Human flesh is opaque. As any good dictionary will tell you, that means it has the quality of "not transmitting light; being impenetrable to sight." Well, forget that: scientists now can use light to see inside objects that were traditionally off-limits to the human eye—including our bodies.
Of course it's already possible to use X-rays, MRI, ultrasound and the like to peer inside human beings, but results are never as crisp and clear as those acquired using visible light imaging. Not only that: optical wavelengths also interact with organic molecules—the one we're made of—so visible light could also contain vital information about the tissue it travels through. It might reveals abnormalities in cells, say, or use information about bodily functions—something that other imaging techniques, such as MRI, resort to complex chemical tracers to achieve. And, perhaps most importantly, it's also non-ionising, which is to say that, unlike X-rays and CT scans, it doesn't increase cancer risk at the intensities used for imaging.