Visible light

The electromagnetic spectrum covers a broad range of phenomenon with radio waves at one end and gamma rays at the other, visible light is somewhere towards the middle of this spectrum tucked in between infrared waves and ultra violet.

Light itself is not visible we only see reflected objects not what actually illuminates them. The beams of light you see from a cinema projector are caused by the light shining through the atmosphere in the room and illuminating dust particles or smoke if smoking is permitted. At rock concerts smoke machines use this effect so you can see the beams of light coming from a laser.

We view our world through a narrow, visible slit in the electromagnetic spectrum.

Visible Light in the em spectrum

visible light waves

Electromagnetic  wavelengths [nm = Nanometres ]

Wavelength
Frequency
Colour
10nm to 400nm
750 to 1500 THz
Ultra violet
380nm to 450nm
668 to 789 THz
Violet
450nm to 495nm
606 to 668 THz
Blue
495nm to 570nm
526 to 606 THz
Green
570nm to 590nm
508 to 526 THz
Yellow
590nm to 620nm
484 to 508 THz
Orange
620nm to 750nm
400 to 484 THz
Red
665nm to 900nm
120 to 400 THz
Near-infrared

Em spectrum - Plate 3
Electromagnetic Transverse wave Plate 4

 

Reflected Light

You can see an object only if light from it enters your eye's.
Objects such as the sun are called "Luminous Sources". Most objects reflect light from a luminous source.

These sources radiate light when their atoms are excited as a result of receiving energy. In a light bulb the energy comes from electricity, these excited atoms give off their light in a haphazard way and is said to be incoherent light.

Light rays

The direction of the path in which the light is travelling is called a "Ray" and is represented in the diagram [to the right] by a straight line with an arrow on it.

A beam is a stream of light and is shown by a number of rays, it may be Diverging, Parallel or Converging.

light beams

 

Laws  of reflection.
 

incident ray
The Incident Ray, the Reflected Ray and the Normal all lie in the same plane
 
 
 
parallel beam of light
If a parallel beam of light falls on a plane mirror it is reflected as a parallel beam of light and "regular" reflection is said to happen. Most surfaces reflect light in an irregular way and the rays in an incident parallel beam are reflected in many directions.
 

 

The angle of incidence is equal to the angle of reflection
 

angle of incidence
The perpendicular to the mirror at the point where the "incident ray" is called the "normal". note, the angle of incidence (i) is the angle between the incident ray and the normal, similarly for the angle of reflection (r)
 
Irregular reflection
Irregular or diffuse reflection is due to the surface of the object not being perfectly smooth like a mirror. At each point on the surface the laws of  reflection are obeyed, except the angle of incidence, so the angle of  reflection varies point to point.
 

 

 

 

 

Nanometres

The wavelength of light is measured in Nanometres. One nanometre is equal to one-billionth of a metre. The width of a strand of hair is about 100,000 Nanometres wide.

Lightwave

Nanometres in SI units

1x10-9m
1x10-3μm
US
Imperial units
3.281x10-9 ft
39.37x10-9 in

 

 

 

 

The wavelength of a light wave is the distance between the beginning and the end of a cycle.
The higher the Frequency, the longer the Wavelength