Chapter 10 Light
  1. Luminous and illuminated bodies
  2. Luminous flux and illuminance
  3. Colors
  4. Reflection of light
  5. Refraction of light
  6. Chapter Quiz
  Support by visiting our advertisers' websites.


    Of all the electromagnetic waves, light is the only portion of waves that can be detected by the human eye.
Electromagnetic waves and light


  Section Section 1. Luminous and illuminated bodies.


Light is produced by a luminous body. A light bulb is a luminous body that emits light in almost every direction.

Light travels in straight lines at 299,792,458 m per sec in vacuum. When light hits an object, it is reflected. An illuminated body reflects light.

When a ray of light reaches our eyes, the receptors in our eyes will produce a different color sensation depending on the wavelength of the light wave.


  Section Section 2. Luminous flux and illuminance.


The amount of light produced per unit of time is called the luminous flux, P. The unit for this is the lumen, lm.

If we only want to know the amount of light produced by a source on a specific object (table, chair), we have to use the formula for illuminance, E. This formula tells us the amount of light per meter square at a certain distance.

E = P/4(pi)dē

Thus, the illuminance is just the luminous flux divided by the area of a sphere. Why a sphere? For example, lets put two objects in different places, both of them two meters away from a source of light.

Same distance

As you can see the illuminance on these objects is the same. This is because almost every source of light emits light in all directions and two objects within the same radius receive the same amount of light.

However, objects that are placed at different distances have different illuminations.

Different distance

The closer an object is to a source of light, the more illuminated it will be.


  Section Section 3. Colors

Red, green and blue are known as primary colors, because when they are added together white light is formed.

Red, green, and blue

By mixing primary colors in pairs we obtain secondary colors. Red and green produce yellow. Blue and red produce magenta, and blue and green produce cyan.


  Section Section 4. Reflection of light
    If we draw a line perpendicular to a surface, this line is the normal of the surface. When a ray of light hits the surface of an object, part of the light is reflected. If the ray of light is in angle with the surface, then the angle between the incident ray and the normal will be the same angle between the normal and the reflected ray.

Normal, incident, and reflected rays

They are not completely flat surfaces. When millions of rays of light hit the rough surface of an object, they are reflected in all directions. This is how we can see illuminated objects.

How does a mirror reflect light?


  Section Section 5. Refraction of light
    When a ray of light passes from one medium to another, it bends. Depending of the new medium the light will travel faster or slower. If the light travels faster in the second medium, then this medium is called the rarer medium. On the other hand, the medium in which the light travels slower, in this case the first one, is called the denser medium.
  • When a ray of light enters a denser medium, it is bent towards the normal.
  • When a ray of light enters a rarer medium, it is bent away from the normal.
Denser medium Rarer medium


  Check! There is an index of refraction (n) between the two mediums. To get a value, we have to divide the sine of the angle in vacuum or air by the sine of the angle in the denser medium.

In the example above, the index of refraction would be

n = sin a / sin b


  Check! Some of the light is always reflected. However, when a ray of light goes from a denser medium to a rarer medium, all the light will be reflected if the angle of incidence is greater than the critical angle. The critical angle is the angle of incidence for which the refracted ray is at 90 degrees with the normal.

Critical angle

How do lenses refract light?



Section 6. Chapter 10 Quiz

Easy... All right. Try this quiz.

[Ch8] - [Ch9] - [Ch10] - [Next Lesson]