How the Focal Length of a Lens Affects Temperature at the Focal Point

Pages: 3 (737 words) Published: September 23, 2013
Investigation into how the focal length of a lens affects the temperature at the focal point Research & Rationale:

In my project I will be looking in to how different lenses with varying thicknesses and therefore focal points magnify and intensify light passing through it. To do this I will be measuring the temperature of an object behind the lens, at the focal point. The study and research of lenses was not part of the course this year, so I thought it would be more interesting to study a new topic. I started by using an ordinary filament lamp and lenses of varying diameters, but the lenses were so large, the focal points would have been too far from the light source. I am now using a projector light so the focused light is more intense, and now the lenses are much smaller, so their focal points are now a much more convenient and measureable distance. An infra-red thermometer will be used to measure the temperature in a precise spot, and to a much more precise value. Plan:

List of Apparatus:
Ruler (1mm increments)

Lens stand

Temperature probe (0.1 °c increments)
Variety of lenses
Data logger

The independent variable is the focal point. The laboratory measurements were used for these, as there was not sufficient time to undertake an independent investigation to measure them. The dependant variable was the maximum temperature reached by the temperature probe. This was measured using a data logger to reduce human error. The logger displayed the temperature to a 0.1 °c level of accuracy. The control variables were:

Distance of lens from projector
Diameter of lens
Intensity of light from projector
Time taken for probe to heat up
Room temperature
Material of lens


1. A lens was selected, and placed 1m away from the source of light, ensuring the light was level with the light. 2. The temperature probe was inserted level with the focussed light, but at a distance behind the lens equal to the focal point...
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