During the recent total solar eclipse in the US, I overlapped my hands, using the gaps between my fingers to form 鈥減inholes鈥�. Just before the start of totality, someone noticed that the shadows formed through this 鈥減inhole camera鈥� onto a whiteboard were so sharp that the individual hairs on my arms were visible. How thin must the solar crescent be for the shadows to be this sharp, and how soon before totality would this occur?
鈥� The size of the light source doesn鈥檛 actually determine the sharpness of the pinhole image. The major factors are the size of the pinhole and the amount of ambient light 鈥渘oise鈥�. In effect, the pinhole allows through rays of light, each one connecting just one pixel of the scene onto a matching pixel on the whiteboard. Of course, a small pinhole and minimal diffraction (scattering of light by the edges of the hole) will give fine light rays, with sharp distinction between adjacent light sources and high resolution. But small pinholes also mean dim images.
The diameter of the ideal pinhole is roughly the wavelength of the light. Any object with a larger diameter will block at least one ray and remove that pixel from the image no matter how large the light source behind the pinhole. It is unrealistic to expect to have a perfect pinhole, but objects such as human hairs or dust particles are typically about 100 times the wavelength of visible light, so they can block pixels from 鈥渞ealistic鈥� pinholes or blur pixels with the light they scatter.
Advertisement
You don鈥檛 see hair shadows in full sunlight because light scattered from a sunlit sky washes out fine detail in the faint pinhole image. But they would show up in a pinhole image projected into a darkened chamber.
Jon Richfield, Somerset West, South Africa
We pay 拢25 for every answer published in 快猫短视频. To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
快猫短视频 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
You can also submit answers by post to: The Last Word, 快猫短视频, 25 Bedford Street, London WC2E 9ES.