How to play Laser Slit
- Choose a test and read the wavelength, the slit width, and the screen distance on the card.
- Use y = n·λ·L ÷ a for the n-th dark band, with every length in the same unit.
- Type your prediction and press Start. The light reaches the screen, then the meter moves out from the centre.
- Compare the orange marks with the dark bands on the screen and in the graph.
Diffraction of light
Light is a wave too, but its wavelength is tiny: from about 400 nm for violet to 700 nm for red, less than a thousandth of a millimetre. So light only spreads noticeably when it passes through a gap almost as narrow, such as a slit a tenth of a millimetre wide. On a screen far away, a slit makes a wide bright band in the middle, with dark bands where the light from different parts of the slit cancels, and fainter bright bands between them. The n-th dark band is at y = n·λ·L ÷ a from the centre: a narrower slit or a longer wavelength spreads the pattern wider. The same spreading limits how sharp a telescope or a microscope can be, and it lets scientists measure the wavelength of light with nothing more than a slit and a ruler. You can see it yourself by looking at a bright lamp through a narrow gap between two fingers.
A focused learning model
The screen is far from the slit compared with the slit's width, and the angles are small, so y = n·λ·L ÷ a. The brightness is (sin β ÷ β)² with β = π·a·y ÷ (λ·L); the screen shows its square root, closer to what the eye sees, so the faint bands stay visible. The drawing is not to scale: the beam is metres long, the pattern a few millimetres. Answers within 2% are right.