LightLight27.4-27.627.4-27.6

Review

• Are light waves and sound waves part of the same scale?– No. Sound waves are mechanical, need a medium, and

vibrate matter. Light waves are part of the electromagnetic scale and do not need a medium.

• What is the speed of light?– 300,000 km/s

• Does light make up a large part of the electromagnetic spectrum?– No. Less than 0.1%

Objectives

1. Explain how the f of light affects what happens when it enters a substance.

2. Interaction of UV rays and glass

3. Opaque vs. transparent materials

4. Solar and lunar eclipses

What is light?

• Light:– vibrating electric charges

Light interacts with matter

• Just as a sound wave can force a sound receiver into vibration, a light wave can force charged particles in materials into vibration

• When incident upon matter --> electrons in matter vibrate

Response of Receiving Material

• Depends on f of light & natural f of material

• Light acts differently when it is incident upon a window compared to a rock

Transparent Materials• Transparent:

– Allows light to pass through

• Visualize this…• Electrons of an atom in glass connected to springs (fig 27.6)• Light wave hits them they vibrate

Transparent MaterialsCan UV light pass through glass?

• E’s in glass have same natural f as UV B light• What happens when you have two things with identical natural

frequencies?• Resonance!• Large amplitude causes atoms to hold on to energy for long time

(100 millionth of a second!)• Atom make many collisions with other atoms --> gives up energy

in form of heat• Most UV B light can NOT pass through!

Transparent Materials Can UV light pass through glass?

1. UV B & glass --> same natural f2. Resonance3. Atoms collide4. Energy loss --> heat5. Most UV B light can’t pass!

Does that mean I can’t get sunburned through glass?

NO!•You CAN get sunburned through glass

•Let’s learn the basics about UVA and UVB rays first

UVB rays

•UVB rays:•Short wavelength (290-320 nanometers)•Responsible for sunburn•Main cause of basal cell and squamous cell skin cancers•Most of these rays are absorbed by glass, but NOT ALL!

UVA Rays•UVA rays:•Long wavelength (320-400 nanometers)•Penetrates more deeply on skin•Responsible for aging, wrinkling and leathering (breaks down collagen and elastin)•Exacerbates UVB’s effects•May directly cause melanoma and other cancers•Passes through glass!

Bottom Line

• You can get sunburned through glass, just not as quickly

• Glass is just like sunscreen, it protects you pretty well, just not perfect

• Most glass blocks out UVB radiation, but not UVA

• Where sunscreen that protects against both UVA and UVB radiation!

Transparent Materials Visible light Has Lower Natural Frequencies than UV light

• Visible light• Smaller amplitudes -->• Atom holds energy for less time• Less collision• Energy reemitted as visible light w/ slight time delay

Transparent Materials (glass) Light Has Lower Natural Frequencies

• Visible light -->

• Lower f & smaller amplitude -->

• Less collisions -->

• Reemitted from glass with slight delay

Why the time delay in glass?

Checkpoint1. What happens when light is incident upon matter?

• Causes the electrons to vibrate

2. Why are most UVB rays not able to pass through glass?

• UV has same natural frequency as glass, atoms resonate and collide, turned into heat.

3. Why is there a time delay when visible light passes through glass? And which color, red or violet, takes longer to get through the glass?

• there is a delay in between atoms in the glass. Light wave hits one atoms, it vibrates, then spits out the light to the next atom . . .etc. Violet takes longer to get through the glass b/c it’s natural frequency is closer to glass than red light.

When light is incident upon a material it can . . .

1. Bounce off (reflect)

2. Be transmitted through

3. Be absorbed by the material

Opaque Materials• Opaque:

– Absorb light w/o reemission

• What happens if energy is not reemitted?

• Vibrations given to electrons by incident light can last for a long period of time --> many collisions --> energy converted to heat

• Material becomes warmer

Atmosphere and Clouds

• Does our atmosphere let UV light in?

• Some, but most absorbed by ozone

• Can we get sunburned on a cloudy day?

• Yes, clouds are semitransparent to UV light

Why is the sky blue? Why are sunsets red?

• Why is the sky blue and a sunset red?

Water tank/powdered milk demo

Checkpoint• Why is a black tar road hotter to the touch than a

pane of window glass?– Sunlight is absorbed and converted to internal energy

in the road surface, but is transmitted through the glass to somewhere else

• Why is glass transparent to visible light, but opaque to most UVB rays?

• Natural frequency of electrons in the glass match that of uv light --> resonance occurs. This generates heat instead of wave remission --> opaque to uv light.

• Different frequency for visible light --> transmits, not absorbed

Shadows•Ray:

•thin beam of light•Shadow:

-formed where light rays can NOT reach

Umbras vs. Penumbras

• Umbra:– Total shadow

• Penumbra:– Partial shadow

– Some light blocked, other light fills in

Solar Eclipse

• Moon between earth and sun

• Moons shadow barely reaches earth

• If directly in umbra--> total darkness

• If in penumbra --> partial eclipse

• When can I see one?• Flash animation on cd-

ROM (Ch. 26 Properties of Light fig. 26-12 and 26-13)

Lunar Eclipse• Earth between sun and moon• Earth casts shadow to space• Which are more common?