How do doctors examine our eyes?

Author: Shanghai Institute of Optics and Precision Mechanics, Chinese Academy of Sciences

The article comes from the Science Academy official account (ID: kexuedayuan)

In the last issue of the "Chasing Lights and Pursuing Dreams" column, we introduced "Optics in the Sky". In our daily lives, we are bound to get sick. But did you know that when doctors treat us, they also rely on the help of optics! So, how does optics protect our health?

When you go to the hospital for an eye examination, the doctor will ask you to put your chin on an instrument. This "instrument" is a "slit lamp microscope", an eye examination instrument invented by Swedish ophthalmologist Gullstrand.

In fact, the slit lamp mainly examines the anterior segment of the eye. Generally, other accessories are added during the examination, such as a gonioscope, which can be used to examine the angle of the eye, so that a series of eye disease symptoms can be examined. Therefore, the slit lamp is a necessary process for ophthalmological examinations! The slit lamp is the "stethoscope" of the ophthalmologist, which plays a role in preliminary judgment. So how does the slit lamp work? This requires the magical knowledge of "optics" to explain!

Slit lamp, as the name implies, is a light that shines through a narrow slit to illuminate the eye, which is then observed using a microscope. The light that shines through the slit is called a "light knife".

Since our cornea, lens and other eye tissues are actually "colloid", the tissues through which light passes will show different degrees of Tyndall phenomenon when illuminated, showing a light gray translucent form that is different from the surrounding darkness. In this way, by slowly adjusting the angle of the incident light, you can see the eye structure conditions in different positions.

We can do a small experiment to deepen our understanding of the principle of the slit lamp. The materials needed for the experiment are very simple, but it should be noted that in order to simulate the eyeball environment, the experiment needs to be completed in a dark space and on a dark table. Pour milk and water into an empty cup, place a flashlight in front of the slit plate, and then observe from the side.

Since the diameter of colloid particles is smaller than the wavelength of visible light, when visible light passes through the colloid, it will produce obvious scattering effect, and we can see a clear light path. Next, keep the observation angle unchanged, move the flashlight and slit plate around the cup, and you can see that the light path is also changing. Keep the light source and observation angle unchanged, put a piece of crystal at the bottom of the cup filled with milk, set a magnifying glass at the human eye observation position, move the magnifying glass back and forth, and you can observe the different levels of the crystal. In the slit lamp, the image seen is equivalent to a pathological section, which can show the optical section of the eye tissue.

Learning about optics will help you better understand your daily life. Next time you have a slit lamp eye exam, you can relax and put your chin on the shelf! After the exam, you can have an in-depth conversation with the doctor to fully understand the true condition of your eyes.

Welcome to ask questions in the comment area, we will @experts to answer~

(To be continued)

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Acknowledgements: Bureau of Science Communication, Chinese Academy of Sciences, Shanghai Science and Technology Commission, Shanghai Association for Science and Technology, Shanghai Jiading District Science and Technology Commission