Analyzing Light 1
Analyze Visible Light
Nuclear fusion reactions in the core of the sun transform matter into energy in the
form of photons. A photon is a basic unit of electromagnetic radiation. Electromagnetic
radiation produced by the sun includes gamma ray, ultraviolet light, visible light,
infrared radiation and radio waves. Each type of electromagnetic radiation has a
specific range of wavelengths, energy and frequencies.
The photovoltaic cells of a Solar Learning Lab transform the energy in photons of
visible light into electrical energy. A spectrometer can be used to study the wavelengths
and energy of photons of visible light.
Goals/Objectives
Use a spectrometer to compare the wavelengths and energies light emitted by
an incandescent light bulb and sunlight reflected from a white surface.
Determine the relationships among the wavelength, frequency and energy of
electromagnetic radiation.
Materials
A spectrometer
Sources of visible light that include 100 watt and 15 or 25 watt light bulbs
A reflective surface
Spectrometers
Raindrops separate sunlight into the different colors of visible light. The gradual
change from one color to another in a rainbow is called a continuous spectrum.
Spectrometers use a diffraction grating or a prism to separate the different wavelengths
of visible light into a continuous spectrum. The Project STAR Spectrometer shown below
is one example of a spectrometer that can be used to analyze visible light.
Analyzing Light 2
Wavelength and Energy Measurements
The following procedure can be used to analyze visible light.
Hold the spectrometer so it is horizontal and the data table is facing up.
Aim the square opening on the wider end of the spectrometer toward a light
source.
Look into the circular opening on the smaller end of the spectrometer.
Slowly move the spectrometer from side to side until a spectrum appears onto
the two spectrometer scales. See examples of the scales below.
If a scale is difficult to read, move the spectrometer up or down.
Source: http://home.comcast.net/~mcculloch-brown/astro/spectrostar.html
The Wavelength Scale:
Wavelengths of photons of different colors of light are measured in hundreds of
nanometers. A nanometer is one billionth of a meter. Use a spectrometer or the figure
shown above to answer Questions 1 and 2.
Question 1: A red photon has a wavelength of 642 nanometers. What is its wavelength
in meters?
The Energy Scale:
The energy of photons of each color of visible light is measured in units called
electron volts. A Joule of energy is the amount of energy needed to accelerate a 1.0
kilogram object at a rate of 1.0 m/s2 as it moves a distance of one meter. An electron
volte is a much smaller amount of energy. One electron volt of energy is equal to only
1.6 x 10-19 Joules of energy.
Question 2: What color of light has photons with energy values of 2.9 electron volts?
Analyzing Light 3
Analyze Incandescent Light
Incandescent light bulbs transform electrical energy into infrared radiation (heat)
and visible light. Filaments in incandescent light bulbs need to reach temperatures of
390⁰ C to begin to glow. Light bulbs that reach temperatures as high as 3000⁰ C glow
brightly.
Use a spectrometer to observe and record examples of a wavelength for each color
in the spectrum of visible light emitted by an incandescent light bulb.
Color Observed Example of a wavelength
Red
Orange
Yellow
Green
Blue
Violet
There are many different shades of each color of visible light. Use a spectrometer to
observe and record an example of an energy value of photons for each color.
Color Observed Example of an Energy Value
Red
Orange
Yellow
Green
Blue
Violet
Question 3: How does the pattern of change in the energy of photons of visible light
compare with the pattern of change in the wavelength?
Analyzing Light 4
Analyze Reflected Sunlight
You have probably seen the continuous spectrum of light produced as sunlight
passes through raindrops to produce a rainbow. The following procedure can be used
to study reflected visible sunlight on a sunny day.
Safety Precaution: Do not look directly at the sun with the spectrometer.
Put a smooth, flat, white sheet of paper on level ground on a sunny day.
Move into a position so you can aim the spectrometer toward the paper and
study reflected sunlight
Record observed wavelengths and energies for each color of reflected sunlight.
Color Observed Example of a wavelength
Red
Orange
Yellow
Green
Blue
Violet
Color Observed Example of an Energy Value
Red
Orange
Yellow
Green
Blue
Violet
Question 4: How was the visible spectrum of sunlight different from the visible
spectrum of an incandescent light bulb?
Question 5: How was the visible spectrum of sunlight similar to the visible spectrum of
an incandescent light bulb?
Analyzing Light 5
Compare the Spectrum of Different Wattage Light Bulbs
The wattage of a light bulb indicates the rate at which electrical energy is
transformed into thermal energy and visible light. Higher wattage light bulbs produce
hotter filaments. Use a spectrometer to compare the visible spectrum produced by light
bulbs with different wattages.
Use a spectrometer to analyze either the wavelengths or the energy of different
colors in the visible spectrum.
Use this data table if you analyze wavelengths.
Color Observed Lower wattage bulb Higher wattage bulb
Red
Orange
Yellow
Green
Blue
Violet
Use this data table if you analyze the energy of photons
Color Observed Lower wattage bulb Higher wattage bulb
Red
Orange
Yellow
Green
Blue
Violet
Question 6: How does the temperature of a light bulb’s filament affect the visible
spectrum that it produces?
Analyzing Light 6
Wien’s Law
Wilhelm Wien discovered the relationship between the temperature of a hot object
and the wavelength of the most intense color of light produced by that hot object. This
relationship is called Wien’s Law.
An instrument called a Bolometer is used to determine the wavelength of the most
intense light emitted from the surface the sun. The formula below the graph shown
below can be used to calculate the temperature on the surface of the sun.
Source: http://www.exposeknowledge.com/kb/2392-technique-to-measure-temperature-sun.aspx
Question 7: How can the Wien’s Law formula be written so that the temperature of a
light source can be calculated?
Question 8: A peak intensity of light from a star has a wavelength of 500 nanometers.
500 nanometers is equal to 500 x 10-9 meters. What is that wavelength in centimeters?
Question 9: What would be the temperature of a light source if the wavelength of
maximum intensity is 500 nanometers?
Question 10: What would be the wavelength of light with the maximum intensity for a
light source that had a temperature of 2000⁰ Kelvin?