Light and Illumination Presented to: Waverly Honors Physics May 11, 2018

Objectives: After completing this lesson, you will be able to: Define light, discuss its properties, and give the range of wavelengths for the visible spectrum. Apply the relationship between frequencies and wavelengths for optical waves. Define the concepts of light rays & shadows, sensitivity curves, and luminous flux. Solve Light Intensity problems.

A Beginning Definition All objects are emitting and absorbing EM radia-tion. Consider a poker placed in a fire. As heating occurs, the emitted EM waves have higher energy and eventually become visible. First red . . . then white. 1 2 3 4 Light may be defined as electromagnetic radiation that is capable of affecting the sense of sight.

Electromagnetic Waves Wave Properties: c E B Electric E Magnetic B Waves travel at the speed of light c. Perpendicular electric and magnetic fields. Require no medium for propagation (to be transmitted). 3 x 108 m/s

The speed of light The speed at which light travels through air is approximately 300 million meters per second. Light travels almost a million times faster than sound.

Example #1: Calculate time 1) You are asked for time. 2) You are given distance and you may find the speed of sound and light. 3) t = d ÷ v 4) For sound: t = (1,609 m) ÷ (340 m/sec) = 4.73 seconds For light: t = (1,609 m) ÷ (3 x 108 m/sec) = 0.0000054 seconds (5.4 x 10-6 sec) Calculate the time it takes light and sound to travel the distance of one mile, which is 1,609 meters.

The Wavelengths of Light The electromagnetic spectrum spreads over a tremendous range of frequencies or wavelengths. The wavelength l is related to the frequency f: c = fl c = 3 x 108 m/s Those EM waves that are visible (light) have wave-lengths that range from 0.00004 to 0.00007 cm. Red, l 0.00007 cm Violet, l 0.00004 cm

The EM Spectrum A wavelength of one nanometer 1 nm is: 1024 1023 1022 1021 10201019101810171016 101510141013101210111010109 108 107 106 105 104 Frequency wavelength f (Hz) l ( nm) 10-7 10-6 10-4 10-3 10-1 1 10 102 103 104 105 106 107 108 109 1010101110121013 Gamma rays X-rays Infrared rays Short Radio waves Broadcast Radio Long Radio waves Ultraviolet A wavelength of one nanometer 1 nm is: 1 nm = 1 x 10-9 m 400 nm  700 nm Visible Spectrum Red 700 nm  Violet 400 nm c = fl c = 3 x 108 m/s

Example 2: Light from a Helium-Neon laser has a wavelength of 632 nm Example 2: Light from a Helium-Neon laser has a wavelength of 632 nm. What is the frequency of this wave? The Helium Neon Laser Wavelength l = 632 nm Laser f = 4.75 x 1014 Hz Red light

Properties of Light Any study of the nature of light must explain the following observed properties: Rectilinear propagation: Light travels in straight lines (Huygens’ Principle) and can be represented as rays. Reflection: Light striking a smooth surface turns back into the original medium. Refraction: Light bends when entering a transparent medium.

The Nature of Light Physicists have studied light for centuries, finding that it sometimes behaves as a particle and sometimes as a wave. Actually, both are correct! Reflection and rectilinear propagation (straight line path) Dispersion of white light into colors.

Photons and Light Rays Light may be thought of as little bundles of waves emitted in discrete packets called photons. photons The wave treatment uses rays to show the direction of advancing wave fronts. Light rays are convenient for describing how light behaves. Light ray

Light Rays and Shadows A geometric analysis may be made of shadows by tracing light rays from a point light source: shadow screen Point source The dimensions of the shadow can be found by using geometry and known distances.

Example 3: The diameter of the ball is 4 cm and it is located 20 cm from the point light source. If the screen is 80 cm from the source, what is the diameter of the shadow? 4 cm 20 cm 80 cm h The ratio of shadow to the source is same as that of ball to source. Therefore: h = 16 cm

Shadows of Extended Objects Extended source penumbra umbra The umbra is the region where no light reaches the screen. The umbra is the region where no light reaches the screen. The penumbra is the outer area where only part of the light reaches the screen.

The Sensitivity Curve Human eyes are not equally sensitive to all colors. Sensitivity curve Wavelength l Sensitivity 555 nm Eyes are most sensi- tive in the mid-range near l = 555 nm. 400 nm 700 nm 40 W 40 W Yellow light appears brighter to the eye than does red light.

Luminous Flux Luminous flux (P) is the portion of total radiant power that is capable of affecting the sense of sight. It is the rate at which light is emitted form a source. Typically only about 10% of the power (flux) emitted from a light bulb falls in the visible region. The unit for luminous flux is the lumen.

The Lumen as a Unit of Flux One lumen (lm) is the luminous flux emitted from a 1/60 cm2 opening in a standard, spheroid light source (think of a light-bulb). In practice, sources of light are usually rated by comparison to a commercially prepared standard light source. A typical 100-W incandescent light bulb emits a total radiant power of about 1750 lm. This is for light emitted in all directions.

The Lumen in Power Units Recalling that luminous flux is really radiant power allows us to define the lumen as follows: One lumen is equal to 1/680 W of yellow-green light of wavelength 555 nm. A disadvantage of this approach is the need to refer to sensitivity curves to determine the flux for different colors of light. Wavelength l Sensitivity curve

Light Intensity (A.K.A. illuminance) The Light Intensity I of a surface A is defined as the luminous flux per unit area (P/A). The units for Intensity is lumens per square meter which is also named a lux (lx) or in units of Watts per square meter. For the purposes of today’s lesson, we will use the units Watts per square meter.

Light Intensity obeys the Inverse Square Law

Example #4 What is the illumination (light intensity) on a surface 3.0 meters below a 150 watt light source that emits a luminous flux of 2275 lumens? Calculate the answer in lumens per square meter (lux) and in watts per square meter.

Example #5 At the surface of the Sun the intensity of the solar radiation is about 6.33×107 W/m2! What is the average yearly intensity of solar radiation if the Earth is an Astronomical Unit (1.5x1011 m) away and the Sun has a luminous flux of 6.808x1027 Lumens? 1 Watt = 17.50 Lumens