Fundamentals of Light & Optics

Properties of Light - Reflection

The refection of light is the change of direction of incident (original) wavefronts as they intercept a reflective surface.

Reflection of Light
Fig 2.0 - Reflection of Light

Depicted in Fig 2.0, the process involved with the reflection of a light wave. This model also applies to other waves in the electromagnetic spectrum. In this case, the reflective surface is a mirror, the incident ray represented by the red line strikes the mirror, the reflective properties of the mirror will act on the incident beam and reflect (bounce) incident ray away from the mirror as represented by the blue line.

If we draw a line perpendicular (90°) to the surface of the mirror, as represented by the green dashed line, we refer to this line as the 'normal'. Using the normal as a reference, we are then able to measure the angle of incidence (θi) and angle of reflection (θr).

The Law of Reflection states that:

θr = θi

The angle of reflection is equal to the angle of incidence from the normal

A typical mirror will reflect between 90-95% of the incident light, typical glass will reflect around 4% of the incident light, while very specialised dielectric mirrors are able to reflect around 99.999% of incident light falling upon them over a very narrow range of wavelengths. The most typical application of dielectric mirrors, are in lasers, forming the laser cavity itself.

The percentage of energy that is not reflected is absorbed by the reflective material, and contributes to heating of the material.

Classification of Reflection

Reflection is classified into two distinct types; specular reflection and diffuse reflection.

Specular Reflection is mirror-like, and therefore capable of producing an image, such as in a reflective telescope.

Diffuse Reflection is from an uneven or granular surface giving rise to multiple angles of reflected light. Diffused reflections spread energy over a greater area, and lose the ability to form images.

Diffuse Reflection
Fig 2.1 - Diffuse Reflection

Reflections In Mirrors

The following images detail the basic principles of reflection in mirrors.

Plain Mirror Reflection
Fig 2.2 - Reflection, Plain Mirror

Fig 2.2 shows the principle of reflections in a plain mirror. Plane mirrors have no focusing ability, therefore, the image is the same shape and size of the object the mirror is reflecting. The image formed by a plain mirror is virtual (i.e cannot be projected), and appears to be coming from behind the plane of reflection.

Convex Mirror Reflection
Fig 2.3 - Reflection, Convex Mirror

Fig 2.3 shows the principle of reflection from a convex mirror, again the virtual image appears behind the mirror. The images seen in a convex mirror appears smaller than actually is in reality, and are typically used to view around corners or behind, such as a rear-vision mirror.

Concave Mirror Reflection
Fig 2.4 - Reflection, Concave Mirror

Fig 2.4 shows the principle of a concave mirror. Concave mirrors have the ability to focus light to a point in front of the mirror. Major applications of this type of mirror include, telescopes, lamp reflectors and solar furnaces. Concave mirrors produce real images, therefore can be projected.

Total Internal Reflection

Total internal reflection occurs when light strikes the boundary of a medium with a higher Index of Refraction than the medium its travelling from.

We will cover total internal reflection in more detail in the next section as this requires the understanding of refraction.

In the next section, we will cover the refractive properties of light.

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