Light

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 Materials Can 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 f 2.Resonance 3.Atoms collide 4.Energy loss --> heat 5.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 ( 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 ( 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?

Checkpoint 1.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?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 and )

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