How does an ophthalmoscope allow a doctor to examine the inside of the eye?

The ophthalmoscope, an instrument famously developed by the German scientist Hermann von Helmholtz in 1851, allows a doctor to inspect the interior of the eye by utilizing a directed light source.1

The device functions by projecting a strong light into the patient's eye using a small mirror or prism. This light reflects off the retina and travels back through a small aperture in the instrument. By looking through this hole, the examiner can view a magnified, nonstereoscopic image of the structures located at the back of the eye.1

This process enables the observation of critical anatomical features, including the optic disk, retinal blood vessels, the macula, and the choroid. Because it provides a clear view of these internal structures, the ophthalmoscope serves as a vital screening tool for diagnosing various ocular conditions, such as diabetic retinopathy.1