SCIE 4001
2.5 Electromagnetic Radiation

Electromagnetic Radiation (EMR)

Simple Properties of Light
Without light, you wouldn't see anything at all.
Light travels in straight lines

  • Sharp shadows when light is far away or small
  • Diffuse shadow when light is near or large.  It is really a number of overlapping shadows.

Experiment to prove light travels in straight lines

  • Light is nature's way of transferring energy through space.
  • Light travels very rapidly, but it does have a finite velocity. In vacuum, the speed of light is 300,000 kilometers per second.
  • It takes about eight minutes that long for the light to reach the Earth from the Sun
  • Light is more generally referred to as electromagnetic radiation (EMR).
  • The full range of EMR is gamma-rays, X-rays, ultraviolet, optical, infrared, and radio.
  • Light is a kind of energy that travels in waves but can also be thought of as particles

Two kinds of sources of EMR.
Luminous (these sources convert EMR from another form of energy like a candle or bulb)
Non-Luminous (these sources only reflect light like the walls, floor, etc… They are not visible without luminous sources…)
Sun is luminous
Moon is non-luminous

Absorption and Emission of Light: Transparent, Translucent and Opaque


An object that lets light pass through and maintain the image it is called transparent
An object that lets light pass through but does not maintain the image it is called translucent
An object that does not let light pass through is called opaque


  • Reflection is when light or an image bounces off an object’s surface
  • If light reflects from a smooth surface, we may see a reflection of objects like in a mirror
  • If light reflects from a rough surface, we may just see the surface

Reflection and color

  • white light contains all colors
  • Why does a red dress look red in the sunshine?
  • Does it look different when it’s in the shade?
  • Well, in the dark, what is the color of the dress? black? No light reflects off it.
  • A red dress reflects only red light; it absorbs all the other colors.

Plane Mirrors – Perfectly flat

  • Virtual – Image is “Not Real” because it cannot be projected
  • Erect – Image is right side up
  • Laterally Inverted - left to Right
  • As far behind the mirror as the object is in front.
  • Same size as Object.

Absorption and Color
Why is a black pavement feel hotter than a white pavement in the sun?
Remember light is energy. Heat is another form of energy.
A white car reflects all wavelengths of light.
A black car absorbs all wavelengths of light,
he absorbed light energy is converted to heat.

Color Spectrum

  • Light in the environment often appears to be white.
  • This light is made up of many colors. 
  • These colors can be seen when light is broken up.
  • There are seven colors of light in a spectrum: red, orange, yellow, green, blue, indigo (purple), and violet.
  • These colors are always seen in the same order.


A rainbow appears in the sky when the sun shines through droplets of water in the air, creating a spectrum of color.
The white light is broken up into its component colors.

Refraction of Light

1. ponds appear to be shallower than they are actually.


(i) Light rays travel in straight lines.
(ii) The rays are bent away from the direction in which they were going (called bent away from the normal- the red line)
(ii) The eye sees the coin in the new position where the rays that enter the eye intersect when projected backwards.
(iv) Since the rays are bent away from the normal the projection appears closer to the surface than the actual coin.

Straight pencil appears bent under the water surface.


Primary and Secondary Colors of Light
Three colors that can be mixed to produce any other colored light
Red + blue + green = white light
These are called the Primary colours of light.

Secondary colours are colours resulting from the mixing of two primary colours.

  • green + red = yellow
  • red + blue = magenta
  • blue + green = cyan


Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.