13.1 - Reflection

What reflection is

Reflection happens when a wave hits a boundary between two materials and bounces back, instead of being absorbed or passing into the new material.

Reflection happens with every kind of wave

• Light reflects off surfaces - it's how we see objects and how mirrors form images.
• Sound reflects off hard surfaces such as walls and cliffs, producing echoes.
• Water waves bounce back off a harbour wall or the side of a tank.

Hard surfaces reflect sound strongly, while softer surfaces absorb more of it and reflect less.

The law of reflection

The law of reflection applies to every reflected wave:

Angle of incidence = angle of reflection

• Both angles are measured from the normal - an imaginary line drawn perpendicular (at right angles) to the surface at the point of incidence. It's usually drawn as a dotted line.
• Angle of incidence - The angle between the incident ray and the normal.
• Angle of reflection - The angle between the reflected ray and the normal.

Drawing a reflection ray diagram

• A ray diagram shows the path a wave travels along. Rays are drawn as straight lines with arrows showing their direction.
• Draw the normal and the incident ray meeting it at the surface, then draw the reflected ray so the angle of reflection equals the angle of incidence.

Reflection of white light and water waves

• White light is a mixture of colours with different wavelengths. Every wavelength obeys the law of reflection, so all colours reflect at the same angle - white light does not split into colours when it reflects.
• Water waves obey the same law: plane waves in a ripple tank reflect off a straight barrier with the angle of incidence equal to the angle of reflection.

Images formed by a plane mirror

A plane (flat) mirror forms an image of an object by reflecting light. The image is:

• Virtual - It can't be projected onto a screen.
• The same size as the object and upright.
• Laterally inverted - Left and right are reversed.
• The same distance behind the mirror as the object is in front.

Why the image is virtual

• Light rays reflecting off the object and into the eye are diverging (spreading out).
• The eye traces these rays back in straight lines to where they appear to meet behind the mirror.
• The light appears to come from behind the mirror, where no actual rays meet.

Drawing the ray diagram

1. Draw the virtual image first, the same size as the object and as far behind the mirror as the object is in front.
2. Draw a reflected ray from the top of the image to the eye - solid between mirror and eye, dotted between mirror and image.
3. Draw an incident ray from the top of the object to the point where that reflected ray meets the mirror.
4. Repeat for the other end of the object.