1. How does light propagate?
2. How can we see?
3. The eye
4. Light reflection - how does a mirror work? - angle of incidence, angle of reflection
5. Refraction of light
   * Air - Water
   * Air - Glass
   * Glass - Water
6. Convex and concave lenses

How does light propagate?

Light consists of electromagnetic waves. These waves consist of electric and magnetic parts. The light travels at a speed of 300 000 km/sec. This is how light is envisioned.


How can we see?

All electromagnetic waves carry a certain amount of energy. This energy is greater the shorter the distance between the wave crests. In a certain range of wavelengths, the energy is sufficient for certain substances in the eyes to temporarily detach. These substances are the eye cones and eye rods. These proteins are to be found in the eye retina. The eye rods respond to dim light. The eye cones translate various electromagnetic wavelengths to the colors represented in the rainbow. All these signals are then passed from the retina at the back of the eye, through the optic nerve. The optic nerve is connected to the visual cortex, located at the back of our brain. In this way we become aware of what we see.

We can see the rainbow colors. UV (ultra violet) radiation and IR (infrared) radiation are also electromagnetic waves, but we cannot see them. Sunburn is the result of the sun's UV rays. Infrared radiation also comes from the sun. IR-radiation is perceived as heat. The shorter the wavelength, the more energy the electromagnetic radiation contains. UV radiation is therefore more energetic than IR radiation.


1 nm = 1 nanometer, i.e. one billionth of a meter (1/10⁹ m)

The eye


Light reflects from the human being and travels towards the eye. The light travels trough the pupil and refracts through the lens. Then the human being is reproduced up side down on the retina.

The retina consists of rods and cones. The cones react on the light rays represented by the rainbow colors. The rods react on dim light. When hit by a sunray the rod or cone falls apart. A nerve impulse arises and travels through the optic nerve to the visual cortex at the back of the brain. We thus become aware of what we see.

Light reflection - how does a mirror work? - angle of incidence, angle of reflection


A light ray hits a mirror. The ray reflects and the angle of incidence (i) equals the angle of reflection (r).

The surface normal is an imaginary line. Towards this, the angle of incidence and reflection are depicted.

Refraction of light - "Air - Water" - "Air - Glass" - "Glass - Water"

i = angle of incidence
b = angle of refraction
Note the vertical lines are the surface normals.


Light reflects from the clam and refracts through the water and finally hits the eye. The brain behind the eye thinks the clam is in the light ray prolongation below the water surface. When man is to catch the clam with his/her hand it will grab above the place where the clam actually is. This fake image is called a virtual image.

Note - the ray in the example, goes from water to air.

When light goes from a lighter to a denser medium, the rays refract towards the surface normal. (Figure 1 and 2).


When light goes from a denser to a lighter medium, the light refracts from the surface normal. (Figure 3).


Convex and concave lenses


To the left is shown light refraction through a condenser lens and to the right a diverging lens. The condenser lens is also called a “convex lens” and the diverging lens is called a “concave lens”.