Light, shadows and reflection

We see the world because of light. An object is visible only when light from it enters our eyes. Some objects, such as the sun, a flame and a bulb, give out their own light; they are luminous. Most objects, such as a book, a tree and the moon, are non-luminous. We see them because they send back, or reflect, light that falls on them.

Light travels in straight lines

Look at a candle flame through a straight pipe and you see it. Bend the pipe and the flame disappears. Light does not bend around corners; it travels in straight lines. This is called the rectilinear propagation of light.

Transparent, translucent, opaque

• Transparent objects let almost all light pass: clear glass, clean water, air. We can see clearly through them.
• Translucent objects let some light pass: butter paper, frosted glass, thin cloth. We see through them only dimly.
• Opaque objects let no light pass: wood, metal, a wall, your body.

Shadows

Because light travels in straight lines, an opaque object blocks it and leaves a dark patch behind. This is a shadow. Three things are needed: a source of light, an opaque object and a screen such as a wall or the ground. A shadow is always dark, whatever the colour of the object. It shows only the outline. Its size changes: the nearer the object is to the light, the bigger the shadow. Your own shadow is long in the morning and evening and shortest at noon.

Eclipses are shadows on a grand scale. In a solar eclipse the moon's shadow falls on the earth. In a lunar eclipse the earth's shadow falls on the moon.

Reflection

When light strikes a surface and bounces back, it is called reflection. A smooth, shiny surface such as a mirror reflects light regularly and forms an image. A rough surface scatters the light in all directions, so we see the surface itself and not an image.

The ray that falls on a mirror is the incident ray. The ray that bounces back is the reflected ray. An imaginary line at right angles to the mirror at the point where the ray strikes is called the normal.

The laws of reflection

1. The angle of incidence is equal to the angle of reflection. (Both are measured from the normal.)
2. The incident ray, the reflected ray and the normal all lie in the same plane.

Image in a plane mirror

The image you see in a flat mirror is upright and of the same size as you. It appears as far behind the mirror as you stand in front of it. It cannot be caught on a screen, so it is called a virtual image. And it is reversed sideways: your right hand looks like the left hand of the image. This is called lateral inversion. It is why the word AMBULANCE is painted in mirror writing on the front of the vehicle: the driver ahead reads it correctly in the rear-view mirror.