Answer:

1.Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power.

2.The units of measured brightness are candela per square metre and the apostilb being the lumens emitted per square metre. Luminance and illuminance are not to be confused. For example, if a sheet of paper has a reflectance of 80% and an illuminance of 100 lux, its luminance will be 80 apostilbs.

3.Violet has the shortest wavelength, at around 380 nanometers, and red has the longest wavelength, at around 700 nanometers. (Left) Isaac Newton's experiment in 1665 showed that a prism bends visible light andthat each color refracts at a slightly different angle depending on the wavelength of the color.

4.The wavelength of visible light determines the color that the light appears. Light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between are the wavelengths of all the other colors of light. A prism separates visible light into its different colors.