Light

April 3, 2014

EM SpectrumWhat we call “light” is merely a small fraction of the total electromagnetic spectrum.

The electromagnetic spectrum Consists of transverse waves of varying frequenciesOccur due to oscillation of electric and magnetic fieldsDo not require a mediumTravel at the speed of light (c = 3.00 X 108 m/s) in a vacuum

Like any wave, an EM wave carries what?

Energy!

VisionAncients believe that we could see because of light beams that left our eyes and hit objects. What day-to-day experiences tell you that isn’t so?

VisionWe see when light when visible light enters our eyes and focuses on our retinas. The light may come from a luminous source (something that emits light, such as the sun or a light bulb) or may bounce (reflect) off an illuminated object.

Light

Light

Light

Luminous source

Illluminated object

VisionWe have photoreceptor cells in our retinas that send a signal to our brain whenever light hits them. • Rods are sensitive to all wavelengths of visible light

provide black and white vision

• Cones are sensitive to only certain wavelengths provide color vision

We have three cones types: blue, green, red.

Our brain interprets color based on the combinations of cones that are triggered by certain light. … if only the red cone is triggered, we see

… if red and green are triggered equally, we see

red

yellow

VisionDifferent materials absorb, reflect, and/or transmit different wavelengths of light.

• Absorption – light energy that has the same natural frequency as the electrons in the object will be absorbed – the energy will be converted into vibrational motion of the electrons.

• Reflection – the light ‘bounces off’ the surface of the object• Transmission – the light goes through the object and out the

opposite side

Which type of light hits our eyes? Reflected or transmitted light!

Transparent objects transmit most of the light that hits them. Translucent objects transmit some of the light that hits them.Opaque objects reflect or absorb most of the light that hits them.

VisionHow would the following objects appear?1. An object that reflects blue and green light; absorbing other

wavelengths.

2. An object that reflects all visible light waves.

3. An object that absorbs all visible light waves.

4. An object that reflects pink light but transmits all other wavelengths.

5. An object that transmits most of the light hitting it.

•

Opaque blue-green

Opaque white

Opaque black

Translucent pink

transparent

Barrier Behavior of WavesWaves of any sort – light, sound, water, etc. – exhibit different behaviors when they encounter a barrier.

• Reflection – bouncing back of a wave as it encounters a new medium

• Refraction – the bending of a wave as it transmits through a different medium

• Dispersion – The separation of a wave into its component frequencies

• Diffraction – the spreading of a wave behind an obstruction

Law of ReflectionAngle of reflection is

equal to angle of incidence.

All waves, including light, sound, water obey this relationship, the law of reflection.

(the angles are measured to the normal to the barrier).

Reflection

i = r

When a wave passes from one medium to another, its velocity changes. The change in speed results in a change

in direction of propagation of the refracted wave.

Refraction

As a toy car rolls from a hardwood floor onto carpet, it changes direction because the wheel that hits the carpet first is slowed down first.

Visualization of refraction

When a wave passes from one medium to another, its velocity changes. The change in speed results in a change in direction of propagation of the refracted wave.

Light – faster in air than water

Sound – Faster in water than air

Frequency is determined by the source so it doesn’t change. Only wavelength changes. Wavelength of the same wave is smaller in the medium with smaller speed.

1 2

1 2

v vf = =

λ λ

Refraction

θ1

θ2

is constant for the given frequency. The Snell’s law is of course valid for all types of waves.

Greater speed – greater angleSlower speed – slower angle

Snell’s LawWe can predict the degree of refraction using Snell’s Law.

Snell’s Law states that the ratio

Turn & Talk

1) Why is there no bending of light in the first animation?

2) Is the block a slow medium or a fast medium? Justify your response.

www.le.ac.uk/ua/mjm33/wave2/images/Snell.gif More Refraction animations!

Exit Ticket

1) You see a translucent purple ruler. What frequencies of light are absorbed? What are reflected? What are transmitted?

2) Draw a reflected wave, labeling the angle of incidence and angle of reflection. Also compare the size of the two angles.

3) Draw a light wave passing through two different mediums, first fast then slow. Label the angles of incidence and angles of refraction.