Physics: Week 31

This week in the Ron Paul Curriculum physics course was an introduction to geometric optics.

The Ray Model of light:

It is a useful model for analyzing geometric optics such as lenses and mirrors. It assumes that light traveled in a straight line. 

Speed of light:

The speed of light depends on the material it passes through.  It is fastest in a vacuum.


This occurs when a wave passes through a medium and bends.  It is due to parts of the wave fronts traveling at different speeds.

Each material has an index of refraction, which is the ration of speed of light in a material to the speed of light in a vacuum: n = v/c

Snell’s law:

Snell’s law is used to calculate the angle of refraction, which is based on the angle of incidence and the different indices of refraction.

Light only bends if it enters a new medium at an angle other than 90 degrees.

If light enters a material and passes out the other side on the same angle, it is because the two refractions have canceled each other out.

Internal reflection:

Occurs when the index of refraction is such that a light ray bounces off a medium instead of passing through it.

Critical angle:

When a light ray goes to pass through a medium at the critical angle all the light is reflected back instead.

Calculation of critical angle equation:

Fiber optics:

Allows cables to transmit light.  They can do this because the light angle does not exceed the critical angle.


There are two kinds of reflection; specular and diffuse

  • Specular reflection: follows the law of reflection.
  • Diffuse reflection: scatters the incoming light ray in all directions.

Flat mirrors:

Flat mirrors are able to form a virtual image behind the mirror.

Curved mirrors:

Always make a real image and make an image larger than the original subject.

Spherical mirrors:

A spherical mirror is a kind of curved mirror that can be either concave or convex.

Concave spherical mirror:

If parallel rays of light fall on a concave spherical mirror, the mirror will approximately focus all the rays on a single point called the focal point.  The focal length is ½ the radius of curvature.

Mirror equations:

Lens types:

  • Converging lenses: parallel beams of light are converged when passing through the lens.
  • Diverging lenses: light passing through the lens will diverge

Lens equations: are the same as mirror equations

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