Introduction to Light

pg 4

This text is meant to accompany class discussions. It is not everything there is to know about energy. It is meant as a prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations linked below under unit resources.

Click here for questions that go along with this energy section.

Index of Refraction ...Continued

From the previous page...

The slowest a photon has every traveled is 38 mph, 17 m/s. So the question is if the speed of light can only be 3.00 x 10⁸ m/s then how can it be 17 m/s?

Sometimes when "the speed of light" is reported it is not the speed of the photons being reported, but the AVERAGE SPEED of light from location "A" to location "B." Below is an animation showing this. As a photon travels from the left to the right in a piece of glass, it strikes many molecules and gets redirected. This takes time at every collision. In between collision with the molecules, the photon does travel at 3.00 x 10⁸ m/s.

The animation above shows how the speed of light is less than "the speed of light," 3.00 x 10⁸ ^m/s. The photon travels between the atoms at the speed of light. But the atom absorbs the photons energy and emits the energy while sending the photon in the different direction. Eventually the photon will travel across the two sides of the substance. But its average speed will be less than "the speed of light." Unfortunately it is typical to just say the "speed of light" in a substance is "x" when we should actually say the "average speed of light" in a substance is "x."

The average speed of light through a material is related by the "index of refraction."

Where:

n = the index of refraction, [ No units ] (Always greater than 1 in this course.)
ν_c= the speed of light in vacuum, 3.00 x 10⁸m/s. (Memorize this number.)
ν_m= the speed of light in medium it is passing through measured in m/s.

This also means

Where the λ's are the wavelengths.
λ_c = wavelength in a vacuum.
λ_m = wavelength in a medium.

What happens to light as it travels between mediums?

This video can be found on YouTube at https://youtu.be/JaOjEzCU6jw

Snell's Law

As light travels between mediums with different indexes of refraction, it path will be altered at the boundary to meet the condition below.

The angles are all measured from the normal line. The "normal line" is perpendicular the boundary between two substances.

This video can be found on YouTube at https://youtu.be/Llj8vaD_CCs

As light passes between two substances,

it does not bend if the the two indexes of refraction are the same or if it impacts the boundary perpendicular to the surface.
It bends AWAY from the normal line if it travels from a high index to a lower index of refraction.
It bends CLOSER to the normal line if it travels from a low index to a higher index of refraction.
The bigger the ratio of the indexes of refraction, (higher index/lower index,) the greater the bending. [Compare the two images on the left above.]

Example

Question
Solution
Video Solution

What is the refracted angle of the light after passing between the two different materials?

This video can be found on YouTube at https://youtu.be/jnD3bdK25_s

Critical Angle

This video can be found on YouTube at https://youtu.be/Ul-lRCiEI9A

Example

Question
Solution

Will the incident light ray reflect or refract and at what angle?

Strategy:

Find the critical angle: If the 28° is great than this angle the the ray is reflected at 28°. If 28° is smaller than the critical angle, then the light is refracted.

by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)

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