All “visible” phenomena around us involve light. Let us organize, using the language of physics, the facts that light travels in straight lines and is reflected by objects.
Basic Knowledge
Light has the property of traveling in a straight line. Light emitted from a light source travels straight without bending in an empty space, so shadows are formed. When light strikes the surface of an object, it is reflected, and the angle of incidence = angle of reflection, known as the law of reflection, holds.
There are regular reflection (specular reflection) from a flat mirror surface and diffuse reflection from a rough surface. We can see objects from various directions because of diffuse reflection. The image seen in a mirror has left and right reversed and is a virtual image located at a position equal to the distance from the mirror surface to the object.
Light source（An object that emits its own light. The Sun・light bulbs・LEDs, etc.）
Straight-line propagation of light（Light travels straight through a uniform material）
Angle of incidence・Angle of reflection（The angles formed by the normal to the mirror surface and the light ray. They are equal）
Law of reflection（Angle of incidence=angle of reflection; the incident ray, reflected ray, and normal lie in the same plane）
Diffuse reflection（The phenomenon in which light is reflected in all directions from a rough surface）
Virtual image（An image that is visible to the eye even though light is not actually gathered there. A mirror image, etc.）
Further Exploration (Background・Significance)
The speed of light in a vacuum is approximately c = 3.0 × 108 m/s (300,000 km/s). This is the fastest speed in the universe, and even a round trip between Earth and the Moon (760,000 km) takes only approximately 2.5 seconds.
The law of reflection is applied in familiar devices such as mirrors, telescopes, car side mirrors, and corner mirrors. Because diffuse reflection occurs, we can read books and notebook pages from various angles and stay in a room that is not dark. The straight-line propagation of light is also related to the shapes of shadows, solar and lunar eclipses, and the principle of the pinhole camera.
- Light source＝something that emits its own light (the Sun・light bulb・LED)
- Light travels straight through a uniform material
- Law of reflection: angle of incidence=angle of reflection (measured from the normal)
- Difference between regular reflection (mirror) and diffuse reflection (paper・wall)
- A mirror image is a virtual image with left and right reversed; its distance is symmetrical
- Speed of light is approximately 3.0 × 108 m/s
- Shadows and eclipses are caused by the straight-line propagation of light
Points to Note (Easily Confused)
① Measure the angle of incidence and angle of reflection from the normal (perpendicular line), not from the mirror surface. ② Both regular reflection (a smooth mirror) and diffuse reflection (a rough surface) obey the law of reflection (only the normal differs for each uneven part). ③ A mirror image is a virtual image; light does not actually exist behind the mirror. ④ Distinguish a light source from a reflecting object (the Moon is not a light source; it only reflects sunlight).
Practice
- State the law of reflection using an equation or a sentence.
- Approximately how many m/s is the speed of light in a vacuum?
- What type of reflection allows us to see objects on rough surfaces?