Principles of Light

What is this going to cover? Properties and Sources of Light Color and Vision

8.4.1e Objectives Describe at least five properties of light. Use the speed of light to calculate the time or distance traveled by light. Explain how we perceive color in terms of the three primary colors. Arrange the colors of light in order of increasing energy, starting with red. Describe light in terms of energy and color.

Properties and Sources of Light Key Question: What are some useful properties of light?

Properties and Sources of Light Light travels almost unimaginably fast and far. Light carries energy and information. Light travels in straight lines. Light bounces and bends when it comes in contact with objects. Light has color. Light has different intensities, it can be bright or dim.

Electric Light The process of making light with heat is called incandescence. Incandescent bulbs generate light when electricity passes through a thin piece of metal wire called a filament. The filament heats up and gives off light.

Electric Light The other common kind of electric light is the fluorescent bulb. Fluorescent bulbs convert electricity directly to light without generating a lot of heat. Fluorescent bulbs use high-voltage electricity to energize atoms of gas that fill the bulb.

Light carries energy and power The intensity of light is the amount of energy per second falling on a surface. Most light sources distribute their light equally in all directions, making a spherical pattern. Because light spreads out in a sphere, the intensity decreases the farther you get from the source.

Light intensity The intensity of light from a small source follows an inverse square law because its intensity diminishes as the square of the distance.

The speed of light The speed at which light travels through air is approximately 300 million meters per second. Light travels almost a million times faster than sound.

The speed of light The speed of light is so important in physics that it is given its own symbol, a lower case c. The best accepted experimental measurement for the speed of light in air is 299,792,500 m/sec. For most purposes, we do not need to be this accurate. Just know it’s darn fast! The fastest thing in the universe!

Calculate time Calculate the time it takes light and sound to travel the distance of one mile, which is 1,609 meters. 1,609 / 340 = 4.7 So, sound travels about .2 miles per second 1) You are asked for time. 2) You are given distance and you may find the speed of sound and light. 3) t = d ÷ v 4) For sound: t = (1,609 m) ÷ (340 m/sec) = 4.73 seconds For light: t = (1,609 m) ÷ (3 x 108 m/sec) = 0.0000054 seconds (5.4 x 10-6 sec)

Reflection and refraction When light moves through a material it travels in straight lines. When light rays travel from one material to another, the rays may reflect.

Reflection and refraction Objects that are in front of a mirror appear as if they are behind the mirror. This is because light rays are reflected by the mirror. Your brain perceives the light as if it always traveled in a straight line.

Reflection and refraction Another example of refraction of light is the twinkling of a star in the night sky As starlight travels from space into the Earth’s atmosphere, the rays are refracted.

Reflection and refraction The light that bends as it crosses a surface into a material refracts and is shown as a refracted ray.

Color and Vision Key Question: How do we see color?

Color and Vision When all the colors of the rainbow are combined, we do not see any particular color. We see light without any color. “The absence of color.” We call this combination of all the colors of light "white light".

Color and Vision We can think of different colors of light like objects with different kinetic energies. Blue light has a higher energy than green light, like the objects that make it into the top window. Red light has the lowest energy, like the objects that can only make it to the lowest window.

How the human eye sees color The retina in the back of the eye contains photoreceptors. These receptors release chemical signals. Chemical signals travel to the brain along the optic nerve. optic nerve

Photoreceptors in the eye Cones respond to three colors: red, green and blue. Rods detect intensity of light: black, white, shades of gray.

How we see colors Which chemical signal gets sent depends on how much energy the light has. If the brain gets a signal from ONLY green cones, we see green.

How we see other colors The three color receptors in the eye allow us to see millions of different colors. We don’t see every combination as white because the strength of the signal matters. All the different shades of color we can see are made by changing the proportions of red, green, and blue.

How we see the color of things When we see an object, the light that reaches our eyes can come from two different processes: The light can be emitted directly from the object, like a light bulb or glow stick. The light can come from somewhere else, like the sun, and we see the objects by reflected light.

Why are plants green? Plants absorb energy from light and convert it to chemical energy in the form of sugar (food for the plant). Chlorophyll is an important molecule that absorbs blue and red light.

Why is the sky blue? The atmosphere scatters short wavelengths the most (like violet and blue). Our eyes are more sensitive to blue than violet (so the blue matters more than violet to us). Sunrise and sunset are reddish because at that angle sunlight is going through much more atmosphere and only long wavelengths (like red and orange) can make it through to our eyes.