Reflection and refraction describe what happens when light reaches the boundary between two different media. Some of the light can bounce back into the original medium, which is reflection, while some can pass into the new medium and change direction, which is refraction. These ideas are essential for understanding mirrors, lenses, eyeglasses, cameras, and many natural optical effects.
Snell's law gives the mathematical rule that predicts how much a light ray bends.
The key to both processes is the boundary and the normal line drawn perpendicular to it. The angle of reflection always equals the angle of incidence, but the refracted angle depends on the refractive indices of the two media. When light enters a medium where it travels more slowly, it bends toward the normal, and when it enters a medium where it travels faster, it bends away from the normal.
This behavior helps explain image formation in lenses, fiber optics, and why objects under water appear shifted.
Understanding Reflection & Refraction
Light changes direction at a boundary because its speed changes, but its frequency stays the same. Frequency is set by the source, such as a laser or a glowing bulb. Since speed equals frequency times wavelength, a lower speed in glass or water means a shorter wavelength there.
This is a wave effect, not a force pulling the ray sideways. One side of a slanted wavefront enters the new material first and changes speed first.
That uneven change turns the whole wavefront. Drawing rays is useful, but remembering the wavefront picture helps explain why the bending happens.
The refractive index tells how strongly a material slows light compared with light in empty space. It is not simply a measure of how dense or heavy a material feels. Water, glass, air, and clear plastic each have different indices because their atoms interact differently with the electric field of light.
The index can change slightly with colour. Blue light usually bends more than red light in ordinary glass.
A prism spreads white light into colours for this reason. This effect, called dispersion, is part of why rainbows form when sunlight enters, reflects inside, then leaves raindrops.
At most boundaries, light does not choose only one path. A fraction reflects and another fraction enters the second material. The amount in each path depends on the materials and on the angle.
Glare on a lake, a window, or a shiny road comes from reflected light. Polarised sunglasses reduce some of this glare because reflection near certain angles produces light with a preferred vibration direction. Reflection becomes especially important when light travels from a higher index material toward a lower index material.
As the incident angle increases, the transmitted ray moves closer to the boundary. At one limiting angle, it would travel along the boundary. Beyond that angle, no transmitted ray travels into the second medium.
All of the light reflects back inside. This is total internal reflection.
Optical fibres use repeated total internal reflection to guide light through long, thin glass strands. The central core has a slightly higher refractive index than the surrounding cladding, keeping signals trapped even when the fibre bends gently. This carries internet data and lets doctors view inside the body with endoscopes.
Diamonds sparkle partly because their high refractive index gives a small critical angle, so light can reflect many times before leaving. When solving ray diagrams, measure every angle from the normal, never from the surface. Label which medium the ray starts in and check whether it is entering a higher or lower index.
A diagram is often the best error check. A ray bending the wrong way usually shows that the reference line or the two media were mixed up.
Key Facts
- Law of reflection:
- Snell's law:
- Refractive index:
- If n2 > n1, the refracted ray bends toward the normal
- If n2 < n1, the refracted ray bends away from the normal
- Critical angle for total internal reflection: , for
Vocabulary
- Incident ray
- The incoming light ray that strikes a boundary between two media.
- Normal
- An imaginary line perpendicular to the surface at the point where the ray hits.
- Reflection
- The bouncing of light back into the original medium after hitting a surface.
- Refraction
- The bending of light as it passes from one medium into another with a different refractive index.
- Refractive index
- A number that shows how much light slows down in a material compared with its speed in vacuum.
Common Mistakes to Avoid
- Measuring angles from the surface instead of from the normal, which gives the wrong values for both reflection and refraction calculations. Always measure every ray angle relative to the perpendicular line.
- Assuming light always bends toward the normal, which is wrong because the direction depends on the refractive indices of the two media. Light bends away from the normal when it enters a lower index medium.
- Using Snell's law with the indices or angles swapped, which leads to incorrect answers. Match n1 with theta1 in the first medium and n2 with theta2 in the second medium.
- Thinking reflected and refracted rays are the same process, which hides the fact that one ray stays in the original medium and the other enters a new medium. Keep track of which side of the boundary each ray occupies.
Practice Questions
- 1 A light ray travels from air with n1 = 1.00 into glass with n2 = 1.50 at an incident angle of 30 degrees. Find the refracted angle.
- 2 A ray in water with n1 = 1.33 strikes an air boundary with an incident angle of 40 degrees. Calculate the refracted angle in air.
- 3 A light ray goes from glass into air and bends away from the normal. Explain what this tells you about the relative refractive indices and the speed of light in the two media.