Snell's law and the focal length of a lensStarter example
A starter example, adapted by LABKickstart from the OpenStax College Physics for AP Courses Lab Manual and shared under CC BY 4.0. It is not a classroom submission from a teacher on Benchtop.
Two classic optics measurements in one session: bend a laser through a glass block to find its index of refraction, then use a converging lens to form a real image and pull the focal length out of the data.
Materials
- Laser pointer
- Half-cylinder glass block
- Protractor, paper and a pencil to trace the block
- Clear lamp or candle as a light source
- Converging lens with a focal length between 15 and 25 cm, plus a holder
- Metre stick
- Index card to act as a screen, 5 by 7 inches or larger
Procedure
- Trace the outline of the glass block on paper and mark the normal to its flat face.
- Shine the laser at the flat face at a measured angle of incidence and mark where the beam leaves the block. Measure the refracted angle.
- Repeat for at least five different incident angles, then plot the sine of the incident angle against the sine of the refracted angle. The slope is the index of refraction.
- Now set the lamp, lens and screen in a line on the metre stick. Move the screen until the image is sharp, and record both distances.
- Repeat for several source distances, keeping the source within about a metre of the lens.
- Plot the inverse of the image distance against the INVERSE of the source distance. The thin lens equation is 1/f = 1/do + 1/di, so this plot is a straight line of slope -1 whose y-intercept is 1/f: the focal length is one over that intercept.
Expected results
The sine plot is a straight line through the origin, giving an index near 1.5 for ordinary glass. Push the incident angle far enough from inside the glass and the refracted beam disappears entirely, which is total internal reflection. The lens forms an inverted real image, and the focal length from your intercept should match the value printed on the lens to within a centimetre or so. Note: the OpenStax handout plots the inverse image distance against the raw source distance and reads the focal length straight off the intercept. That is not linear and the intercept is 1/f, not f, so step 6 above corrects it.
Safety
Never aim the laser at anyone's eyes or at a reflective surface at eye level: the retinal damage is permanent and painless at the time. Work with the beam below eye level. The cut edge of the glass block is sharp, and if you use a candle keep the flame clear of paper and hair.
Source
Adapted from OpenStax, College Physics for AP Courses Lab Manual (CC BY 4.0). Access for free at openstax.org. AP is a trademark registered by the College Board, which was not involved in and does not endorse this adaptation.
Downloads
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