Electromagnetic Waves and Light

Electromagnetic Waves and Light • EM Waves and the speed of light • The EM Spectrum • Reflection • Refraction • Visible Light • Wave Nature of Light

Some light bounces off the surface: Reflection Some light travels into the other material:Refraction Some light is absorbed by the material Light Hitting a Boundary

Reflection • Light bounces off an object • Two kinds: regular (or specular) and diffuse Specular: Reflection off a very smooth surface (like a mirror) -- Light comes in, bounces off in 1 direction -- What we usually call reflection Diffuse: Reflection off irregular surfaces -- Light comes in, bounces in various directions -- How we see most objects

Reflection • Angle of incidence = angle of reflection • Angles are measured from the normal (perpendicular) Normal θi θr

Mirror Images • Images that a flat mirror creates are called virtual images • This means that the image is not a real image– would not appear on a screen if it were placed there • We see an image, it seems to be behind the mirror, but there’s no real image at that spot Object Image

Drawing Mirror Images • Draw incident and reflected rays from object to observer • (Make sure the angle of incidence = angle of reflection) • Project a line from the observer through the mirror to find the direction of the image • Distance from mirror to object is same as distance from mirror to image (behind the mirror) Object Image Object Distance = Image Distance

Drawing Mirror Images • Or, you can draw the position of an object by extending the rays of multiple observers • They will intersect at the image location Object Image Object Distance = Image Distance

Drawing Mirror Images • Just repeat for different parts of one object Object Image Object Distance = Image Distance

Concave Mirror • Concave: “like a cave” • Near objects look bigger • Far objects look upside down • No image in the middle Object Image Object Distance = Image Distance

Curved Mirrors • Two key parts: • Principal axis: the axis of symmetry • Focal point: where all rays parallel to axis of symmetry will reflect towards f

Finding Images on Curved Mirrors • Draw two sets of rays from object • One parallel to axis, reflecting through f • One through f, reflecting parallel i i f Image will be at the intersection

Convex Mirror • Objects look smaller Object Image Object Distance = Image Distance

Electromagnetic Waves and Light • EM Waves and the speed of light • The EM Spectrum • Reflection • Refraction • Wave Nature of Light • Visible Light

Some light bounces off the surface: Reflection Some light travels into the other material:Refraction This light goes in a slightly different direction because the speed of light is different in some materials Light Hitting a Boundary

Speed of Light • In a vacuum: • About 3 x 108 m/s in a vacuum • Symbol: c • Can be considerably less in different materials

Speed of Light Index of refraction: n = _______c___________ speed of light in material • Water n =3 x 108 = 1.333 2.25 x 108 • Higher n: slower medium

In air: v = c = 3 x 108f = 6.4 x 1014 Hzλ = v/f = 4.7 x 10-7 m In glass: v = c/1.67 = 1.8 x 108f = 6.4 x 1014 Hzλ = v/f = 2.8 x 10-7 m Does your swimsuit change color underwater? Speed Changes, Frequency Doesn’t • Consider blue light entering glass Air (n=1) Glass (n = 1.67)

Reading assignment • Read “The Light of Other Days” (excerpt) • Write a paragraph on how something like “slow glass” could ever exist. Explain in terms we’ve used to discuss refraction. • How slow do you think a material could make light travel? (1/2 c? 1/10 c? smaller?)

Some of the light reflects Refraction: How Light Bends • When light waves enter a new medium, the rays bend Air Glass Entering slower medium, light bends toward the normal Air Entering faster medium, light bends away from the normal Applet Applet 2

Refraction: Calculating the Angles • Measure the angles from the normal (perpendicular) • Snell’s Law n1 sin(θ1) = n2 sin (θ2) Here, if θ1 = 45º, 1.0 * sin(45º) = 1.67 sin (θ2) θ2= 25º Air n = 1.0 θ1 Glass n = 1.67 θ2

Snell’s Law Lab

Mirages • Hot day: the ground absorbs light from the sun and radiates heat • Really hot low, less hot higher COOLER AIR (light moves slower) HOT AIR (light moves faster) HOT ROAD

Mirages • Since light travels faster in hotter air, the light curves • Sky and tree appear to be reflected off the road • We think there’s water doing the reflecting COOLER AIR HOT AIR HOT ROAD

Mirages

Refraction • What happens when light moves from dense to less-dense? • Light bends away from normal Air n = 1.0 θ2 Glass n = 1.67 θ1 Example

What if it’s already a big angle? • Snell’s Law n1 sin(θ1) = n2 sin (θ2) Here, if θ1 = 45º, 1.67 * sin(45º) = 1.0 sin (θ2) 1.18 = sin (θ2) Error: Sin can’t ever be more than 1! So it CAN’T refract! Air n = 1.0 θ2 Glass n = 1.67 θ1

TIR Applet

Total Internal Reflection • When angle of incidence is large, no light refracts • All light is reflected • Can bounce on practically forever • The idea behind fiber optics Air n = 1.0 Glass n = 1.67 θ1 Example

Remember: • Snell’s Law: n1 sin(θ1) = n2 sin (θ2) • When you go from fast medium (air) to slow medium, light bends toward the normal • When you go from slow medium to fast medium, light bends away from the normal • Frequency doesn’t change: red light in means red light out (but wavelength changes!)

Rearview mirror: Reflection and refraction • In the rearview mirror, you’re just toggling between • seeing the reflection off the mirror on the back surface • and seeing the reflection off the front surface of the glass

The index of refraction is slightly different for different colors (dispersion) Blue light travels more slowly in glass than red light So when white light passes through a glass prism, blue is bent more than red. Dispersion and prisms

Dispersion in Rainbows • White(ish) light from the sun refracts in water droplets, separating the spectral colors • The higher you look, you see the less-refracted (longer wavelength) colors

Electromagnetic Waves and Light