The goal of this lab is to experiment with some basic optics to
learn how they work and to learn about light.
Break up into groups of two. Obtain a lens set from the instructor.
The lens set should contain three lenses labeled A,B, and C. Note
the shapes of the lenses which are inscribed to the side of the lens.
Which (and why) of these do you expect will focus the light? Measure
the focal length of each of the lenses that forms a focused image using the
bright light source provided.
The relation between object and image distance and focal length is
give by 1/(object distance) + 1/(image distance) = 1/(focal length) (hint:
for object distance >> image distance you can ignore the 1/(object distance)
term). Use a sheet of paper behind the lens to see at what separation
you get a well-focused image.
Measure the focal length three times for each lens that forms a real
image
(calculate the mean and dispersion for your final answer).
Start with lens A or B near the page of paper with writing, move the lens
slowly away and describe the behavior of the image seen through the lens
as the lens is moved away (is the image bigger or smaller than the original
writing, is it inverted in any way?). Now using the lens describe the
appearance of a distant (> several feet) object. Use a ray diagram to
explain why the image orientation is the way it is.
Now use lens A and B in combination to make a telescope (you want to
magnify distant objects and have them in good focus). Describe your
solution (lens order and separation,
again at least three measurements to produce a mean and dispersion).
Estimate the magnification (how
many times larger do things appear). If you've made a telescope that
works using lenses A and B, this is a standard design for refracting
telescopes.
Pick up a hand-held spectrograph from the instructor. Record the
spectra of He, Ne, H, and Ar (draw the spectra and measure the
wavelengths of the brightest lines in each spectrum). Then
use your known spectra to classify the unknown spectrum.
Advanced: Examine one of the optical fibers and shine light
into one end. Can you think of an application for optical fibers
in spectrographs? Hint: identify a limitation of the standard
slit spectrograph.