```Focal Length                                                Wave-Lengths of Different Color Lights
It is the distance from the center of the lens to           The colours of the spectrum of white light are
the focal point.                                            violet, indigo, blue, green, yellow, orange and
red (VIBGYOR). The white light is a mixture
Principal Axis                                              of different colours.
It is a horizontal imaginary line drawn through                Colour                 Wavelength
the center of the lens.                                        Violet                400 – 440 nm
Indigo                 440 - 460 nm
Power of a Lens or Optical Power                               Blue                   460 - 500 nm
Green                 500 – 570 nm
    It is the degree to which a lens, mirror, or
Yellow                 570 - 590 nm
other optical system converges or diverges                Orange                 590 - 620 nm
light.                                                    Red                    620 - 720 nm
    The power of a lens is defined as the
reciprocal of its focal length in meters.
1                        Colour Mixing
Power of Lens =
Focal lenght in meters
                                                           RGB Colour Combination: Additive Mixing
    The unit of power is diopter (D).
1                                     1.  Red + Green + Blue = White
1 dioptre(D)         1 m1
                    meter
2.  Red + Blue = Magenta
Prism                                                       3.  Blue + Green = Cyan
4.  Red + Green = Yellow
A transparent object with smooth and polished
surfaces which refract the light is known as
prism.                                                      CMYK Colour Combination: Subtractive
Dispersion of Light by Prisms                               Mixing
When a narrow beam of white light is passed                 1.  Cyan + Magenta + Yellow = Black
through a prism, it splits into seven colours               2.  Magenta + Cyan = Blue
known as spectrum. The colours of the                       3.  Yellow + Magenta = Red
spectrum are violet, indigo, blue, green,                   4.  Yellow + Cyan = Green
yellow, orange and red (VIBGYOR).
HUMAN EYE
    The human eye is an optical instrument just like a photographic camera.
    It forms the real image of the object on retina of the eye.
    The human eye has a 200-degree viewing angle and can see 10 million colors and shades.
    For the normal eye, the range of vision is from 25 cm to infinity.
Defects of Vision
1. Myopia : A person cannot see the distant                 2. Hypermetropia (Hyperopia): A person
object. Image is formed before the retina.                 cannot see near object. Image is formed
Concave lens is used for correcting this                   behind the retina. Convex lens is used for
defect.                                                    correcting this defect.
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