Light

EM Spectrum What we call “light” is merely a small fraction of the total electromagnetic spectrum. The electromagnetic spectrum Consists of transverse waves of varying frequencies Occurs due to oscillation of electric and magnetic fields Does not require a medium Travels at the speed of light (c = 3.00 X 108 m/s) Longer (lower frequency) waves carry less energy than shorter (higher energy waves) Light is both a wave and a particle (called photon) but today we will focus on the wave characteristics of light. Like any wave, an EM wave carries what? Energy!

Vision Ancients 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? We can’t see in a dark room Iris contracts in bright light, dilates in dark areas

Vision We 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 Longer (lower frequency) waves carry less energy than shorter (higher energy waves) Light is both a wave and a particle (called photon) but today we will focus on the wave characteristics of light. Luminous source Light Light Illluminated object

Vision We 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 Ask questions: If only red cone is triggered, what color do we see? If red and green are triggered equally, we see… , etc. red yellow

Vision Different 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. Reflection – light is briefly absorbed then reemitted by atoms on the surface of the object Transmission – light energy is passed molecule to molecule through the medium, then reemitted on the opposite side.

Vision How would the following objects appear? An object that reflects blue and green light; absorbing other wavelengths. An object that reflects all visible light waves. An object that absorbs all visible light waves. An object that reflects red light but transmits all other wavelengths. An object that transmits most of the light hitting it. Opaque blue-green Opaque white Opaque black Translucent red transparent

Barrier Behavior of Waves Waves 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

Reflection Law of Reflection Angle of reflection is equal to angle of incidence. i = r (the angles are measured to the normal to the barrier). All waves, including light, sound, water obey this relationship, the law of reflection.

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.

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. Visualization of 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.

Refraction light waves sound waves Use the car example to predict motion! 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.

Snell’s Law Snell’s Law states that the ratio θ1 We can predict the degree of refraction using Snell’s Law. Snell’s Law states that the ratio θ1 is constant for the given frequency. The Snell’s law is of course valid for all types of waves. Derived experimentally in 1621, can also be proved using geometry N1sin1 = n2sin2 θ2 Greater speed – greater angle Slower speed – lower angle

More Refraction animations! www.le.ac.uk/ua/mjm33/wave2/images/Snell.gif Turn & Talk Why is there no bending of light in the first animation? Is the block a slow medium or a fast medium? Justify your response. No refraction if angle of incidence is zero. Fast – angle becomes less, think of car example

Exit Ticket 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. No refraction if angle of incidence is zero. Fast – angle becomes less, think of car example