A single thin ray of light strikes a glass plate and seemingly breaks into two rays. One ray bounces back into the air; the other passes through the glass but travels in a different direction. Reflection and refraction can be confusing because both phenomena might occur simultaneously at the same interface but are very distinct phenomena.
Take a flat mirror and a glass or acrylic block and place them on a piece of paper. Shine a low-power light source that emits a thin ray of light. Do not look directly into the light source. Observe how a light ray behaves when it hits the mirror. Where does it come from, and where does it go after hitting the mirror? Does it pass through another medium or bounce back into the same medium? It bounces back into the air, which is the same medium. This phenomenon is called reflection.
Draw a line on the paper perpendicular to the mirror at the point of incidence. This line is called a normal. Measure the angle between the incident ray and the normal, and measure the angle between the reflected ray and the normal. You’ll find that both angles are roughly equal. Make sure to measure the angles from the normal, not the mirror itself, because the angle from the mirror itself would give you incorrect measurements. Label the normal before measuring any angle.
Now, replace the mirror with the glass or acrylic block. Does the ray of light pass into another medium or return to the same medium? The ray of light passes into the glass or acrylic block. It changes direction because light moves at different speeds in air and in glass or acrylic. This change in direction is called refraction. Mark the incident ray, point of incidence, and the ray of light inside the block. The angle between the refracted ray and the normal is called the angle of refraction.
A transparent surface can reflect some light while refracting the rest. When looking through a window, you can see both what’s outside and what’s inside the room. Both the reflection and the refraction are occurring. Refracted light goes through the boundary and changes its direction. Reflected light bounces off the surface and comes back to you. Just because you see both effects doesn’t mean they’re the same thing.
Don’t just look at the intensity of the light to determine whether it’s reflection or refraction. Follow the light ray. Did it return to the same medium it came from, or did it enter a different medium? If it returned to the same medium, it’s reflection. If it entered a different medium and bent, it’s refraction. In your next observation, mark the boundary and the normal. From there, you can easily tell whether the light is reflecting or refracting.
