Legs in water look short, and a straw appears bent—these are all phenomena called refraction. They occur when light enters a different material.
Basic Knowledge
When light travels from air into a material with a different density, such as water or glass, its direction bends at the boundary. This is called refraction.
When entering water (glass) from air: angle of incidence > angle of refraction (moves closer to the normal)
When emerging into air from water (glass): angle of incidence < angle of refraction (moves farther from the normal)
When light emerges into air from water or glass, if the angle of incidence exceeds a certain angle (critical angle), all the light is reflected at the boundary. This is called total internal reflection and is used in optical fibers and internal reflection in binoculars.
Refraction（The phenomenon in which light changes direction at the boundary between different materials）
Angle of refraction（The angle formed by the refracted ray and the normal）
Critical angle（The angle of incidence at which total internal reflection begins）
Total internal reflection（The phenomenon in which all light is reflected at the boundary without being refracted）
Optical fiber（A cable that uses total internal reflection to carry light signals over long distances）
Dispersion（The phenomenon in which white light separates into rainbow colors through a prism）
Further Exploration (Background・Significance)
Refraction occurs because the speed of light changes depending on the material. Light travels fastest in a vacuum; in water it slows to approximately 2.25 × 108 m/s, and in glass it slows to approximately 2.0 × 108 m/s.
When white light passes through a prism, it separates into the spectrum of red・orange・yellow・green・blue・indigo・violet because the refractive index differs slightly for each wavelength (color) (dispersion). This was discovered by Newton, and rainbows also result from refraction and dispersion in raindrops.
Optical fibers using total internal reflection support modern Internet communication and are also used in endoscope light sources and illuminations.
- Refraction = light bends at a boundary
- Air→water・glass: angle of incidence > angle of refraction
- Water・glass→air: angle of incidence < angle of refraction
- Total internal reflection occurs when the critical angle is exceeded
- Applications of total internal reflection: optical fibers, binoculars, endoscopes
- Dispersion in a prism separates white light into rainbow colors
- Rainbows result from refraction・reflection・dispersion in raindrops
Points to Note (Easily Confused)
① Refraction occurs only at a boundary (light travels straight inside a material). ② Total internal reflection occurs only when light travels from an optically denser material to a less dense material (water→air, glass→air). ③ Reflection and refraction often occur at the same time (at a water surface, some light is reflected and some is refracted). ④ Light travels more slowly in materials with a greater refractive index.
Practice
- When light emerges from water into air, which is greater, the angle of incidence or the angle of refraction?
- What optical phenomenon is used in optical fibers?
- What is the phenomenon called in which white light separates into rainbow colors through a prism?