A narrow beam of light passes through a slab obliquely and is then received by an eye9figure 18-Q1). The index of refraction of the material in the slab fluctuates slowly with time. How will it appear to the eye? The twinkling of stars has a similar explanation.

As the refractive indices will change with time, so the ray of light will get refracted differently and will get concentrated on different points, not on eye. So, the person will see the starts and then again won’t. Like, the twinkle of stars.