http://www.vtorov.com/interf.htm

Diffraction Gratings

A multitude of closely spaced parallel slits makes up what is called a diffraction grating

a diffraction grating separates colors by interference

Interference From Thin Films

Interference From Thin Films

As the thickness of the soap layer changes, different different wavelengths of light are canceled

Checkpoint 3

If the rays of monochromatic light from the top surface of a thin film travel one full wavelength less than the rays from the bottom surface, will there be destructive interference or constructive interference?

 Polarization

Polarization

If a rope is shaken up and down, a vertically polarized wave is produced.

If a rope is shaken from side to side, a horizontally polarized wave is produced

A vertically vibrating electron emits vertically polarized light.

A horizontally vibrating electron emits horizontally polarized light

Polarization

Polarization

Reflection of light from non-metallic surfaces results in polarization parallel to the surface  

Most glare is horizontally polarized 

Polarized Light and 3-D Viewing

A 3-D slide show uses polarizing filters. The left eye sees only polarized light from one projector; the right eye sees only polarized light from the other projector

The End

Checkpoint 

If you want to spear a fish from above the water, do you have to aim higher or lower?