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Light, reflection and seeing
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Light is how we see the world. It travels incredibly fast in straight lines and transfers energy.
How light travels
Light travels in straight lines (rays). It is the fastest thing there is: about 300 000 km every second (3 × 108 m/s) in a vacuum.
Unlike sound, light does not need a medium: it crosses empty space from the Sun.
When light hits an object it can be transmitted (through glass), absorbed (by a black surface) or reflected. Opaque objects block light, making shadows.
Unlike sound, light does not need a medium: it crosses empty space from the Sun.
When light hits an object it can be transmitted (through glass), absorbed (by a black surface) or reflected. Opaque objects block light, making shadows.
Reflection
Law of reflection: angle of incidence = angle of reflection. Both are measured from the normal, a line at 90° to the mirror.
A smooth surface like a mirror gives specular reflection (a clear image). A rough surface like paper scatters light in all directions: diffuse reflection.
The image in a plane mirror is the same size, the same distance behind the mirror, and the wrong way round.
A smooth surface like a mirror gives specular reflection (a clear image). A rough surface like paper scatters light in all directions: diffuse reflection.
The image in a plane mirror is the same size, the same distance behind the mirror, and the wrong way round.
Seeing and cameras
We see objects because light from them enters our eyes.
In a pinhole camera, rays pass through a tiny hole and make an upside-down image on a screen.
In the eye, the cornea and lens focus light onto the retina, which has light-sensitive cells. They send signals to the brain along the optic nerve.
Light transfers energy: it causes chemical changes (photographic film) and electrical effects (camera sensors, solar cells).
In a pinhole camera, rays pass through a tiny hole and make an upside-down image on a screen.
In the eye, the cornea and lens focus light onto the retina, which has light-sensitive cells. They send signals to the brain along the optic nerve.
Light transfers energy: it causes chemical changes (photographic film) and electrical effects (camera sensors, solar cells).
A ray hits a mirror at an angle of incidence of 35°. What is the angle of reflection, and what is the angle between the ray and the mirror surface?
- Angle of reflection = angle of incidence = 35°.
- The normal is at 90° to the mirror.
- Angle to the mirror = 90 − 35
Answer: Reflection 35°; 55° to the mirror surface
Light travels in straight lines at 300 000 km/s and needs no medium. Angle of incidence = angle of reflection, measured from the normal. Mirrors give specular reflection; rough surfaces give diffuse reflection. The lens focuses light onto the retina.
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