Rays of light

Overview of rays of light

A light ray is a line (straight or curved) that is perpendicular to the light’s wavefronts. In optics a ray is an idealized model of light, obtained by choosing a line that is perpendicular to the wavefronts of the actual light that points in the direction of the light (Moore, 2005). A light ray is an idealized model of light, which is drawn as a straight line. Light rays allow us to draw clear diagrams showing the motion of light, including reflection (like bouncing off mirrors) and refraction (the bending of light when moving from one transparent material to another) (Greivenkamp, 2014).

Diagrammatic representation of light rays

Types of light rays

According to Stewart (2006), there are numerous names for types of light rays, the most common ones are:

  • Incident rays,
  • Reflected rays
  • Refracted rays

Incident rays: Incident rays are the rays that approach and hit a particular surface – they are said to be ‘incident’ on the surface.

Reflected rays: Reflected rays are rays that come out if a surface is in some way reflective, as in the case of a plane mirror. The ray that bounces off the surface at an angle is known as the reflected ray.

Refracted rays: Refracted rays are rays that pass through a surface, bending due to change of material (or medium). For example, if a ray of light travels from air into water, or into a block of glass, the light ray will bend as a result. This effect is called refraction, and the ray that bends as it moves through the material is called a refracted ray.

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Diagrammatic representation of different types of light rays

Characteristics of light rays in ophthalmic lens

Refraction of light rays in converging lens

Suppose that several rays of light approach the lens; and suppose that these rays of light are travelling parallel to the principal axis. Upon reaching the front face of the lens, each ray of light will refract towards the normal to the surface. At this boundary, the light ray is passing from air into a more dense medium (usually plastic or glass). Since the light ray is passing from a medium in which it travels fast (less optically dense) into a medium in which it travels relatively slow (more optically dense), it will bend towards the normal line. This is shown for two incident rays on the diagram below (Hecht, 2012). Once the light ray refracts across the boundary and enters the lens, it travels in a straight line until it reaches the back face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. Since the light ray is passing from a medium in which it travels slow (more optically dense) to a medium in which it travels fast (less optically dense), it will bend away from the normal line (Yao, 2008).

Diagrammatic representation of light rays in a converging lens

The above diagram shows the behaviour of two incident rays approaching parallel to the principal axis. Note that the two rays converge at a point; this point is known as the focal point of the lens.

Suppose that the rays of light are travelling through the focal point on the way to the lens. These rays of light will refract when they enter the lens and refract when they leave the lens. As the light rays enter into the more dense lens material, they refract towards the normal; and as they exit into the less dense air, they refract away from the normal. These specific rays will exit the lens travelling parallel to the principal axis (Valentine, 2008).

 

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The above diagram shows the behaviour of two incident rays travelling through the focal point on the way to the lens. Note that the two rays refract parallel to the principal axis. A second generalization for the refraction of light by a double convex lens can be added to the first generalization (Grbic, 2014).

Refraction of light rays in diverging lenses

Suppose that several rays of light approach the lens; and suppose that these rays of light are travelling parallel to the principal axis. Upon reaching the front face of the lens, each ray of light will refract towards the normal to the surface. At this boundary, the light ray is passing from air into a more dense medium (usually plastic or glass). Since the light ray is passing from a medium in which it travels relatively fast (less optically dense) into a medium in which it travels relatively slow (more optically dense), it will bend towards the normal line. This is shown for two incident rays on the diagram below. Once the light ray refracts across the boundary and enters the lens, it travels in a straight line until it reaches the back face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. Since the light ray is passing from a medium in which it travels relatively slow (more optically dense) to a medium in which it travels fast (less optically dense), it will bend away from the normal line (Vincent, 2007). These principles of refraction are identical to what was observed for the converging lens above.

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The above diagram shows the behaviour of two incident rays approaching parallel to the principal axis of the double concave lens. Just like the converging lens above, light bends towards the normal when entering and away from the normal when exiting the lens. Yet, because of the different shape of the diverging lens, these incident rays are not converged to a point upon refraction through the lens. Rather, these incident rays diverge upon refracting through the lens. For this reason, a diverging can never produce a real image. Diverging lenses produce images that are virtual.

 References

Greivenkamp, J. E. (2014). Field Guide to Geometric Optics. London: SPIE Field Guides

Grbic, A. (2014). Planar Transmission-line Lens. Physical Review Letters 92 (11): 117-120.

Hecht, E. (2012). Optics (4th ed.). New York: Addison Wesley.

Moore, K. (2005). “What is a ray?”. ZEMAX Users’ Knowledge Base 23 (9): 87-90.

Stewart, J. E. (2006). Optical Principles and Technology. California: CRC Press.

Valentine, J. (2008). Three-dimensional optical lens refraction. Nature 455 (7211): 376–379.

Yao, J. (2008). Optical Refraction in Lenses. Science 321 (5891): 930.