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Light often travels in straight lines, but its path changes when it meets a surface or enters a new material. Reflection sends light back into the original medium, while refraction bends light as it crosses into a medium where its speed is different. These effects explain mirrors, lenses, eyeglasses, rainbows, fiber optics, and many everyday visual phenomena. Understanding the geometry of rays helps students predict where light will go and how images form.
At a boundary, angles are measured from the normal line, which is perpendicular to the surface. The law of reflection says the incoming and reflected angles are equal, while Snell's law connects the angles of refraction to the refractive indices of the two media. When light moves from air into glass or water, it slows down and bends toward the normal. When it moves from glass or water into air, it speeds up and bends away from the normal, and total internal reflection can occur if the angle is large enough.
Key Facts
- Law of reflection: theta_i = theta_r
- Snell's law: n1 sin(theta1) = n2 sin(theta2)
- Index of refraction: n = c / v
- Light bends toward the normal when it enters a medium with a higher refractive index.
- Light bends away from the normal when it enters a medium with a lower refractive index.
- Critical angle for total internal reflection: sin(theta_c) = n2 / n1, valid when n1 > n2
Vocabulary
- Incident ray
- The incoming ray of light that strikes a surface or boundary.
- Reflected ray
- The ray of light that bounces back from a surface into the original medium.
- Refracted ray
- The ray of light that passes into a new medium and changes direction because its speed changes.
- Normal
- An imaginary line drawn perpendicular to a surface at the point where a ray strikes it.
- Index of refraction
- A number that compares the speed of light in a vacuum to the speed of light in a material.
Common Mistakes to Avoid
- Measuring angles from the surface instead of from the normal is wrong because reflection and refraction laws use angles measured from the perpendicular normal line.
- Assuming light always bends toward the normal is wrong because light bends toward the normal only when it enters a medium with a higher refractive index.
- Using degrees in a calculator set to radians is wrong because Snell's law calculations require the sine of the angle in the correct angle mode.
- Forgetting that frequency stays constant during refraction is wrong because the wave speed and wavelength change, but the frequency is set by the light source.
Practice Questions
- 1 A light ray in air strikes a flat mirror at an angle of 35 degrees from the normal. What is the angle of reflection?
- 2 A ray of light travels from air with n = 1.00 into glass with n = 1.50 at an incident angle of 40 degrees from the normal. Use Snell's law to find the refracted angle.
- 3 A light ray passes from glass into air and bends away from the normal. Explain what this tells you about the speed of light in each medium and why the ray changes direction.