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Light Transmission, Reflection & Absorption

Instructor: Matthew Bergstresser
Light is an electromagnetic wave that can travel through nothing. In this lesson, we will learn how light travels, and what happens to it when it hits something. Will it be absorbed by a material, reflected off of a material or pass through a material?

What Did She See?

''But grandmother! What big eyes you have,'' said Little Red Riding Hood. ''The better to see you with, my dear,'' replied the wolf. Sight is the sense that fills the world with color and shadows. What about the science behind light? It has its own story that we will learn about now.

What is Light?

Light is a broad term referring to any electromagnetic wave in the electromagnetic spectrum ranging from gamma rays to radio waves. Visible light is more or less in the middle of the electromagnetic spectrum.


The electromagnetic spectrum
EM_spec

Electromagnetic waves are generated by an accelerating charged particle, and they have two wave fronts. One is an electric field wave (E), and the other is a magnetic field wave (B). They both oscillate out of phase and are perpendicular relative to each other.


Electromagnetic wave
EM_wave

They do not need a medium to travel, which is good for us because otherwise we wouldn't see the sun or even be here on Earth for that matter! The light ray that leaves the sun and reaches Earth, has traveled roughly 1.5 billion meters (93 million miles) at 3.0 x 108 m/s in a straight line pretty much through nothing. We call this emptiness a vacuum. That speed is known as the speed of light, and all electromagnetic waves travel at that speed through a vacuum.

Reaching Earth

When light comes into contact with Earth's atmosphere it encounters our atmosphere first, which is mostly composed of gas molecules. Depending on the angle it hits the atmosphere, it can be absorbed, reach the Earth's surface, or be reflected back into space. When it is absorbed by gas particles, it is re-emitted.

When light is absorbed by gases in the atmosphere, or Earth's surface, it causes them to heat up. Ever feel an asphalt parking lot during a sunny day? It gets hot! A portion of light is reflected by our atmosphere and by the Earth's surface. This reflected light is how astronauts have taken pictures of Earth from space.

The color of objects you see is actually the color that is not absorbed by pigments in that material. For example, the pigment in a red shirt absorbs every other color except red.

Reflection

''Magic mirror, on the wall - who is the fairest one of all?, asked the Evil Queen in Snow White and the Seven Dwarfs. We don't have a magic mirror, because they defy physics, but we all have plane mirrors. A plane mirror is a very smooth surface that is usually an alloy of metallic materials, which reflects light almost perfectly. When light hits the mirror, it bounces off of it at the same angle that it hit with. The angle I am referring to is measured with respect to the normal line to the mirror.

The normal is an imaginary perpendicular line to the mirror's surface. The angle between the incoming light ray (incident ray) and the normal is called the incident angle, and the angle between the reflected ray and the normal is known as the reflected angle.


Diagram 1. Incident angle = Reflected angle
angles

Light bouncing off of a mirror or another really flat, smooth surface like a calm pond, is called specular reflection. Reflection off of rougher surfaces is called diffuse reflection because the light is diffused, or spread out in all directions. Think of looking at an image of yourself on a shiny wooden surface. You don't look yourself. You look kinda of fuzzy.

Refraction

Light traveling through air that reaches a translucent or transparent medium such as water or glass, not only reflects a little, but penetrates the boundary between the air and the medium. The light rays actually bend or refract at these boundaries because the light's speed, and wavelength change. Its frequency remains the same, though.


Refraction
refraction

Light rays slow down and bend towards the normal when entering a denser medium. Diagram 2 shows θi is greater than θr.


Diagram 2
angles

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