Light meter bench – transparent, translucent and opaque materials, reflection and the inverse square law
PhysicsOscillations & WavesAges 16–17
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Sign in to playA virtual practical with a lamp, a sample holder and a light meter (lux meter). On the Materials screen, shine a beam on clear glass, frosted glass, tracing paper, white and black card, glossy paper, a mirror and colour filters, and swing the meter round the sample to measure transmitted and reflected light and to tell specular from diffuse reflection. On the Distance screen, move the meter away from a small lamp to test the inverse square law, with background subtraction, a results table and linearised graphs.
Lesson: Transmission, reflection and absorption of light; transparent, translucent and opaque materials; specular and diffuse reflection; colour filters; the inverse square law for light from a point source
What it shows
When light meets a material, some is transmitted, some is reflected and the rest is absorbed. Transparent materials let light pass straight through, translucent ones scatter it, and opaque ones transmit none. Smooth, glossy surfaces reflect light in one direction (specular reflection), while rough, matt surfaces scatter it in all directions (diffuse reflection). A colour filter transmits mainly its own colour. Light from a small lamp spreads out over a sphere, so the illuminance falls with the square of the distance: E = I/r². A light meter measures illuminance in lux.
How to use
On Materials, choose a Sample, set Meter to Behind (transmitted) or In front (reflected), and move Angle φ or drag the meter round the sample; press Record. Record the empty holder at φ = 30° as I₀, and switch off Lamp on to record the background. On Distance, set Distance r or drag the meter, press Record at several distances, and choose a Graph to test the inverse square law.
Parameters you can change
- Screen Materials: transmission and reflection, Distance: inverse square law
- Sample in the holder No sample (empty holder), Clear glass, Frosted glass, Tracing paper, White card (matt), Black card (matt), Glossy white paper, Plane mirror, Red filter, Green filter, Blue filter
- Second filter behind the sample None, Red, Green, Blue
- Meter position Behind the sample (transmitted light), In front of the sample (reflected light)
- Meter angle from the normal φ 0–85 °
- Room lights on
- Distance from lamp to meter r 0.1–1 m
- Luminous intensity of the small lamp I 10–100 cd
Questions to explore
- Why does a mirror give a large reading at only one angle, while white card gives a small reading at every angle?
- What happens to the reading when you put a red filter and a green filter together, and why?
- If you double the distance from the lamp, by what factor does the reading change, and why?
