- Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.
- There is one special case you need to know. Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in that case they carry straight on.
When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is called diffraction.
The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves. Significant diffraction only happens when the wavelength is of the same order of magnitude as the gap. For example:
- a gap similar to the wavelength causes a lot of spreading with no sharp shadow, eg sound through a doorway
- a gap much larger than the wavelength causes little spreading and a sharp shadow, eg light through a
Sound waves and light waves reflect from surfaces. When waves reflect, they obey the law of reflection:
the angle of incidence equals the angle of reflection
- The normal is a line drawn at right angles to the reflector
- The angle of incidence is between the incident (incoming) ray and the normal
- The angle of reflection is between the reflected ray and the normal.
Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images The waves can:
- appear to come from a point behind the mirror, for example a looking glass
- be focused to a point, for example sunlight reflected off a concave telescope mirror.
Rough surfaces scatter sound and light in all directions. However, each tiny bit of the surface still follows the rule that the angle of incidence equals the angle of reflection.
Constructing a ray diagram
In a ray diagram, the mirror is drawn a straight line with thick hatchings to show which side has the reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel. Light rays that appear to come from behind the mirror are shown as dashed straight lines.
The image in a plane mirror is virtual, upright and laterally inverted
Make sure that the incident rays (the solid lines) obey the law of reflection: the angle of incidence equals the angle of reflection. Extend two lines behind the mirror. They cross where the image appears to come from.
The image in a plane mirror is:
- virtual (it cannot be touched or projected onto a screen)
- upright (if you stand in front of a mirror, you look the right way up)
- laterally inverted (if you stand in front of a mirror, your left side seems to be on the right in the reflection).
This is the end of the topic!
Drafted by Cherry (Chemistry)