The Properties of Light Rectilinear Propagation: Light travels in straight lines. The Pinhole Camera:

Diffraction: The spreading of waves as they pass through a slit.

Illumination, intensity and luminous flux Intensity is the constant brightness of the light source. Measured in candelas 40watt=35 cd 60watt= 70 cd 100 watt =130 cd 15 watt flor = 70 cd

Luminous Flux Luminous flux is a measure of the power of visible light P= 4  *I (cd)= lumens

Illuminance is a measure of photometric flux per unit area, or visible flux density. Illuminance is typically expressed in lux (lumens per square meter) or foot-candles (lumens per square foot).

The Inverse Square Law The inverse square law defines the relationship between the irradiance from a point source and distance. It states that the intensity per unit area varies in inverse proportion to the square of the distance. E = I / d2 In other words, if you measure 16 W/cm2 at 1 meter, you will measure 4 W/cm2 at 2 meters, and can calculate the irradiance at any other distance. An alternate form is often more convenient: E1 d12 = E2 d22 Distance is measured to the first luminating surface - the filament of a clear bulb, or the glass envelope of a frosted bulb.

Example: You measure 10.0 lm/m2 from a light bulb at 1.0 meter. What will the flux density be at half the distance? Solution: E1 = (d2 / d1)2 * E2 E0.5 m = (1.0 / 0.5)2 * 10.0 = 40 lm/m2

The Spectral Lines in the stars show the Doppler Shift due to relative motion with the earth. v = c x (wavelength shift) / (wavelength)