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Light

9 min read

How light travels

We see an object when light from it enters our eyes. A luminous object, such as the Sun or a lamp, makes its own light. A non-luminous object, such as the Moon or this page, is seen because it reflects light. Light travels in straight lines, called rays, and it travels very fast: about 300 000 km every second.

Shadows: umbra and penumbraA large light source on the left, an opaque object in the middle and a screen on the right. Directly behind the object is a dark shadow called the umbra, where no light arrives. Around it is a lighter ring called the penumbra, where only part of the source is visible.large sourceopaque objectscreenpenumbraumbra
A large source makes a soft-edged shadow: a dark centre (umbra) with a lighter ring around it (penumbra).
MaterialWhat happens to lightExample
Transparentpasses through clearlyclean glass, air
Translucentpasses through but scatteredfrosted glass, tissue paper
Opaquedoes not pass throughwood, metal

Reflection of light

When light hits a smooth surface such as a mirror it bounces off. This is reflection. The normal is a line at right angles to the surface at the point where the ray hits.

Laws of reflection
1. The angle of incidence equals the angle of reflection (). 2. The incident ray, the reflected ray and the normal all lie in the same plane.
Terms in the reflection of lightA ray of light hits a plane mirror and bounces off. The normal is a line at right angles to the mirror at the point of incidence. The angle of incidence i between the incident ray and the normal equals the angle of reflection r between the reflected ray and the normal.irincident rayreflected raynormalplane mirrorpoint of incidence
Try itAim the ray

Change the angle of incidence . The angle of reflection always matches it. Angles are measured from the normal, not from the mirror.

The image in a plane mirror

The image in a plane mirrorAn object in front of a plane mirror and its image behind the mirror, the same distance away on the other side. The image is virtual: light only appears to come from it. Rays from the object tip reflect into the eye, which sees the image tip.eyeobjectimage (virtual)mirrordd
Property of the image
It is the same size as the object
It is as far behind the mirror as the object is in front
It is virtual: it cannot be shown on a screen
It is laterally inverted: left and right are swapped
Worked exampleA ray meets a mirror

A ray makes an angle of with the surface of a plane mirror. Find the angle of reflection.

  1. The angle with the mirror is 30°, so the angle with the normal is 90° − 30° = 60°.
  2. The angle of reflection equals the angle of incidence.

Answer: The angle of reflection is 60°.

Check yourself An object is 2 m in front of a plane mirror. How far is the image from the object?
Check yourself The angle of incidence is 25°. What is the angle of reflection?

Key terms

Ray
A straight line showing the path of light.
Normal
A line at right angles to a surface.
Opaque
Does not let light through.
Virtual image
An image that cannot be formed on a screen.

Summary

  • Light travels in straight lines and forms shadows.
  • Laws of reflection: i = r, and ray, normal and reflected ray are in one plane.
  • A plane mirror gives a virtual, upright, same-size, laterally inverted image as far behind the mirror as the object is in front.
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