Lab-4 blackbody and Filters (1)

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Name: Lab - Blackbody Curves & UBV Filters Background Material Thoroughly review the “Spectra” and “Filters” background pages. The color index page may also be helpful to review. Here the links: http://astro.unl.edu/naap/blackbody/spectra.html http://astro.unl.edu/naap/blackbody/filters.html Filters Simulator Overview The filters simulator allows one to observe light from various sources passing through multiple filters and the resulting light that passes through to some detector. An “optical bench” shows the source, slots for filters, and the detected light. The wavelengths of light involved range from 380 nm to 825 nm which more than encompass the range of wavelengths detected by the human eye. The upper half of the simulator graphically displays the source-filter-detector process. A graph of intensity versus wavelength for the source is shown in the leftmost graph. The middle graph displays the combined filter transmittance – the percentage of light the filters allow to pass for each wavelength. The rightmost graph displays a graph of intensity versus wavelength for the light that actually gets through the filter and could travel on to some detector such as your eye or a CCD. Color swatches at the far left and right demonstrate the effective color of the source and detector profile respectively. The lower portion of the simulator contains tools for controlling both the light source and the filter transmittance. ∙ In the source panel perform the following actions to gain familiarity. o Create a blackbody source distribution – the spectrum produced by a light bulb which is a continuous spectrum. Practice using the temperature and peak height controls to control the source spectrum. o Create a bell-shaped spectrum. This distribution is symmetric about a peak wavelength. Practice using the peak wavelength , spread , and peak height controls to vary the source spectrum. o Practice creating piecewise linear sources. In this mode the user has complete control over the shape of the spectrum as control points can be dragged to any value of intensity. ▪ Additional control points are created whenever a piecewise segment is clicked at that location. ▪ Control points may be deleted by holding down the Delete key and clicking them. ▪ Control points can be dragged to any location as long as they don’t pass the wavelength value of another control point. ∙ In the filters panel perform the following actions to gain familiarity.

NAAP – Blackbody Curves & UBV Filters 1/7 o Review the shapes of the preset filters (the B, V, and R filters) in the filters list . Clicking on them selects them and displays them in the graph in the filters panel . o Click the add button below the filters list . ▪ Rename the filter from the default (“filter 4”). ▪ Shape the piecewise linear function to something other than a ﬂat line. o Click the add button below the filters list . ▪ Select bell-shaped from the distribution type pull down menu. ▪ Alter the features of the default and rename the filter. o If desired, click the remove button below the filters list . This removes the actively selected filter (can’t remove the preset B, V, and R filters). Filters are not saved anywhere. Refreshing the ﬂash file deletes the filters. Click (or copy/paste) here for simulator: http://astro.unl.edu/naap/blackbody/animations/filters.html Filters Simulator Questions ∙ Use the piecewise linear mode of the source panel to create a “ﬂat white light” source at maximum intensity. This source will have all wavelengths with equal intensity. ∙ Drag the V filter to a slot in the beam path (i.e. place them in the filter rack ). ∙ Try the B and the R filter one at a time as well. Dragging a filter anywhere away from the filter rack will remove it from the beam path. Question 1: Sketch the graphs for the ﬂat white light and V filter in the boxes below. What is the effective color of the detected distribution? Green source distribution combined filter transmittance detected distribution Question 2: With the ﬂat white light source, what is the relationship between the filter transmittance and the detected distribution? The plain white light doesn’t transmit when the green light is added and the combined filter transmittance graph and the detected distribution graph are the same. NAAP – Blackbody Curves & UBV Filters 2/7 ∙ Add a new piecewise linear filter. ∙ Adjust the filter so that only large amounts of green light pass. This will require that addition of points. Question 3: Use this green filter with the ﬂat white light source and sketch the graphs below.

source distribution combined filter transmittance detected distribution ACME Source FILTER RACK ACME Detector Question 4: Use the blackbody option in the source panel to create a blackbody spectrum that mimics white light. What is the temperature of this blackbody you created? 6000k ∙ Add a new piecewise linear filter to the filter list . ∙ Modify the new filter to create a 40% “neutral density filter”. That is, create a filter which allows approximately 40% of the light to pass through at all wavelengths ( transmittance ) ∙ Set up the simulator so that light from the “blackbody white light” source passes through this filter. Question 5: Sketch the graphs created above in the boxes below. (This situation crudely approximates what sunglasses do on a bright summer day.) source distribution combined filter transmittance detected distribution Question 6: Remove all filters in the filters rack. Place a B filter in the beam path with the ﬂat white light source (about 75% intensity). Then add a second B filter and then a third. Describe and explain what happens when you add more than one of a specific filter. By doing so, the wavelengths get smaller and ﬂatter and the color turns really dark. NAAP – Blackbody Curves & UBV Filters 3/7

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