IGCSE Online Tutorial - Jesmul Sir's Portal

IGCSE Online Tutorial - Jesmul Sir's Portal

Physics GCSE: Properties of Waves

Reflection of waves

Light waves and the rules of reflection

When you see an object, light is bouncing off that object into your eye. Rough and smooth surfaces look different because of the way light bounces off them.
When light hits a mirror it bounces off the mirror. This is called reflection. The ray of light hitting the mirror is called the incident ray. The ray of light bouncing off is called the reflected ray. There are three rules of reflection:
Rule 1: The angle of incidence is always the same size as the angle of reflection. (Angle A = Angle B)

The angles are always measured from the ray to the normal line. The normal line is a line at right angles to the mirror.

Watch out. Many students measure angles from the mirror to the ray. This will give the wrong answer.

Plane (flat) mirrors produce virtual images. This means that the images are not real, they don't exist. We can see these images because our brains think that they exist.
The rays of light reflect off the mirror (obeying rule 1), back into the eye. The brain thinks that light only travels in straight lines, so tracks the lines back to the point behind the mirror. This point is the virtual image. It doesn't really exist, but the brain thinks it does.

Here are the other rules of reflection:

Rule 2: The image is always the same distance behind the mirror as the object is in front (distance C = distance D).

Rule 3: The image is always:

  • The same size as the object.
  • Laterally inverted (left becomes right and right becomes left), which is why writing is back to front in a mirror.

Reflection of water waves

For water wave diagrams you need to draw in the wave direction and the wave fronts. The wave fronts are the peaks of the waves and will always beat right angles to the wave direction.
The distance between the wave fronts represents the wavelength. The closer the wave fronts are together the shorter the wavelength of the water wave.

Water waves obey the first rule of reflection, so that the angle of incidence of the waves is the same as the angle of reflection.
If water waves hit a curved surface the wave fronts become curved. This can make the wave fronts go to a point. The more curved the surface is, the quicker the waves come to a point after hitting the surface.
A raindrop hitting a puddle produces curved wave fronts. If these hit a plane (straight) surface the shape of the wave fronts curves the opposite way.
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